Frogtech, UK OGL 21CRM East Shetland Platform SEEBASE Phase 1 by Frogtech Geoscience

21CXRM EAST SHETLAND PLATFORM PROJECT PHASE 1 – SEEBASE® STUDY

A GLOBAL LEADER IN GEOSCIENCE

TABLE OF CONTENTS

21CXRM EAST SHETLAND PLATFORM PROJECT PHASE 1 – SEEBASE STUDY March 2017 Table of Contents EXECUTIVE SUMMARY

I. INTRODUCTION Objective of the Study Frogtech Geoscience’s Approach SABRE Methodology SEEBASE Workflow

5 7 8 9 11

II. DATA COMPILATION AND PROCESSING Core and Constraining Datasets Custom Projected Coordinate System Properties Core Dataset Digital Elevation Model (DEM) Gravity Magnetics Surface Geology Calibration Dataset 2D Seismic and Cross-sections Wells Depth Modelling Data Confidence Map

13 15 16 17 17 18 22 24 25 25 26 27 28

III. BASEMENT INTERPRETATION Importance of Basement Present-day Distribution of Basement Terranes Terrane Unit Summaries Basement Assembly Basement Character Gravity Magnetics

29 31 32 33 36 38 38 39

IV. TECTONIC EVENTS Tectonic Evolution Summary (Paleozoic to Recent) Overview: Stratigraphic Chart Interpreted Structures and Events

41 43 46 49

V. SEEBASE AND DERIVATIVES SEEBASE SEEBASE: Basins and Terranes 3D SEEBASE: Terranes and Structure 3D SEEBASE: Basins Sediment Thickness Depth to Moho Basement Thickness Crustal Thickness Crustal Extension Crustal Architecture: Cross-Sections Interpretation Confidence

61 63 65 66 67 68 69 70 71 72 73 74

VI. GRAVITY MODELLING

EAST SHETLAND PLATFORM Project Team Geodynamics: Cedric Jorand, PhD (Senior Geoscientist) Jia-Urnn Lee, PhD (Geoscientist)

Geophysics: Zhiqun Shi, PhD (Principal Geophysicist)

VII. HEAT FLOW Importance of Basement Components and Approach Calibration Dataset Heat Flow Component I Heat Flow Component II Present-day Basement Heat Flow Model

81 83 84 85 86 92 94

VIII. DISCUSSION

IX. CONCLUSIONS

103

X. REFERENCES

107

APPENDICES

Digital File Only

PROJECT CODE: UK703 © All Rights Reserved The conclusions and recommendations expressed in this report represent the opinions of the authors based on the data available to them. No liability is accepted for commercial decisions or actions made resulting from this report. Please cite this work appropriately if all or parts of it are used or altered for use in other documents. The correct citation is:

Frogtech Geoscience, 2017, 21CXRM East Shetland Platform Project Phase 1 - SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

Adam Kroll, BSc(Hons) (Senior Geophysicist)

Stratigraphy: Jen-Deng Lee, BSc(Hons) (Geoscientist)

GIS: Grace Westerman, BSc (GIS Specialist)

Reviewers: Lynn Pryer, PhD (Principal Structural Geologist) Karen Connors, PhD (Principal Structural Geologist) Karen Romine, PhD (Director and Principal Geoscientist) Donna Cathro, PhD (Senior Geoscientist)

Office: Suite 17F, Level 1 2 King Street Deakin West ACT 2600 AUSTRALIA

Post:

PO Box 250 Deakin West ACT 2600 AUSTRALIA

T: E: W:

+61 02 6283 4800 info@frogtech.com.au frogtechgeoscience.com.au

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

The conclusions and recommendations expressed in this report represent the opinions of the authors based on the data available to them. No liability is accepted for commercial decisions or actions made resulting from this report. SEEBASE® and SABRE® are registered trademarks of Frogtech Pty Ltd. Please cite this work appropriately if all or parts of it are used or altered for use in other documents. The correct citation is: Frogtech Geoscience, 2017, 21CXRM East Shetland

Frogtech Geoscience Post: PO Box 250, Deakin West ACT 2600, AUSTRALIA Office: Suite 17F, Level 1, 2 King Street, Deakin West ACT 2600, AUSTRALIA T: +61 02 6283 4800, E: info@frogtech.com.au W: frogtechgeoscience.com.au

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

EXECUTIVE SUMMARY Introduction

Deliverables

The 21CXRM East Shetland Platform Project Phase 1 – SEEBASE Study is an integrated geophysical and geological study of basement and basin evolution of the Greater East Shetland Platform and surrounds on the UK Continental Shelf (UKCS). The study was undertaken in support of the United Kingdom Oil and Gas Authority’s (UK OGA) 21st Century Exploration Road Map Initiative (21CXRM) to stimulate future exploration activity and support improvements in regional understanding of this area.

Basement Geology: Analysis and description of basement terranes, assessment of terrane boundaries, and interpretation of basement composition.

The depth and geometry of the SEEBASE basement surface (lower right) provides a new interpretation of geological basement for the Greater East Shetland Platform and surrounds. Basement structure and composition have a fundamental control on the location, formation, timing and geometry of basins and depocentres. The SEEBASE structural model is the result of integration of multiple datasets using Frogtech Geoscience’s unique workflow, and advances the understanding of the economic potential of the underexplored East Shetland Platform region. It provides a geologically-constrained, spatially-continuous depiction of the basement surface, overcoming the challenge of poor imaging of basement on 2D seismic data, thus reducing the exploration risk on the UK Continental Shelf. This study is a cost-effective regional evaluation tool that gives a first-pass perspective on the geology and regional prospectivity of this complex area. It provides a basis for focusing future exploration and data acquisition strategies to reduce exploration risk and underpin petroleum systems analysis.

Depth-to-Basement Model - SEEBASE: 3D geometry and depth of the basement surface highlighting basin depocentres and highs.

The East Shetland Platform SEEBASE study is an integrated interpretation of public domain and UK OGAsupplied potential field datasets, geological maps, plate reconstructions, literature, published cross-sections and seismic sections. The study benefits from Frogtech Geoscience's state-of-the-art geophysical processing of public domain gravity and magnetic data to produce the highest possible resolution from multiple datasets, including several Frogtech Geoscience proprietary enhancements and suite of filters. Integrated evaluation of the SEEBASE surface, regional tectonic synthesis, basement composition and accompanying heat flow modelling highlights the regional crustal architecture and present-day basement heat flow distribution at a much higher resolution than available from published models. These deliverables emphasise the important relationship between basement geology and the location and geometry of overlying basins. Results presented in this atlas-style report are accompanied by an ArcGIS project containing interpretation layers as well as the processed and enhanced potential field datasets (grids and images). The processing and enhancements of the datasets are documented in Appendices I and II with this report.

Tectonostratigraphic History: Plate reconstructions and summary of key tectonic events; implications for basin development; interpretation of major basement faults, and fault event maps for key events.

Depth to Moho: Geometry and depth of the Moho surface. Maps of Sediment, Crustal and Basement Thickness, and Beta Factor: Derivative maps created using the SEEBASE and Moho models. Inferred Basement Heat Flow: Computation of radiogenic heat production and heat flow, integrating basement geology and architecture, as well as timing and intensity of regional tectonothermal events. Potential Field Datasets: Frogtech Geoscience's state-of-the-art geophysical processing and enhancements.

The East Shetland Platform SEEBASE Study provides a first-pass regional assessment, the resolution of which can be improved with integration of higher resolution potential field datasets. Frogtech Geoscience also recommends integration of higher quality calibration data such as wells and seismic, to further improve and customise the SEEBASE depth-to-basement model in areas of interest.

Depth msl (m) 3000 500 -2500 -5000 -7500 Oblique 3D View of the East Shetland Platform SEEBASE.

-9500

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

EXECUTIVE SUMMARY Key Results

21CXRM and Proposed Licensing Round

The 21CXRM East Shetland Platform Project Phase 1 – SEEBASE Study is an integrated geophysical and geological study of basement and basin evolution for the East Shetland Platform Area of Interest. The SEEBASE 3D model provides a geologically-constrained, spatially-continuous illustration of basement surface, and overcomes challenges of poor basement imaging on 2D seismic data. The project has yielded new insights and key results.

The East Shetland Platform SEEBASE study has been designed: i)

to support the UK Oil and Gas Authority’s 21st Century Exploration Road Map Initiative to stimulate future exploration activity and support improvements in regional understanding in the Greater East Shetland Platform and surrounds;

• The Greater East Shetland Platform is not a broad structural high as traditionally perceived. It comprises numerous basement highs, early Paleozoic half-graben and en-echelon basins of varying depths.

ii)

for companies with interest in the proposed 31st Frontier Licensing Round for the UK Continental Shelf. The UK OGA’s unlicensed blocks are shown in yellow (figure below).

• Basement and basin geometry in the SEEBASE highlight the difference in structural grain orientation of the Grampian Highlands to that of the Northern Highlands and Hebrides terranes. Basement structures are predominantly oriented NE-SW in the northern and western part of the AOI, corresponding broadly to the Hebrides, Northern Highlands and Faroes-Shetland terranes. In the Grampian Highlands Terrane, basement structures are predominantly oriented NW-SE on the East Shetland Platform.

Up to date information on the status of licensing rounds on the UKCS can be found at https://www.ogauthority.co.uk/.

• The contrast in basement fabric, and hence basin orientation, is controlled by: i) the complex interaction of the lower crust of the Baltica indentor with Laurentia below the Grampian Highlands; and ii) terrane boundaries and major faults in the upper crust and basement surface.

More detailed assessments of individual basins can be achieved with higher resolution geophysical datasets and/or additional calibration data. Frogtech Geoscience recommends the integration of proprietary data such as well formation tops and better quality seismic data, which will significantly improve the SEEBASE model, as well as the interpretation of the thermal history of the different basins. Frogtech Geoscience’s unique services, geological expertise and data management capabilities can be contracted for further enhancements and improvements to the East Shetland Platform SEEBASE Study.

• Gravity modelling demonstrates that both basement depth and compositionplay a major role in producing the observed gravity response over the East Shetland Platform. The models are consistent with potential field interpretation that the platform is cored by numerous intrusives, resulting in rheological variations.

• The rheological variations have controlled which areas persisted as sub-aerial highs (such as the Fladen Ground Spur (FGS) and Kraken High (KH)) and which areas developed into intra-platformal basins (such as the East Shetland Platform Basin (ESPB) and Crawford-Skipper Basin (CSB)). • Numerous felsic to intermediate intrusives below the platform are interpreted to generate locally high radiogenic heat flow values and to support the preservation of basins during tectonic inversion events.

Shetland Islands

• Potentially prospective zones in frontier areas can be inferred from the SEEBASE interpretation, based on similarities in tectonic style to known analogues. Examples include: • Interpreted Devonian syn-rift wedges and/or depocentres within the revised extent of the CrawfordSkipper Basin (CSB), and East Shetland Platform Basin (ESPB);

East Shetland Platform

• Formation of Early Paleozoic depocentres alongside major structures and terrane boundaries via transtension and reworking of basement structures, such as within the East Fair Isle Basin (EFI) and Dutch Bank Basin (DBB), on-trend with discoveries in the Witch Ground Graben (WGG); and • Potential preservation of early Paleozoic intra-platform depocentres within fault zones during inversion events, such as within the East Fair Isle Basin, Crawford-Skipper Basin and East Orkney Basin (EOB).

EFI

• Many Mesozoic-Cenozoic fields in the Witch Ground Graben, Inner Moray Firth Basin (IMF), and Viking Graben (VG) are located above Early Paleozoic structures and depocentres, with contributions from a possible Early Paleozoic source. • The regional depth-to-basement model provided by the SEEBASE, complemented by structural analysis over successive regional tectonic events, can be used to infer the trend of structural traps and depocentres on the East Shetland Platform. Many of the geological features highlighted in the SEEBASE are undrilled and present new opportunities for exploration.

ESPB

CSB

DBB FGS

EOB

IMF

WGG

Image of the East Shetland Platform SEEBASE. Yellow and black polygons show the UK OGA’s unlicensed and licensed blocks respectively on the UKCS current as at 01 March 2017. Frogtech Geoscience Post: PO Box 250, Deakin West ACT 2600, AUSTRALIA Office: Suite 17F, Level 1, 2 King Street, Deakin West ACT 2600, AUSTRALIA T: +61 02 6283 4800, E: info@frogtech.com.au W: frogtechgeoscience.com.au

SEEBASE EAST SHETLAND PLATFORM

I. INTRODUCTION

FROGTECH GEOSCIENCE

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

I. INTRODUCTION United Kingdom Oil and Gas Authority Government Funded Exploration License Competition In November 2016, the United Kingdom Oil and Gas Authority (UK OGA) launched an Exploration Licence Competition to identify innovative interpretations and products that will stimulate offshore oil and gas exploration activity in the UK Continental Shelf (UKCS). The competition sought proposals that used recently acquired datasets from the UK Government-funded gravity and magnetic surveys of the UKCS to complete petroleum system related studies for the East Shetland Platform area (Figues 1.1a and 1.1b). The Exploration Licence Competition for the East Shetland Platform package is phased and tendered as a series of lots. In December 2016, Frogtech Geoscience was awarded Phase 1, Lot 1 of the tender package to undertake an evaluation of the structural evolution of the East Shetland Platform. This report and accompanying GIS presents the final results of the 21CXRM East Shetland Platform Project Phase 1 – SEEBASE Study.

Faroes Islands

Norway Shetland Islands

Legend Geophysics AOI SEEBASE AOI Coastline United Kingdom

UKCS Offshore Quadrants

Open Blocks Licensed Blocks

Background and Area of Interest The main objective of the 21CXRM East Shetland Platform Project Phase 1 – SEEBASE Study is to provide a regional structural model and tectonic overview of basement evolution of the Shetland Platform and Northern North Sea regions on the UK Continental Shelf. It is undertaken in support of the UK OGA’s 21st Century Exploration Road Map initiative (21CXRM), and which aims to stimulate future exploration activity and support improvements in regional understanding of this area. This study evaluates the structural evolution of the East Shetland Platform area, and includes a discussion of basement terranes, basement composition, tectonics, basement depth and architecture, gravity modelling, and heat flow, to provide a geological context in which to place petroleum systems observations. Integration of these geological elements will help explorationists predict the occurrence and distribution of petroleum system play elements. Frogtech Geoscience’s innovative methodology to map depth-to-basement has produced a 3D basement surface at a significantly higher resolution than previously depicted. The depth and geometry of existing basins, as well as revision of the extent of depocentres, are revealed across an uninterrupted view of basement in the Area of Interest (AOI, red dashed polygon in Figure 1.1). A review of the tectonostratigraphic evolution of the region is integrated into the depth-to-basement interpretation, which fundamentally improves geological understanding of this area. The SEEBASE structural model illustrates the economic potential of the under-explored East Shetland Platform region. It reduces the risk and challenges of uncertainties in seismic interpretation, and improves targeted oil and gas exploration on the UK Continental Shelf. The East Shetland Platform SEEBASE provides a first-pass regional assessment at 1:2,000,000 scale, the resolution of which can be improved with integration of higher resolution potential field datasets. Frogtech Geoscience recommends integration of client proprietary datasets, including higher quality calibration data such as wells and seismic, to further improve and customise the SEEBASE depth-to-basement model in areas of interest.

Figure 1.1: a) Location map showing the Geophysics Area of Interest and SEEBASE Area of Interest for the 21CXRM East Shetland Platform Project Phase 1 – SEEBASE Study. b) Digital Elevation Model showing the SEEBASE project area (dotted red polygon) for the East Shetland Platform SEEBASE study. The light grey polygons show the United Kingdom Continental Shelf (UKCS) offshore quadrants downloaded from https://www.gov.uk/guidance/oil-and-gas-offshore-maps-and-gis-shapefiles. Yellow polygons represent Open Blocks, and dark grey polygons represent licensed blocks, downloaded from https://www.ogauthority.co.uk/data-centre/, current as of 01 March 2017.

I. INTRODUCTION

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

Frogtech Geoscience’s Approach The evolution of sedimentary basins is controlled by a response in the crust and lithosphere to tectonic forces. The nature of this response depends both on the magnitude of the tectonic forces and on the character and kinematic response of the underlying basement. The strength, composition and fabric of basement at the time of a tectonic event controls crustal response, while sediments record the resultant changes in basin morphology. A rigorous model for basin evolution can be developed through an understanding of basement character beneath and adjacent to sedimentary basins, coupled with a knowledge of tectonic events that were responsible for basin formation (i.e. basin phases). This model (Figure 1.2) provides a basis for more accurate prediction of the occurrence and distribution of petroleum play elements throughout basin evolution. Individual basin phases are separated from one another by changes in the type of subsidence mechanism or the magnitude or rate of subsidence. Basin phase boundaries correspond to plate-scale tectonic events and in turn to major megasequence boundaries. Stresses operating during each basin phase cause reactivation of basement structures and reactive fabrics, as well as the development of new structures. Understanding the kinematics of each tectonic event allows a predictive model for structural reactivation to be applied to the interpreted faults from fault history data calibrated with geological observations (e.g. seismic, maps). Potential field data (principally gravity and magnetic data) provide a window to the basement that can cover a wide area with uninterrupted data at constant resolution. Such ‘map view’ interpretation contrasts with the ‘cross-section view’ interpretation that often results from only working with more conventional petroleum industry datasets such as wells and seismic data. But the combination is a highly successful one, enabling extraction of more geological information than is possible with either dataset on its own. This is the basis of the Frogtech Geoscience approach that has been developed over many years.

1 4 2

® Figure 1.2: Frogtech Geoscience’s SABRE methodology

Frogtech Geoscience’s SABRE Methodology SABRE®: Systematic Approach to Basin Resource Evaluation Frogtech Geoscience has developed SABRE® - Systematic Approach to Basin Resource Evaluation – as the backbone of its rapid basin evaluation workflow. The SABRE methodology uses efficient and effective techniques to fasttrack the evaluation and ranking of play elements using regional-scale datasets and constraining information. The SABRE methodology (Figure 1.2) begins with a big-picture analysis of plate reconstructions, structural provinces, basement terranes and regional kinematics (left side of diagram). This regional analysis provides a foundation to move rapidly into basin and play-scale evaluations (right side of diagram) – from Plate to Pores! Frogtech Geoscience’s SABRE methodology develops a comprehensive structural model which is built on the integration of all readily available geophysical and geological datasets and other constraining information. While individual datasets used in isolation can produce ambiguous results, the integration of multiple datasets yields a more tightly constrained model that is consistent across all available data. Frogtech Geoscience experts in geophysics, structural and basement geology, geodynamics and basin analysis work as a team and undertake multiple iterations to build the integrated structural model. The big-picture and ‘bottom-up’ approach of Frogtech Geoscience’s SABRE methodology is also described in Figure 1.3, and illustrated using a geological example from Australia’s Southern Margin (Figure 1.4). Frogtech Geoscience’s signature product is a depth-to-basement model called SEEBASE®– Structurally Enhanced view of Economic BASEment. The SEEBASE product is the culmination of the integrated geophysical and geological analysis undertaken by Frogtech Geoscience interpreters. Key elements of the SEEBASE interpretation and integration are shown diagrammatically in Figure 1.5.

I. INTRODUCTION

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

SABRE Methodology SABRE Methodology and SEEBASE Workflow

BASIN PHASES Petroleum Systems and Play Evaluation Sediment Provenance and Supply Palaeogeography Stratal Geometry Accommodation History

• Compile and process (data and images) all available potential field datasets • Compile and process all constraining dataset, such as but not limited to: wells, seismic, geology, cross-sections, stress, heat flow, radiometric dates, seismicity, refraction, detrital minerals, isopachs, etc. • Identify tectonic events (inter- and intra-plate) and responses (deformation events) that have influenced basement and basin evolution • Calibrate geophysical/remote sensing data interpretation to regional and local geology • Identify basement geology and terrane boundaries – attribute in GIS • Interpret georeferenced datasets • Define succession of structural events in area of interest – attribute in GIS

BASIN ARCHITECTURE SEEBASE™ Depth to Basement Structural Analysis

• Develop a kinematic and mechanical model for the structural evolution of the area • Confirm main events that control timing and duration of basin phases • Estimate depth to basement from all appropriate datasets • Combine structural model and depth to basement estimates to generate SEEBASE image • If contracted, undertake evaluation of petroleum systems, plays and play elements within SEEBASE framework – this may include the following steps:

BASEMENT GEOLOGY Terranes Type and Age Composition Structure Crustal Architecture Heat Flow

• Constrain basement heat flow history • Constrain paleogeography of potential source rocks, reservoirs and seals • Define areas of likely fractured reservoir • Predict basement-involved trap types, size, timing, and integrity • Develop new exploration and acquisition strategies appropriate for the exploration risk

WHAT IS BASEMENT? SEEBASE maps depth to economic basement. Economic basement is defined by Frogtech

TECTONIC HISTORY Precambrian and Phanerozoic Basement Evolution and Assembly Plate Reconstructions Tectonic Events and Response

Geoscience interpreters as the base of all sedimentary rocks, which would generally be the base of all potential petroleum systems other than fractured basement reservoir. In most cases, economic basement is also metamorphic or crystalline basement defined as the top of metamorphic or igneous lithologies. Mapping basement with gravity data relies on the density contrast between sediments and basement lithologies. Magnetic basement is defined as the top of magnetic lithologies with the exception of intra-sedimentary sources including volcanic rocks. The combination of gravity and magnetic data allows identification of basement lithologies.

Figure 1.3: Frogtech Geoscience’s “Bottom-Up” Rapid Basin Evaluation Process. Green colours indicate workflows usually undertaken in a standard SEEBASE study. Workflows shown in red were not undertaken during this study.

I. INTRODUCTION 115 °

SEEBASE in SABRE Workflow

120 °

125 °

130 °

135 °

140 °

145 °

Penola Trough

-30 °

Robe Trough -35 °

Megasequences

Otway Ranges Torquay

Recherche

Gippsland

Ceduna

Each basin phase is characterised by a specific subsidence mechanism that may vary considerably along an extensional margin.

-40 °

Sorell

Basin Phases

Type 2 Type 5

115 °

Type 1

120 °

110 °

115 °

Type 4

125 ° 120 °

130 °

125 °

130 °

135 ° 135 °

Officer

140 °

145 °

150 °

-30 °

Perth

Polda

Eyre

Stansbury

Ceduna

Basins

Mentelle

Recherche

Bremer

Duntroon Robe Trough

-35 °

Denmark

-35 °

Penola Trough

Recherche

Colac Trough

Beachport Subbasin

Torquay Sub-basin Gippsland

Outer Otway

-40 °

Bass

Sorell 135 ° Christie

140 °

Terrane

-30 °

Gawler 110 °

115 °

120 °

125 °

130 °

Yilgarn Craton Indian Ocean

Volcanics

St Vincent’s 145 ° Terrane

Albany-Fraser Mobile Belt

Highly Extended Continental Crust

Moonta-

Coulta Terrane

Wallaroo

Extended

Domain Kangaroo

Coulta Terrane

(undifferentiated) Transitional Crust

-35 °

Stavely Terrane

AdelaideKanmantoo Fold Belt

King Island Block

-35 °

Pinjarra Mobile Belt

Island

Stawell-

Block

Terrane MelbourneNW Tasmania

(oceanic crust)

Block

-40 °

Terranes can generally be divided into cratons resistant to deformation and mobile belts. The fabric and rheology, or mechanical behaviour of each terrane controls basement response to tectonic stress. Terrane boundaries and the fabric of mobile belts are the primary control on basin location and evolution.

Extended

Tabberabbera-

Adelaide-

Mathinna

Kanmantoo

Terrane

-40 °

Mobile Belts

Figure 1.4: An example of Frogtech Geoscience’s SABRE Methodology from the Australian Southern Margin.

Buchan Terrane

Bendigo

(Continent-Ocean Boundary) Southern Ocean

Cratons

150 °

Nuyts Terrane

(oceanic crust)

Terrane Type

Range 140 °

135 °

Fowler Terrane

145 °

Wilgena Terrane

-30 °

SEEBASE images provide a picture of the present-day shape of the basins. The image is derived from the systematic integration and interpretation of both non115 ° 120 ° 125 ° 130 ° Nawa seismic and seismic data and an underlying structural model for the Munyarai evolution of Terrane Terrane the area. It is a geological interpretation, not a geophysical inversion.

-40 °

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

Type 3

Bremer

-40 °

SABRE

Each basin phase is defined by a tectonic event and the kinematic response (fault reactivation) of the structural fabric of each terrane.

Polda

-35 °

-30 °

Eyre

Perth

Fold Belt

Tynean Block CambroOrdovician Mobile Belts

110 °

115 °

120 °

125 °

130 °

135 °

140 °

145 °

150 °

I. INTRODUCTION

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

SEEBASE Workflow Frogtech Geoscience’s SEEBASE workflow develops a comprehensive depth-to-basement structural model which is built on the interpretation and integration of all readily available geophysical and geological datasets and other constraining information. While individual datasets used in isolation can produce ambiguous results, the integration of multiple datasets yields a more tightly constrained model that is consistent across all available data.

Deliverable

Frogtech Geoscience’s experts in geophysics, structural and basement geology, geodynamics and basin analysis work as a team and undertake multiple iterations to build the integrated structural model. The model is then applied and tested by iterative interpretation and checked against the data interpretation. Adjustments are made as needed to both the model and the data interpretation with each iteration. The SEEBASE model is dynamic and can be updated as new datasets become available.

Process Basement Heat Flow Prediction

Tectonic Events and Fault Maps

Crustal Architecture and Evolution

Derivatives: Depth to Moho, Sediment and Basement Thickness

SEEBASE

Integration and Calibration

Basement Composition

Well and Seismic Interpretation

Tectonic Events and Kinematic Interpretation

Basement Terranes

Gravity Modelling

Magnetic Depth Modelling

Processed Potential Field Datasets

Plate Reconstructions

New Data Acquisition

Interpretation

Project Setup and Management

Data Compilation and Processing

ArcGIS Database and Metadata Construction

Upgrade SEEBASE Figure 1.5: Frogtech Geoscience’s SEEBASE Workflow Chart.

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

SEEBASE EAST SHETLAND PLATFORM

II. DATA COMPILATION AND PROCESSING FROGTECH GEOSCIENCE

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

II. DATA COMPILATION AND PROCESSING

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

Core and Constraining Datasets All readily available, open-file datasets were compiled into GIS format for this study (Figure 2.1). These datasets can be divided into Core Datasets which are interpreted and integrated in detail, and Calibration Datasets which are used to constrain the interpretation. Potential field datasets (gravity and magnetics) are important map-view data that are used extensively by Frogtech Geoscience interpreters as they provide the most continuous coverage of a study area. While the current study utilised existing datasets, the SEEBASE and structural interpretations can be upgraded and further calibrated with the acquisition of new datasets. Frogtech Geoscience can also upgrade this product under separate contract using client-held proprietary datasets.

Core and Constraining Datasets Gravity…………………………………………………………………. Grids and images Magnetics……………………………………………………………. Grids and images Digital Elevation Model (DEM)…………………………….. Grids and images Surface Geology…………………………………………………… Digital coverage and maps

Calibration Datasets Wells……………………………………………………………………. Location, formation tops, basement penetrations, etc. Stratigraphy…………………………………………………………. Published stratigraphic charts, wells and seismic data, paleogeography, tectonostratigraphy Publications, Papers, Maps, Cross-sections………….. Extensive reference list Frogtech Geoscience Regional Knowledge-base…… Global experience and intellectual property

The following section outlines the datasets used for this report and processing undertaken on the data.

Figure 2.1: Example of georeferenced Calibration/Integration/Interpretation of data.

II. DATA COMPILATION AND PROCESSING

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

Project Area The area of processed magnetic, gravity and DEM data covers the Greater East Shetland Platform and its surrounds (red polygon, Figure 2.2). The area covered by the SEEBASE interpretation is shown by the dashed black polygon. The Geophysics AOI covers a wider area than the SEEBASE AOI. The ArcGIS product contains geophysical datasets, interpreted datasets, calibration and reference datasets used in this study. Datasets in the ArcGIS are provided in the European 1950 Datum and UTM Zone 30N Projection.

Custom Projected Coordinate System Properties Custom Projection Projection Name:

European Datum 1950 UTM Zone 30N

False Easting:

500000

False Northing:

Central Meridian:

-3

Scale Factor:

0.999600000000000040

Latitude of Origin:

Linear Unit Name:

Metre

Metres per Unit:

Geographic Coordinate System Name:

GCS_European_1950

Angular Unit:

Degree (0.0174532925199433)

Prime Meridian:

Greenwich (0.0)

Datum:

D_European_1950

Spheroid:

International_1924

Semi-major Axis:

6378388.0

Semi-minor Axis:

6356911.946127947

Inverse Flattening:

297.0

Legend Geophysics AOI East Shetland Platform SEEBASE AOI Coastline

Figure 2.2: Geophysics and SEEBASE Area of Interest shown with coordinates using the European Datum 1950 UTM Zone 30N. Projection as detailed.

II. DATA COMPILATION AND PROCESSING

Digital Elevation Models often show the youngest and/or active geological structures. They are widely used for neotectonic analysis. A DEM may also be used to distinguish different compositional domains where the composition of eroding terrain controls its resistance to weathering.

Frogtech Geoscience Data Processing Source dataset: SRTM15 Plus V1 (released November, 2014) SRTM15 Plus is a global DEM with a horizontal grid spacing of 15 arc-seconds (approximately 500 meters) comprising a combination of land data from the USGS SRTM30 gridded DEM data product created with data from the NASA Shuttle Radar Topography Mission, and ocean data based on the Smith and Sandwell global 1-minute grid. All data are derived from public domain sources. For details see the SRTM Homepage: http://www2.jpl.nasa.gov/srtm/. For source data information refer to http://topex.ucsd.edu/marine_topo/mar_topo.html.

References:

Becker, J. J., D. T. Sandwell, W. H. F. Smith, J. Braud, B. Binder, J. Depner, D. Fabre, J. Factor, S. Ingalls, S-H. Kim, R. Ladner, K. Marks, S. Nelson, A. Pharaoh, R. Trimmer, J. Von Rosenburg, G. Wallace, P. Weatherall., Global Bathymetry and Elevation Data at 30 Arc Seconds Resolution: SRTM30_PLUS, Marine Geodesy, 32:4, 355–371, 2009. Smith, W. H. F. and D. T. Sandwell, Global seafloor topography from satellite altimetry and ship depth soundings, Science, v. 277, p. 1957–1962, 26 Sept., 1997.

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

Core Dataset: Digital Elevation Model (DEM)

Legend Coastline SEEBASE AOI

Figure 2.3: SRTM15 Plus V1 (Becker et al., 2009; Smith and Sandwell, 1997) for the East Shetland Platform study. Note that the Geophysics AOI covers a wider area than the SEEBASE AOI.

II. DATA COMPILATION AND PROCESSING

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

Core Dataset: Gravity Introduction

Frogtech Geoscience Data Processing

Gravity is a very important tool for interpreting basins. It maps subtle changes in the Earth’s gravitational field caused by variations in the density of the underlying rocks, and it provides valuable information on basement topography and the nature of the deeper parts of the crust and mantle beneath the basins. Important intra-basin elements often have an associated gravity signature indicating that each element is related to a deep basement structure.

The coverage used for the East Shetland Platform project (Figure 2.4a) is a stitch of the source datasets below (Figure 2.4b).

In order to interpret the geological source of a gravity anomaly, the data must be calibrated. Gravity images show density contrasts within the crust, but the source of the contrast is not unique. As a regional tool it gives information both on the density of bodies within the crust and on differences in mantle depth and composition. Satellite free-air gravity also has a major contribution from bathymetry. Thus, the nature of each anomaly as crust or mantle must be distinguished. By combining the onshore gravity data with the mapped geology of the same region, the sources of many anomalies can be inferred and extrapolated offshore and/or under sedimentary cover. Others require geophysical modelling which must be constrained by a geological model. Calibrated interpretation of gravity data is a powerful tool for developing an understanding of basin shape.

• British Geological Survey Land Gravity Data (downloaded May, 2016) • Satellite Free-Air Gravity V23.1 (released May, 2014) from SIO and NOAA (Sandwell and Smith, 2009). The satellite gravity data (Free-Air) was downloaded from Scripps Institution Of Oceanography. The data has a grid cell size of 1-minute or 0.0167 degree, equivalent to about two kilometres and is provided in European Datum 1950 UTM 30N projection. For source data information refer to: http://topex.ucsd.edu/WWW_html/mar_grav.html. Details of the data assessment, processing and enhancements are presented in Appendices I and II included as with this report. Reference:

Sandwell, D. T. and W. H. F. Smith, Global marine gravity from retracked Geosat and ERS-1 altimetry: Ridge Segmentation versus spreading rate, J. Geophys. Res., 114, B01411, doi:10.1029/2008JB006008, 2009.

Legend Coastline SEEBASE AOI

BGS Land Gravity Data Satellite Free-Air Gravity V23.1

Figure 2.4a: Colour-shaded relief image of the Frogtech Geoscience Stitched Free-Air Gravity data for the East Shetland Platform study.

Figure 2.4b: Gravity source datasets stitched together for the East Shetland Platform study.

II. DATA COMPILATION AND PROCESSING

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

Core Dataset: Gravity Bouguer Gravity

First Vertical Derivative of Bouguer Gravity

Free-air gravity data is reduced to Bouguer gravity by the calculation and removal of the effects of topography (rock/water or rock/air interface) on the gravity anomaly. Figure 2.5 is a Bouguer correction of the satellite gravity data. In this case, the density used for Bouguer correction was 2.67 – 2.70 g/cm3 onshore and 2.20 g/cm3 offshore.

The first vertical derivative (1VD) filter enhances near surface contrasts in density by amplifying the high-frequency component of the spectrum (linear increasing filter). Noise and merging artefacts are also enhanced in this process. Figure 2.6 is a 1st vertical derivative of the Bouguer-corrected satellite gravity data. In this case, the density used for Bouguer correction was 2.67 – 2.70 g/cm3 onshore and 2.20 g/cm3 offshore.

Legend Coastline SEEBASE AOI

Figure 2.5:

Colour-shaded relief image of the Frogtech Geoscience Stitched Bouguer gravity data for the East Shetland Platform study.

Figure 2.6:

Colour-shaded relief image of the First Vertical Derivative of the Frogtech Geoscience Stitched Bouguer gravity data for the East Shetland Platform study.

II. DATA COMPILATION AND PROCESSING

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Core Dataset: Gravity Isostatic Residual of Bouguer Gravity

Ternary Image of Tilt, Modulus and ZS-Edge

Isostatic residual gravity anomaly maps are produced by subtracting long-wavelength anomalies related to topography. The long-wavelength anomalies are assumed to result from isostatic compensation of topographic loads. Isostatic residual gravity anomaly maps (Figure 2.7) therefore reveal the density distributions within the crust more clearly than Bouguer gravity anomaly maps. The Isostatic Regional field (or Isostatic Correction; not shown) is the calculated gravimetric response to changes in the thickness of the crust (e.g. in a mountain belt with a deep root).

The CMY ternary image (Figure 2.8) combines the results of the Tilt (cyan), Modulus (magenta) and ZS-Edge (yellow) enhancement filters to produce a highly detailed image. The enhancement image highlights the edges of broad anomalies, enhances subtle anomalies in areas of relatively flat response, and shows continuity of major structures while retaining the anomaly amplitude over the source, thereby improving anomaly characterisation and the mapping of source edges. This image is also of use in accurately locating anomalies as no sun angle is applied to the image. Individual grids (tilt, MS, ZS-Edge) can be found in the accompanying ArcGIS project.

Legend Coastline SEEBASE AOI

Figure 2.7:

Colour-shaded relief image of the Isostatic Residual of the Frogtech Geoscience Stitched Bouguer Gravity data for the East Shetland Platform study.

Figure 2.8:

Colour-shaded relief image of the Frogtech Geoscience Stitched Bouguer Gravity Tilt, Modulus and ZS-Edge filters for the East Shetland Platform study.

II. DATA COMPILATION AND PROCESSING

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

Core Dataset: Gravity High-pass (100 km) Applied to Bouguer

Low-pass (100 km) Applied to Bouguer

A residual separation of the short-wavelength components (<100 km) of the Bouguer gravity grid was undertaken to reduce the effects of the shallow Moho. The resulting image is useful for interpreting upper crustal structure and basement relief. A High-pass (HP) filter passes high frequencies but substantially attenuates (or reduces) frequencies lower than the cut-off frequency. The 100 km HP filter (Figure 2.9) is used to enhance the anomalies from rather shallow depths. The wavelengths of the anomalies are within 100 km.

A Low-pass (HP) filter passes high frequencies but substantially attenuates (or reduces) frequencies higher than the cut-off frequency. The 100 km LP filter (Figure 2.10) is used to enhance the anomalies from intermediate depths, as well as the top parts of deep sources. The wavelengths of the anomalies are greater than 100 km. However, caution is warranted when interpreting the image, because the signals may also arise from aliasing of shallower sources.

Legend Coastline AOI

Figure 2.9:

Colour-shaded relief image of the 100 km High-pass filter of the Frogtech Geoscience Stitched Bouguer Gravity data for the East Shetland Platform study.

Figure 2.10: Colour-shaded relief image of the 100 km Low-pass filter of the Frogtech Geoscience Stitched Bouguer Gravity data for the East Shetland Platform study.

II. DATA COMPILATION AND PROCESSING

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Core Dataset: Magnetics Introduction

Total Magnetic Intensity (TMI)

Aeromagnetic data measures variations in the Earth’s magnetic field caused by variations in the magnetic susceptibility of the underlying rocks. It provides information on the structure and composition of magnetic basement and intra-sedimentary magnetic units, if present. Most bodies within the basement have a distinctive magnetic signature which is characterised by the magnitude, heterogeneity, and fabric of the magnetic signal. When calibrated with known geology, magnetic data can be used to map basement terranes under a cover of sedimentary rock, regolith, water, or ice.

The Total Magnetic Intensity (TMI) is the basic magnetic grid before enhancement (Figure 2.11b). The TMI is the measurement of total magnetic field at a location with the International Geomagnetic Reference Field (IGRF) removed. The IGRF is a spatially and time varying quantity. The IGRF is removed so that measurements at different times and locations can be merged and compared. The TMI dataset provides the raw dataset and is therefore a key reference dataset when evaluating the enhanced datasets. However, the TMI dataset does not provide information on the correct position of anomalies and does not reflect the shape of the source at low latitude. For accurate position of source anomalies, refer to the Reduction-to-Pole filter on the next page.

The most important and accurate information provided by magnetic data is the structural fabric of the basement. Major basement structures can be interpreted from consistent discontinuities and/or pattern breaks in the magnetic fabric. Once the structures have been evaluated and combined with those interpreted from gravity data, a model for the evolution of the basement and overlying basins can be developed. Where the source of the magnetic signal is very deep and not resolvable after standard data processing, enhancement techniques are applied that reveal information on the geometry and structure of the basement at depth. Enhancement processing techniques are chosen specifically for each magnetic dataset depending on the type of information that needs to be extracted. Enhancement processing is critical for evaluating deep basins. Magnetic data is also valuable for determining the distribution of magnetic sources within the sediments ranging from heavy mineral deposits (e.g. fans) to basalts.

Frogtech Geoscience Data Processing The magnetic grid for the East Shetland Platform project is a stitch of the following source datasets (Figure 2.11a). • British Geological Survey Legacy Data Mid North Sea High Pre-1990 Magnetic Marine Survey

Legend

• British Geological Survey Aeromagnetic Data UK (downloaded May, 2016)

Coastline

• Geological Survey of Canada Aeromagnetic Data

SEEBASE AOI

BGS Legacy Data Mid North Sea High Magnetic Survey BGS Aeromagnetic Data UK Geological Survey of Canada Aeromagnetic Data

Figure 2.11: a) Magnetics source datasets stitched together for the East Shetland Platform study.

Figure 2.11: b) Frogtech Geoscience Stitched Total Magnetic Intensity image for the East Shetland Platform study.

II. DATA COMPILATION AND PROCESSING

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Core Dataset: Magnetics Reduction to Pole

Ternary Image of Tilt, Modulus and ZS-Edge

To obtain Reduction to Pole (RTP), a transformation from the observed TMI to the predicted magnetic field at the North or South Magnetic Pole is required. The transformation is computed based on a local geomagnetic inclination and declination. The result of the reduction to pole (RTP) operation is a field which would be observed if the given field had been observed with vertical polarisation, that is, as though observed at one of the Earth’s magnetic poles (refer to Appendices I and II for further details). If strong remanent magnetisation is present in directions other than that of the Earth, the transformed field will be in error. TMI data is routinely reduced to the pole to shift anomalies directly over their source and for vertical dipping sources to produce symmetric anomalies. The location of sources, particularly source edges, can more readily be determined when the magnetic data has been reduced to the pole.

The CMY ternary image (Figure 2.13) combines the results of the Tilt (cyan), Modulus (magenta) and ZS-Edge (yellow) enhancement filters to produce a highly detailed image. The enhancement image highlights the edges of broad anomalies, enhances subtle anomalies in areas of relatively flat response, and shows continuity of major structures, while retaining the anomaly amplitude over the source, thereby improving anomaly characterisation and the mapping of source edges. This image is also used in accurately locating anomalies as no sun angle is applied to the image. Individual grids (tilt, MS, ZS-Edge) can be found in the accompanying ArcGIS project.

Legend Coastline SEEBASE AOI

Figure 2.12: Colour-shaded relief image of the Frogtech Geoscience Stitched Reduction to Pole (RTP) magnetic data for the East Shetland Platform study.

Figure 2.13: Ternary image of the Tilt, Modulus and ZS-Edge filters of the Frogtech Geoscience Stitched RTP magnetic data for the East Shetland Platform study.

II. DATA COMPILATION AND PROCESSING

Surface geology (Figure 2.14) is a key dataset for any geological interpretation. Surface geological maps provide calibration for interpretation of DEM, gravity, and magnetic data. Where basement is outcropping, direct correlation of geological units with patterns in geopotential field data is possible. Once the magnetic and/or gravity response of different basement lithologies has been calibrated, it is possible to extrapolate beneath basins to interpret basement character. The main surface geology dataset used for interpretation was compiled from the British Geological Survey public domain geological data. The digital geology was converted in a Frogtech Geoscience standard from the original maps. Data sources: • 1:625,000 Digital Geological Map of Great Britain. British Geological Survey, 2016.

Legend Coastline SEEBASE AOI

Lithology Conglomerate

* * * * * *

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

Felsic Extrusive

E Felsic Intrusive

* * * * * * * * *

Gneiss Mafic Extrusive

E E E Mafic Intrusive BBB E E E Metamorphic BBB BBBMigmatite

Mudstone / Claystone Quartzite Sandstone Surficial E

* * * * * * * * *

Core Dataset: Surface Geology

Ultramafic Intrusive Volcaniclastic !

Figure 2.14: Digital geology map FOR the East Shetland Platform study.

II. DATA COMPILATION AND PROCESSING

Published cross-sections and seismic sections (Figure 2.15) were used to calibrate the structural interpretation and constrain the SEEBASE surface.

2D Seismic Seismic data is important for calibration of basement depth and structure. In particular, the interpretation of fault movement histories, essential for calibration, is not readily obtained from other data sources. Uninterpreted and interpreted seismic sections for the East Shetland Platform study are from publicly available sources. Stratigraphic horizons, as well as any interpreted basement, are compared to potential field data, as well as to other calibration data such as well data to assess their viability.

Published Cross-Sections Published cross-sections provide important regional constraints on the structural geometry of basement blocks and basins and on the movement histories of major structures. Information about the source of individual cross-sections can be found in the cross-sections attribute table in the accompanying ArcGIS project.

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

Introduction

Legend Data Type J J J J J J J

Calibration Dataset: 2D Seismic and Cross-Sections

Cross-Section

Gravity Model

Refraction

Seismic - Interpreted

Seismic - Uninterpreted

Seismic Horizons

Well - Correlation

Figure 2.15: Published cross-sections, seismic reflection and refraction data used in this study. The Digital Elevation Model for the East Shetland Platform is shown as a background.

II. DATA COMPILATION AND PROCESSING

Well data used for the East Shetland Platform project is a compilation of available open source well data in the project area and surrounds, and also provided by the United Kingdom Oil and Gas Authority.

The offshore well dataset (Figure 2.16a) has been used to provide a constraint on the minimum depth to basement in this study. However, total penetrated depth of the wells (Figure 2.16b) is generally at a significant depth above basement depth interpreted from other datasets (e.g. published cross-sections and seismic). Basement for this study is characterised by rocks of Scandian age (~435 Ma) and older (see definition of basement in Section III of this report). Wells that penetrate Devonian lithologies that are not crystalline basement (for example Old Red Sandstone Group) are not considered in this study to be basement wells. Information from published maps and cross-sections has been used to confirm the location of a few key basement wells during interpretation (e.g. Bassett, 2003).

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

Wells

Legend Total Depth (m) MSL -7999 - -7000 -6999 - -6000 -5999 - -5000

Well Location

( !

Basement Well Non-basement Well

-4999 - -4000 -3999 - -3000 -2999 - -2000 -1999 - -1000 -999 - 0

Figure 2.16: a) Location map of compiled well information used in this study. b) Location map of wells showing total depth in meters. The Digital Elevation Model for the East Shetland Platform is shown as a background in both images.

II. DATA COMPILATION AND PROCESSING

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Calibration Dataset: Depth Modelling Magnetic models of depth, dip, width, depth extent, and susceptibility of magnetic sources were calculated using the modelling package ModelVision. 180 lines (Figure 2.17), were modelled in order to assess specific anomalies in the East Shetland Platform AOI. The magnetic model profiles are located within the area covered by BGS aeromagnetic data (western part of AOI). Magnetic depth modelling was not carried out for the broader geophysics area (faded TMI image in Figure 2.17) due to poor resolution. Further detail is provided in Appendix III of this report. First, suitable profiles were selected across the TMI grid (see example below). These profiles were then modelled individually to match observed magnetic field variations using tabular sources of uniform magnetisation and sharp margins with distinct edges. An initial array of bodies is created based on inspection of the profile. These bodies are adjusted using forward modelling, with strike, position, and azimuth corrected to match the anomaly extents mapped by the TMI image.

The model is then adjusted by inversion of the body positions (along profile), widths, dip, susceptibilities, and depths to closely match the observed magnetic field variation along that profile. The source bodies can be converted to depth-point values at the centre of their top faces. With images of the depth sections used for reference, the most reliable depth points are selected based on geological understanding of the study area. These depth points and inferred faults are incorporated into a (possibly discontinuous) surface to represent the faulted top of basement or, where appropriate, of any intrasedimentary surface at which sources generate magnetic field variations. A brief discussion on the use of magnetic depth models for constraining basement depth is included at the end of Section V in this report.

Depth Modelling Process Using a Generic Example

Legend

Step 1: A traverse is selected through the

Magnetic Modelling Depths (m)

anomaly of interest to pass through maxima/minima roughly perpendicular to strike. Bodies created for each anomaly are positioned, strike and azimuth adjusted to match the anomaly extents.

-14394 - -14000 -13999 - -13000 -12999 - -12000 -11999 - -11000 -10999 - -10000 -9999 - -9000

Step 2: A regional field gradient is assigned and

-8999 - -8000

then the bodies are inverted to match the field variation along the extracted traverse.

-7999 - -7000 -6999 - -6000 -5999 - -5000 -4999 - -4000 -3999 - -3000 -2999 - -2000 -1999 - -1000

Step 3: A data point containing the source parameters is generated for each body.

-999 - 0 1 - 1000 Magnetic Modelling Lines

Figure 2.17: Modelled depth points used to help assess depth to basement, against the BGS Aeromagnetic TMI magnetic image, on the broader (faded) stitched TMI magnetic image. Selected depth models were used to constrain the SEEBASE surface. Frogtech Geoscience Post: PO Box 250, Deakin West ACT 2600, AUSTRALIA Office: Suite 17F, Level 1, 2 King Street, Deakin West ACT 2600, AUSTRALIA T: +61 02 6283 4800, E: info@frogtech.com.au W: frogtechgeoscience.com.au

II. DATA COMPILATION AND PROCESSING

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Data Confidence Map The purpose of the Data Confidence map in Figure 2.18a is to communicate the interpreters’ evaluation of the various datasets supporting the development of the SEEBASE model.

Sections - Confidence 10 20

Confidence values have been assessed for the following calibration datasets:

All Datasets

- Mapped basement outcrop (Figure 2.18b)

- Published cross-sections and seismic sections, and wells (Figure 2.18b)

50 60

- Gravity coverage (Figure 2.18c) and magnetic coverage (Figure 2.18d)

During the interpretation process, the interpreter evaluates various datasets available to support the SEEBASE model, and assigns confidence percentages to each feature. Data confidence ranges from 0–100%, with:

Wells (TD Stratigraphic Age) 0 No Information

• a constant value of 100% for basement outcrop,

10 No Lithology Or Formation Data

• 20 to 80% for published cross-sections and seismic sections (depending on whether a basement surface is shown and how reliable the interpretation is considered to be), • 100% for basement-penetrating wells, 90-20% depending on stratigraphic age at total depth and presumed distance from basement and reliability of well information, to 0% for wells with no information, and

a) Summary map displaying data confidence. Note that only the highest confidence data shows up in this image; b) Data confidence for published cross-sections / seismic, wells, and basement outcrop; c) Data confidence for gravity resolution; and d) Data confidence for magnetic resolution.

the

SEEBASE AOI Country Outline

Basement Outcrop 100 Outcropping basement

Gravity Data 50 Moderate resolution (satellite) 60 Good resolution (Survey)

Magnetic Data 20 Poor Resolution 50 Moderate Resolution 60 Good Resolution

40 Jurassic Wells 50 Triassic Wells

70 Carboniferous Wells

The values reflect the resolution and quality of the data. The suitability of the data to provide information on the depth to basement has also been separately assessed and this is summarised by the Interpreter’s Confidence in Section V.

Legend

30 Cretaceous Wells

60 Permian Wells

• 20-60% for magnetic data and 50-60% for gravity data.

Figure 2.18:

20 Paleocene And Younger Wells

Calibration Data

80 Devonian Wells / Potential Basement Wells 90 Potential Basement Wells (Conflicting Data) 100 Basement Wells

Gravity

Magnetics

SEEBASE EAST SHETLAND PLATFORM

III. BASEMENT INTERPRETATION

FROGTECH GEOSCIENCE

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III. BASEMENT INTERPRETATION Importance of Basement

What is “Basement”?

The East Shetland Platform area is situated at the collisional junction between the Laurentia and Baltica megaterranes (thick black line in Figure 3.2). These megaterranes can be further subdivided into basement terranes. The basement of any basin provides the foundation onto which the sediments are deposited. The rheology, or mechanical behaviour of the basement controls the rate of subsidence and geometry of each phase of the evolving basin. The composition (Figure 3.1) of the basement will determine its strength or stiffness. The age and early history of each basement terrane will dictate the intensity and character of the structural fabric. This inherent fabric plays a major role in the manner in which the crust deforms during major periods of extension or compression.

The concept of “what is basement?” is non-trivial, since basement evolution in many terranes post-dates basin evolution in others. Hence the question “what is basement?” must be asked for each basement terrane. Additionally, the distinction between economic, magnetic, acoustic, or metamorphic (“crystalline”) basement must be made. By definition, SEEBASE maps depth to economic basement.

Understanding basement structure allows models to be developed that can predict which structures will reactivate, how they will move under an applied stress, and how they will propagate into the overlying sediment pile. Using plate tectonic reconstructions, the far-field stress state during past events can be estimated and a kinematic reconstruction produced for each event. Since basin sediments deform in response to movements in the basement and to gravity, knowing how and when the basement moves provides a basis for predicting the most likely locations of depocentres and structures (both basement-involved and basement-detached) in the sediments. In addition, basement topography controls the localisation and geometry of many basement-detached systems.

Economic basement is defined as the base of all petroleum systems (with a few exceptions – e.g. fractured basement reservoirs). In general, economic basement is also metamorphic basement. Metamorphic or “crystalline” basement is defined as the top of metamorphosed basement lithologies. Magnetic basement is defined as the top of magnetic lithologies (with the exception of subtle, intra-sedimentary sources). In the East Shetland Platform project, basement age has been defined by the age of the last tectonic event that would have been likely to metamorphose any existing sediments. In the project area, this is dominantly the last event of the Caledonian Orogeny, i.e. the Scandian Orogeny (c. 435 – 395 Ma).

The faults described in this study have been interpreted primarily using non-seismic datasets and are mainly basement-involved. The reactivation history of these faults reflects the changes in the crust’s stress regime in response to specific tectonic events. Event maps show structures at top-basement level that are interpreted (predicted) to have been active during a specific basin phase. Details of the influence of these basement-involved structures on the evolution of structures in the overlying sediments provides the basis for future studies, from prospect to basin scale. By building such a “bottom-up” model for basin evolution and combining it with the “top-down” knowledge generated from seismic and wells, petroleum systems can be better understood and targeted. The characteristics of basement provide a first-order control on basin architecture with the potential for influencing:

Basement Composition (Compostion)

• Source rock distribution and volumetrics

Basement Composition Composition

• Heat flow patterns • Migration focusing and pathways • Trap timing, distribution, type, integrity, and size • Sediment supply and stratal geometry • Distribution and quality of reservoirs and seals

Gneiss E

E Intrusive

E Mafic

intrusive

E E E E BBBB Metamorphic BBBB

Quartzite Ultramafic intrusive

Figure 3.1: Basement composition as interpreted in the present study.

III. BASEMENT INTERPRETATION

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Present-day Distribution of Basement Terranes A basement terrane is defined as a discrete, mappable, structurally bounded block of crust of regional extent with a tectonostratigraphic history different to that of neighbouring terranes (e.g. Jones et al., 1977; Howell, 1995).

Faroes-Shetland

The present-day composition (Figure 3.1) and distribution (Figure 3.2) of basement terranes in the East Shetland Platform has been interpreted from a variety of sources, including magnetic and gravity data, DEM, publications and geological maps. Below is a brief introduction to the interpreted basement terranes. Detailed descriptions of each terrane are provided on the following pages, including references to relevant publications. Terrane boundaries are interpreted based on internally consistent gravity and magnetic properties. Displayed terrane boundaries in this project reflect the terrane geometry at the SEEBASE depth-to-basement surface, which ranges from outcropping basement to basement at depth. Terrane boundary locations may change at depth within the crust. An example is the Grampian Highlands which is interpreted as being partly a nappe situated structurally above Precambrian Baltican basement, as a result of thrusting and stacking of crustal slivers during successive collisional events. Interpreted boundaries will then represent the outer limit of the upper crustal domains. A discussion is provided later in this report concerning the proposed extent of Baltica crust below the East Shetland Platform (See Figure 3.3).

Hebrides

The basement terranes of the project AOI all form part of much larger terranes and our understanding of these terranes is based on data throughout a much larger region. The detailed summaries for each terrane are taken from published descriptions covering mainly Scotland, Ireland, Shetland islands and Norway as these areas have been mapped and interpreted in more detail. Outcrop geology and age dating from the Shetland Islands is consistent with the better known areas to the south.

Northern Highlands

Three basement terrane types are represented in the Area of Interest. Type

Description

Highly Attenuated

Continental crust which has been stretched and thinned as a result of rifting or failed rifting event(s), e.g. the Faroes-Shetland terrane.

Undifferentiated Orogenic Belt

Deformed belt between cratons; usually long, thin, arcuate tracts of rock with a pronounced linear structure, e.g. Northern Highlands.

Undifferentiated Continental

Consists of continental crust which cannot be classified as a “true” craton, but has formed a coherent terrane with a long history, commonly as old as Proterozoic, e.g. Hebrides and Caledonides Belt terranes.

Legend

Grampian Highlands

Caledonides Belt

Basement Terrane Type

Coastline

Highly Attenuated

Project Area (Area of Interest)

Undifferentiated Continental

Megaterrane Boundariy

Undifferentiated Orogenic Belt

Terrane Boundaries

Figure 3.2: Basement terranes identified in the study area.. The dashed black polygon in the centre of the image defines the AOI.

III. BASEMENT INTERPRETATION

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Terrane Unit Summaries Descriptions of the individual terranes are provided below. The abstracts are extracted from the terranes database in the accompanying ArcGIS project. Terranes are grouped and described based on their megaterrane affiliation (Figure 3.3). The two megaterrane groupings in the East Shetland Platform AOI are Laurentia and Baltica. The Caledonides Belt is within the Baltica megaterrane on the eastern side of the AOI. The Laurentia megaterrane is represented by the Grampian Highlands, Northern Highlands, Hebrides and Faroes-Shetland terranes to the west.

Faroes-Shetland

Terrane Unit Summary: Laurentia Terrane (Age)

Abstract

FaroesShetland 2100 to 55 Ma

The Faroes-Shetland Terrane is a highly attenuated terrane interpreted to comprise Greenland/Laurentia Proterozoic basement. It was extensively reworked during the Caledonian Orogeny and extensively modified during Paleocene hotspot magmatism. The terrane comprises granitoids, felsic gneiss/migmatite, metasediments and metavolcanics (Sorensen, 2003).

Hebridean 3125 to 410 Ma

The Hebridean Terrane comprises Archean and Paleoproterozoic continental granulites and gneisses (~3.1 Ga) reworked by the ~2.6 – 2.4 Ga Inverian Orogeny and ~1.8 – 1.6 Ga Laxfordian Orogeny (Trewin and Rollin, 2002). Southwest of the project AOI, the basement is often referred to as “Lewisian Complex” in Northern Scotland where it has been further subdivided into numerous terranes separated by NW-SE oriented Proterozoic shear zones. Kinny et al. (2005) proposed that the outcropping Scottish Lewisian Complex can be subdivided into 10 terranes, from north to south: Rhiconich, Assynt, Gruinard, Gairloch, Iailltaig and Rona terranes for the mainland and Nis, Tarbert, Roineabhal, Uist block terranes for the Hebrides islands. The Lewisian Complex is interpreted as part of an extensive Paleoproterozoic orogenic belt linking the Archean Canadian Shield with Greenland and Scandinavia. NW-SE shear zones were reworked as transfer faults during the Permo–Triassic and subsequent extension phases. In Scotland to the southwest, the terrane is bounded to the east by the Moine Thrust which was active during the Scandian phase of the Caledonian Orogeny (Baltica-Northern Highlands collision).

Hebrides

Legend

Basement Terrane Type

Coastline

Highly Attenuated

Project Area (Area of Interest)

Undifferentiated Continental

Megaterrane Boundariy

Undifferentiated Orogenic Belt

Terrane Boundaries

Figure 3.3: Laurentian terranes.

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Terrane Unit Summary: Laurentia (continued) Terrane (Age)

Abstract

Grampian Highlands 2100 to 55 Ma

The Grampian Highlands Terrane, also known as Central Highlands, is defined from Scotland where it is bounded by the Great Glen Fault to the northwest and to the south by the Highland Boundary Fault, and by the Fair Head-Clew Bay Line in Ireland. The oldest unit of the terrane is defined by the Annagh Gneiss Complex (AGC) in Northern Ireland and the Rhinns Complex on Islay Island. The AGC comprises juvenile Paleoproterozoic crust of the ca. 1753 Ma calc-alkaline Mullet gneisses and Late Mesoproterozoic ca. 1271 Ma and ca. 1177 Ma intrusions (Daly, 1996). AGC was reworked during the Grenville deformation at 1177 – 1015 Ma and 995 – 960 Ma (Daly, 1996). The Rhinns Complex gneisses have been dated at 1779 ± 3 Ma (Daly et al., 1991) and 1782 ± 5 Ma (Marcantonio et al., 1988). In the West Shetland Islands Archean basement is mapped. The Grampian Highlands Terrane is dominated by late Neoproterozoic to late Cambrian lithologically diverse metasediments and mafic volcanics of the Dalradian Supergroup marine successions deposited on the Laurentian margin (Smith et al., 1999; Tanner and Sutherland, 2007). A-type granitic intrusions and basaltic lavas are dated at 601 ± 4 Ma (Oliver et al., 2008). This magmatism is contemporaneous with bimodal magmatism in the Appalachians and the Norwegian Caledonides. The Grampian event of the Caledonian Orogeny resulted in the emplacement of granites during the collision of the Midland Valley Arc Terrane with the Grampian Highlands Terrane. Grampian orogenic peak is dated at c. 470 Ma (Friedrich et al., 1999; Flowerdew et al., 2000; Chew et al., 2003; Daly and Flowerdew, 2005). Early Ordovician accretion complex sequences are identified along the Clew Bay Line including slivers of ophiolites. Calc-alkaline magmatism during the closure of the Iapetus Ocean, started at ca. 430 Ma and lasted at least 22 Ma (Nielson et al., 2009). The Ballachulish and Kilmelford Igneous complexes have been dated at 433 ± 1.8 Ma and 425 ± 1.7 Ma, respectively (Conliffe et al., 2010). The Ballachulish Complex has been related to the active subduction zone, whereas the Kilmelford Complex is interpreted as being related to the slab break off stage (Conliffe et al., 2010). The Ronas Hill granite on West Shetland is dated 427±6 Ma (Lancaster et al., 2016) A last Caledonian event, the Scandian phase, resulting from the collision between Baltica and the Northern Highlands, resulted in important sinistral movement along the Great Glen Fault and the juxtaposition of the Northern Highlands and Grampian terranes that ended by ~410 Ma (Oliver et al., 2008). Lyngsie and Thybo (2007) proposed that north of the Moray Firth basin, the Grampian Highlands would be allochtonous on Baltica lower crust. As such, the exact extent of the terrane is not trivial to interpret and the proposed outlines may encompass fragments of Northern Highlands terrane in its northern part.

Northern Highlands 2100 to 55 Ma

The Northern Highlands Terrane consists of the deformed Laurentian cover sequence of the Moine Supergroup and its probable basement represented by the "Lewisonoid" inliers of the NW Highlands (Chew and Strachan, 2013). UPb zircon dating indicates Neoarchaean protolith ages in the range of 2.9 – 2.7 Ga for basement gneisses (Friend at al., 2008), correlating with the Archean Proterozoic age of the Lewisian Gneiss Complex of the Caledonian foreland (Rathbone and Harris, 1979). Most inliers are dominated by tonalitic to dioritic, hornblende gneisses with subordinate hornblendite, serpentinite and garnet–pyroxene lithologies. Locally, in the Eastern Glenelg inlier, the igneous protolith of eclogitic mafic sheets has been dated at c. 2.0 Ga (Brewer at al., 2003). Eclogite ages of ca. 1050 Ma implies that some of the basement inliers were reworked during the Grenville Orogeny (Sanders et al., 1984). Upper amphibolite facies retrogression occurred at c. 995 Ma (Brewer et al., 2003). The Moine Supergroup is a metasedimentary sequence that was deposited along the eastern margin of Laurentia between c. 1000 Ma, the age of the youngest detrital zircons, and before c. 870 Ma, the age of the oldest intrusive igneous rocks (Friend et al., 1997, 2003; Oliver et al., 2008). It comprises thick, monotonous successions of psammites, semi-pelites and pelites (Holdsworth et al., 1994). The Moine Supergroup has a polyphase history including multiple episodes of lower- to upper-amphibolite facies metamorphism during the Middle Neoproterozoic at c. 830 – 725 Ma, the Early Ordovician at c. 470 – 460 Ma (Grampian Orogeny), and the Silurian at 435 – 425 Ma (e.g. Vance et al., 1998; Kinny et al., 1999, Bird et al., 2013). Syn- to late-thrusting granites at different structural levels have yielded ages of c. 435 – 425 Ma (Kinny et al., 2003; Alsop et al., 2010). The ca. 425 Ma event corresponds to the collision between Baltica and the Northern Highlands Terrane. Regional scale NW-directed ductile thrusting culminated in the development of the c. 425 Ma Moine Thrust that bounds the Northern Highlands Terrane to the west. It is bounded by Great Glen Fault to the southeast. Some plutons older than 425 Ma were involved in the Late Scandian deformation (Neilson et al., 2009). In the later stage of the Scandian event, sinistral movement along the Great Glen Fault resulted in the Northern Highland Terrane shifting westward, bringing together the Northern Highlands and Grampian terranes by c. 428 – 410 Ma (Nielson et al., 2009; Oliver et al., 2008).

Northern Highlands

Grampian Highlands

Figure 3.4: Laurentian terranes (continued). See legend on Figure 3.3.

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Terrane Unit Summaries Terrane Unit Summary: Baltica Terrane (Age)

Abstract

Caledonides Belt 2100 to 420 Ma

The Caledonides Belt results from the continental collision between Baltica, Avalonia and Laurentia megaterranes during the late phase of the Caledonian Orogeny, following the closure of the Tornquist Sea (Shelveian Orogeny; ca. 440 Ma) and Northern Iapetus Ocean (Scandian orogeny, c. 430 Ma; e.g. Torsvik and Cocks, 2005). To the west of Norway, being now partly submerged most information comes from the interpretation of sesmic potential field data and extrapolation of Norway outcropping geology. The Laurentia and Baltica convergence continued for c. 30 Ma into the early Devonian when the high and ultra-high pressure rocks of the Western Gneiss Region reached their maximum burial depth at c. 410–400 Ma (e.g. Hacker et al. 2010). Deep seismic and potential field data modelling suggest that Baltica and Fennoscandia Lower crustal domains underlie most part of the belt to the north and west of Norway (e.g. Christiansson et al. 2000, Lyngsie and Thybo, 2007; McBride and England, 1999; Maystrenko and Scheck-Wenderoth, 2013). This suggests that Baltica was part of the downgoing plate during the collision. West of Norway, along the Teysseire-Tornquist shear zone, and south of the Southern Upland Fault, the upper crustal part of the Belt is interpreted to be composed of accreted fragments from Eastern Avalonia, Iapetus? and Baltica terranes (Lyngsie and Thybo, 2007). North of the Southern Upland Fault and Norwegian Caledonides Front, in the Northern North Sea, fragments of Laurentia-derived Midland Valley and Grampian terranes might be included in the belt. Along the northwest Norway Iapetus suture the near linear 1800 km long belt of basement is mostly composed of south-eastward imbrications of the outer margin of Baltica onto the Precambrian Baltic Shield, overridden by elements of Laurentian affinities and outboard oceanic domains (Andersen et al., 1990) or serpentinised mantle (Andersen et al., 2012). Along the belt, age of the economic basement might vary from Archean to Silurian. In southern Norway, the Caledonian parageneses shows a northwestward increase in pressure, ranging from 15 – 30 kbar implying that Baltic basement was buried to depths of 70 – 100 km (e.g. Hacker et al., 2010). The Belt was affected by lithospheric extension events postdating the Caledonian orogenic extensional collapse during the Permian-Early Triassic and Late Mid-Jurassic to earliest Cretaceous time (Christiansson et al., 2000; Ziegler, 1990). The southern boundary has been interpreted based on gravity and magnetic data.

Caledonides Belt

Legend Coastline

Basement Terrane Type Undifferentiated Continental

Project Area (Area of Interest) Megaterrane Boundariy Terrane Boundaries

Figure 3.5: Baltica terranes.

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Basement Assembly Formation and assembly of the basement terranes in the East Shetland Platform region occurred during the Caledonian Orogeny, that can be divided into three successive accretion events (in the area of interest) depicted below and on the next page. Those accretion events are the Grampian Orogenies I and II, Shelveian Orogeny and the Scandian Orogeny. A detailed tectonic events chart including regional basement-forming events is presented in Figure 4.1 together with plate reconstructions for key events.

Pre-Caledonian Orogenies and Development of Laurentia Margin (> c. 500 Ma) Pre-Caledonide Laurentian foreland is recognised in Scotland and the Shetland Islands to the northwest of the Highland Boundary Fault (HB) (Figure 3.6). Laurentian basement, comprising Archean to Paleoproterozoic high-grade gneisses, is exposed in the Hebridean Terrane (H, Figure 3.6) of northwest Scotland and Shetland, and in basement inliers of the Northern Highlands (NH) and Grampian Highlands terranes (G) (Chew and Strachan, 2013). In the Hebridean Terrane, basement was reworked by the ~2.6 – 2.4 Ga Inverian Orogeny, and ~1.8 – 1.6 Ga Laxfordian Orogeny (Trewin and Rollin, 2002). In the Northern Highlands and Grampian Highlands terranes, Neoproterozoic metasedimentary rocks form basement cover sequences. The mid-Neoproterozoic to lower Cambrian Dalradian Supergroup comprises a thick sequence of metasedimentary rocks in the Grampian Highlands Terrane. The lower Paleozoic rocks of that sequence generally represent the deeper water equivalent to the lower Paleozoic shallow-water shelf sediments of the Hebridean Terrane to the northwest, reflecting a southeast-facing passive margin during the development of the Iapetus Ocean. The southeast boundary of the Grampian Highlands Terrane broadly represents the boundary between elements of Laurentian affinity and those of an exotic or oceanic affinity, the latter comprising relics of serpentinised subcontinental lithospheric mantle and the metamorphic sole of a suprasubduction ophiolite obducted between ~500 Ma and 490 Ma (Chew et al., 2010). In the Shetland Islands, a late Cambrian ophiolite (Shetland Ophiolite Complex) is recognised on the islands of Unst and Fetlar (Chew and Strachan, 2013) and more recently in southeast Shetland (Dunrossness Spilite Subgroup; Day et al., 2016). Those Shetland complexes might constitute parts of an extensive ophiolite sliver, obducted during Iapetus Ocean closure in forearc setting (Day et al, 2016).

FS Unst and Fetlar ~500 Ma ophiolites obducted during ~484 Ma Grampian I Orogeny

G Archean to Paleoproterozoic highgrade gneisses

Shetland East Mainland succession and Yell Gp, equivalent to Dalradian Gp, peak metamorphism 480 – 460 Ma; Grampian I reworked during Grampian II c. 454 – 443 Ma

Grampian Orogenies: Arc - Continent Collision between Laurentian Margin and IntraOceanic Arc(s) (Grampian I, c. 475 – 465 Ma; Grampian II, c. 450 Ma) Between 475 and 465 Ma, the Dalradian Supergroup of the Grampian Highlands Terrane was subjected to polyphase deformation and regional metamorphism to upper-amphibolite facies (peak ~470 – 465 Ma) during the Grampian I Orogeny (Stephenson et al., 2013; Chew and Strachan, 2013). This event is interpreted as being caused by northwestward ophiolite obduction and collision with an intra-oceanic arc (Midland Valley Terrane and equivalent, MV) on the north end of the Iapetus oceanic plate initiating the closure of the Iapetus Ocean (Bird at al. 2013; Domeier, 2016). In the northeast Scottish Highlands, orogenesis was accompanied by syn-orogenic and decompression melts (~465 – 450 Ma), and basic to silicic plutonism (Stephenson et al., 2013; Chew and Strachan, 2013). Rapid, regional postorogenic cooling followed in the Middle to Late Ordovician (c.460 – 450 Ma).

Upper Neoproterozoic to Upper Cambrian Dalradian Gp - upper amphibolite peak metamorphism ~470 – 465 Ma - Grampian I

A distinctly younger Late Ordovician (~450 Ma) phase of deformation and metamorphism, Grampian II Orogeny, has also been reported from the Northern Highlands Terrane. During this phase the Ordovician plutons recently intruded are deformed (Bird et al., 2013; Cawood et al., 2014).

GGF Great Glen Fault WBF Walls Boundary Fault HBF Highland Boundary Fault MT Moine Thrust MFB Moray Firth Basin

FS Faroe-Shetland Terrane H Hebridean Terrane NH Northern Highlands Terrane G Grampian Highlands Terrane MV Midland Valley Terrane SU Southern upland Terrane CB Caledonides Belt Terrane

Maximum Age Period ORDOVICIAN

Megaterrane Laurentia

SUT

CAMBRIAN NEOPROTEROZOIC MESOPROTEROZOIC PROTEROZOIC ARCHAEAN

Ordovician minor felsic and large gabbroic intrusives emplaced between Grampian I and II

Figure 3.6: Terranes in the study area affected by the Grampian I and II orogenies (highlighted in red). Archean to Ordovician basement outcrops are shown classified by maximum age. The black dashed polygon in the centre of the image defines the AOI.

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Basement Assembly (Continued) Shelveian Orogeny: Collision between Baltica and East Avalonia. (c. 445 – 440 Ma) Opening of the Rheic ocean along the north margin of Gondwana c. 500 Ma initiated the drift of the East Avalonia Megaterrane (see Section IV for plate reconstructions) that would ultimately collide with Baltica (in blue in Figure 3.7). It follows the closure of the Tornquist Sea (part of the Iapetus Ocean) c. 440 Ma along the Teysseire-Tornquist Shear Zone (southeast, outside Figure 3.7; Torsvik and Cocks, 2005). This collision resulted in the formation of the southern continuation of the Caledonides Belt (CB) Terrane southeast of the AOI.

Scandian Orogeny: Collision of Avalonia and Baltica with Laurentia (c.435 – 425 Ma)

Newer Granites

Ongoing oblique northward subduction of the Iapetus ocean led to continental collision of Avalonia (green terranes in Figure 3.7) and Baltica (blue terranes) with Laurentia (red terranes), forming the western and northern Norway ‘arms’ of the Caledonides Belt. Regional deformation and metamorphism was largely restricted to the Northern Highlands Terrane of Scotland, where the Neoproterozoic Moine Supergroup was subjected to northwest-verging thrusting and folding. Greenschist to amphibolite-facies metamorphism (Chew and Strachan, 2013) also occurred on the southeast and northern margins of Baltica. Isotopic ages from syn- to post tectonic intrusions (“Newer Granites”; Browne and Locke, 1979) indicate displacement of the Moine Thrust Zone occurred at ~430 Ma, (although deformation continued elsewhere into the late Silurian (Strachan and Evans, 2008; Goodenough et al., 2011). In the Trondheim region, exotic oceanic assemblages were evidently emplaced by ~430 – 425 Ma (metamorphic cooling ages and youngest sedimentary deposits; Nilsen et al., 2003). Mid-Silurian deformation was weak or localised south of the Grampian Highlands Terrane.

G CB

427 – 370 Ma

Between c. 427 Ma and c. 370 Ma, >200 km ductile sinistral strike-slip along the Great Glen Fault (GGF) and Walls Boundary Fault (WBF) juxtaposed the Northern Highlands and Grampian Highlands, coeval with transtensional collapse of the Caledonides (Watts et al., 2007; Stewart et al., 2001; Mendum and Noble, 2010). Emplacement of younger granites and extrusive equivalents (c. 430 – 390 Ma, “Newer Granites”) occurred during regional scale strike-slip faulting following break off of the north-dipping slab. Deposition of the Upper Devonian-Carboniferous continental Old Red Sandstone began near the end of the Scandian Orogeny ending the Caledonian Orogeny.

WBF NH Dalradian Group as an allochthon nappe on Baltica lower crust?

Debate on Baltica and Grampians Highland Terrane Extents The northern and eastern extents of the Grampians Highlands Terrane in the AOI are related to the extent of the lower crust of Baltica under the Laurentian terranes, and therefore difficult to interpret. Based on integrated potential field modelling, Lyngsie and Thybo (2007) show that crust bearing Baltica lower crustal properties is present under the Caledonides Belt Terrane and might extend westward as far as the Walls Boundary Fault in Shetland. The proposed hypothesis is that Avalonian upper crust thrusted over Baltican lower crust via thin-skinned tectonics and that Baltica under-thrusted Laurentia, possibly including the East Shetland Platform, in a crocodile tectonic style (Lyngsie and Thybo, 2007). The latter is also supported by potential field modelling by Maystrenko (2015). The Grampian Highlands Dalradian Supergroup would be an allochtonous sheet on Baltica, situated north of the Moray Firth Basin (MFB) on Figure 3.7. Baltican lower crust would then exert a strong control on the tectonic and magmatic complex development of the Viking Graben, East Shetland Platform and Moray Firth Basin. Such a crustal model is consistent with the contrasting large-scale structural orientations between the onshore Grampian Terrane and the East Shetland Platform, consistent with the Caledonides Belt, and the location of the crustal-scale Walls Boundary Fault (see Section IV – Tectonic Events). Our proposed limit of Baltican lower crust differs slightly from previous authors (see Figures 3.8 and 3.9). FS Faroe-Shetland Terrane H Hebridean Terrane GGF Great Glen Fault NH Northern Highlands Terrane WBF Walls Boundary Fault G Grampians Terrane HBF Highland Boundary Fault MV Midland Valley Terrane MT Moine Thrust SU Southern upland Terrane MFB Moray Firth Basin CB Caledonides Belt Terrane

Maximum Age Period

Megaterrane

Carboniferous

Baltica

Devonian

Iapetus

Silurian and Devonian

Laurentia

Devonian Old Red Sandstone

MFB

Newer Granites

Crustal Domains Baltica Lower Crust Baltica Upper Crust

Lyngsie and Thybo (2007)

Figure 3.7: Terranes involved in the Scandian Orogeny in the study area coloured by megaterranes. Proposed extent of the Baltica upper and lower crust from Lyngsie and Thybo (2007) are also displayed.

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Basement Character: Gravity Gravity data (Figure 3.8) has been critical in the interpretation of basement terranes and basement composition. Many of the terranes have boundaries that are well defined, and/or have distinct “textures” evident on the data. Several different filters were used in the interpretation (see Geophysics Appendix for further details). The area is characterised by widespread negative anomalies of various sizes with a significant part being interpreted as granites. Prominent positive anomalies are interpreted as ophiolites directly east of Shetland Islands, high grade gneisses or shear zones elsewhere. Ophiolite complexes at outcrop.

The Great Glen and Walls Boundary faults, and their splays, are clearly visible in the gravity data. The high-grade metamorphic basement in the Hebridean Terrane is marked by strong NE-SW oriented positive anomalies. The white dashed line on Figure 3.8 marks the possible extent of Baltica lower crust below the Grampian Highlands. Southwest of this line, the Grampian Highlands Terrane is transected by a ~NW-SE structure (Tornquist Line, black dashed line) that separates a southwestern domain characterised by ~EW-oriented anomalies that contrast with an eastern domain characterised by NW-SE and NNW-SSE structures. Along the western terrane boundary the Grampian Highlands Terrane is marked by N-S structural trends, e.g. the Walls Boundary Fault and its splays, and NE-SW trending anomalies and structures such as the Møre-Trondelag Fault Zone (orange dashed line).

GGF Great Glen Fault WBF Walls Boundary Fault MTF Møre-Trondelag FZ

Positive anomaiy signature from high grade Basement.

Negative anomaly associated with Newer Granites (refer to definition on previous page).

Numerous intrusions along major steep structures and thrusts.

Northern Highlands

Northern Highlands Grampian Highlands

Intrusives in the Orcadian Basin.

Numerous intrusives in the East Shetland Platform.

Hebridean

Numerous intrusives in the Orcadian Basin.

Grampian Highlands

Intruded dense basement rocks, potentially derived from high-grade metamorphics similar to Sveconorwegian basement amphibolite to granulite facies rocks.

Tornquist Line.

Figure 3.8: Outlines of basement terranes (black polygons) overlain on an image of the 100 km High-pass filter of Bouguer (A) and on a ternary image of Bouguer gravity B). The dashed black polygon defines the AOI. Dashed white line is the proposed extent of Baltica crust under the East Shetland Platform. Black lines are terrane boundaries. Frogtech Geoscience Post: PO Box 250, Deakin West ACT 2600, AUSTRALIA Office: Suite 17F, Level 1, 2 King Street, Deakin West ACT 2600, AUSTRALIA T: +61 02 6283 4800, E: info@frogtech.com.au W: frogtechgeoscience.com.au

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Basement Character: Magnetics Magnetic data (Figure 3.9) has also been important in the interpretation of basement terranes and composition. Some terrane boundaries are well defined, such as the Walls Boundary Fault, and/or have distinct “textures” that are evident on the images. Several different filters were used in the interpretation (see Appendix II for further details). Due to a larger line spacing, the resolution of offshore magnetic data east of the white dashed line in Figure 3.9a is lower than the magnetic data processed for the onshore and shallow basement areas around the Islands. Although the Grampian Highlands Terrane outlines are similar the outlines proposed by Beamish et al. (2016), alternative outlines can be considered with the Møre-Trondelag Fault Zone demarcating the north end of the terrane. Also the area comprised between the inferred north limit of Baltica lower crust and the MTFZ might constitute a separate individual terrane. Higher Resolution

The low resolution of the data on the eastern part of the AOI (see Section II for survey boundaries) have consequences for basement interpretation: 1) Interpretation of the offshore continuity of the numerous thin features/markers imaged onshore is not possible, and 2) the medium and long wavelength anomalies are generated by deep and/or large features. With a few exceptions, small individual intrusives interpreted from gravity anomalies are difficult to interpret in the magnetic data. 3D magnetic body modelling in this study confirms that most features visible in the south part of the ESP are deeper than 9 – 10 km, i.e. sitting in the middle to lower crust domains.

Lower Resolution WBF Walls Boundary Fault MTF Møre-Trondelag FZ Magnetic Gneisses, west of Shetland (Ritchie, 2011). Negative or weak magnetic anomalies from Caledonian nappes and possible intrusives (similar to onshore Grampian Highlands Terrane) and/or amphibolite facies of Precambrian basement.

Potential ophiolite complexes, or high-grade metamorphic rocks the from Sveconorwegian Orogeny?

Magnetic modelling results indicate that the top depth of this anomaly is lies in the middle crust at ~13 km depth

Grampian Highlands

NW – SE oriented and NW deepening positively magnetised lower crustal domain features (high grade thrusts and /or mafic intrusives from Forties volcanic province?) overlain by weakly or negatively magnetised upper domains (metasediments (Dalradian?) and postDevonian sediments.

Grampian Highlands

East Shetland High comprising intruded magnetic basement similar to visible anomalies on the west side of the Viking Graben associated with Sveconorwegian Orogen granulites along west Norway, east of AOI, may continue in this area. Lossiemouth anomaly – Paleoproterozoic magnetic basement which is observed onshore (Rollin, 2009).

Compound expression of deep (Blue arrows) and shallow (white) of Jurassic Forties Volcanic Province? (Smith and Ritchie, 1993)

Figure 3.9: Outlines of basement terranes (black polygons) overlain on an image of A) the RTP of the TMI and B) a ternary image of the RTP. The dashed polygon in the centre of the image defines the AOI. Dashed white line is the proposed extent of Baltica crust under the East Shetland Platform. Black lines are terrane boundaries. Frogtech Geoscience Post: PO Box 250, Deakin West ACT 2600, AUSTRALIA Office: Suite 17F, Level 1, 2 King Street, Deakin West ACT 2600, AUSTRALIA T: +61 02 6283 4800, E: info@frogtech.com.au W: frogtechgeoscience.com.au

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SEEBASE EAST SHETLAND PLATFORM

IV. TECTONIC EVENTS FROGTECH GEOSCIENCE

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IV. TECTONIC EVENTS

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Tectonic Evolution Summary – Paleozoic 320 Ma

The tectonic events chart below, together with plate reconstructions on the right (Figure 4.1) show the main geodynamic events affecting the East Shetland Platform area during the Paleozoic when first economic basins are developed. Many of the structures formed during the events listed here control the tectonic evolution of future basins. The key tectonic events are described in more detail, and include fault event maps showing basement faults interpreted to have been active during each event, in the following pages.

Siberia

Collision of Siberia/Kazakhstania

Area deformed during Variscan Orogeny

Fennoscandia Laurentia

Paleotethys

Rheic Suture

Gondwana

350 Ma

“Expulsion” of Fennoscandia

Siberia

Fennoscandia Rheic subduction Zone

Laurentia

Gondwana

390 Ma Siberia

Fennoscandia

Laurentia

Gondwana Figure 4.1:

Paleozoic tectonic event chart summarising the large scale and local tectonic evolution of the North Atlantic and Greater North Sea (left) and plate reconstructions by Frogtech Geoscience for key time slices (right).

Area deformed during Caledonian Orogeny

Rheic subduction Zone

IV. TECTONIC EVENTS

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Tectonic Evolution Summary (cont’d) – Mesozoic The tectonic events chart below (Figure 4.2) shows the main geodynamic events affecting the East Shetland Platform during the Mesozoic period. Bay of Biscay Sea-floor Spreading

80 Ma

160 Ma Rifting Central Atlantic Sea-floor Spreading

220 Ma Rifting

Central Atlantic Sea-floor Spreading

Figure 4.2:

Mesozoic tectonic events chart summarising the large scale and local tectonic evolution of the North Atlantic and Greater North Sea (left) and plate reconstructions by Frogtech Geoscience for key time slices (right).

IV. TECTONIC EVENTS

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Tectonic Evolution Summary (cont’d) – Cenozoic The tectonic events chart below (Figure 4.3) shows the tectonic evolution and main geodynamic events affecting the East Shetland Platform during the Cenozoic.

20 Ma Extinct Aegir Ridge

New Kolbeinsey Ridge

Alpine Orogeny

50 Ma

Pyrenean Orogeny

Figure 4.3:

Cenozoic tectonic events chart summarising the large scale and local tectonic evolution of the North Atlantic and Greater North Sea (left) and plate reconstructions by Frogtech Geoscience for key time slices (right).

IV. TECTONIC EVENTS

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Overview: Stratigraphy – Paleozoic Orogenies, Erosion and Extensional Collapse Figure 4.4a shows a generalised stratigraphic chart for the Paleozoic of the East Shetland Platform and surrounds. SW

• The end of the Caledonian Orogeny was marked by closure of the Iapetus Ocean. Deformed basement was unconformably overlain by continental sediments of the Devonian to Carboniferous Old Red Sandstone (ORS), likely derived from erosion of the collapsing Caledonian orogen (Beach, 1985; McClay et al., 1986). Although the Devonian Orcadian Basin covered the entire East Shetland Platform, deeper local depocenters developed as transtensional basins bounded by NS and, NE-SW fault systems in the West Orkney and adjacent basins, (Coward and Enfield, 1987), and NW-SE structures in the Platform region (e.g. Fossen, 2010).

Saalian Unconformity

• During orogenic collapse and extension, felsic to intermediate granitic intrusives were emplaced throughout the AOI, including for example, the Halibut Granite in the Greater Moray Firth area, the Bressay Granite in the Crawford-Skipper Basin on the East Shetland Platform, and probably the Utsira High Granite on the eastern side of the South Viking Graben. Those “Newer Granites” (Brown and Locke, 1979) were emplaced along major SW-NE-trending crustal structures that were reactivated during subsequent tectonic events (e.g. Variscan). Intruded areas are locally strengthened and the overall buoyancy of the crust increased. Crustal blocks cored by the Caledonian intrusives (e.g. East Shetland Platform) were more resistant to deformation during later tectonic events. • Middle Devonian to earliest Carboniferous NE-SW-directed strike-slip movement on NE-trending structures continued between Laurentia and Avalonia-Baltica during the Early Variscan Orogeny, resulting in deposition of coarse non-marine clastics in deep pull-apart basins. Proven source rocks of Middle Devonian age have been identified on the East Shetland Platform and Greater Moray Firth (e.g. Duncan and Buxton, 1995). • Mid-Variscan collision of Gondwana with Laurentia caused the lateral, eastward continental escape of Fennoscandia (Coward, 1993), creating deep early Carboniferous coal-rich basins (e.g. Firth Coal) south of the AOI. • By mid Carboniferous, Late Variscan Ural collision terminated Fennoscandian escape and reversed the lateral movement (Scotese and McKerrow, 1990), causing uplift and inversion and leading to the major late Carboniferous to early Permian stratigraphic hiatus, regionally known as the “Saalian Unconformity” (Seranne, 1992; Zanella and Coward, 2003). Devonian Plays Devonian lacustrine/brackish mudstones have similar geochemical characteristics to the Middle Devonian proven oil-prone source rocks that charge the Beatrice Field in the Inner Moray Firth Basin, as well as Witch Ground Graben and West of Shetlands (Duncan and Buxton, 1995; Peters et al., 1989; Patruno and Reid, 2017a). Potential Devonian plays may exist with potential Devonian reservoir rocks (e.g. ORS sandstones and carbonates) developed in horst and syn-rift wedges overlain by intra-Carboniferous or post-Permian mudstones (acting as a Cretaceous-Tertiary regional seal). Source rock maturation is variable, however there may be ongoing present-day generation and expulsion from Middle Devonian source rocks over the ESP. PostJurassic generation may be coeval with deposition of extensive post-Upper Jurassic syn-rift and Cretaceous seals (e.g. Crawford area; Patruno and Reid, 2017a). The Cairngorm field (South Viking Graben) is reservoired in fractured Silurian basement (Patruno and Reid, 2017a). Figure 4.4:

a) Paleozoic generalised stratigraphic chart of the East Shetland Platform (ESP) (continued next page). Timescale is after Ogg et al. (2016). This stratigraphic chart was compiled from the following references: Cameron (1993), Department of Climate Change (2014), Du and Hosseinzadeh (2014), Du and Key (2015), Duncan and Buxton (1995), Goff (1983), Johnson et al. (2005), Johnson and Lott (1993), Knox and Holloway (1992), Morton (1979), Patruno and Reid (2016a,b; 2017a,b), Pedersen et al. (2004), Peters et al. (1989), Reid and Patruno (2016), Richards et al. (1993), Sarkar and Armstrong (2016), Underhill (2003) and Waters et al. (2007).

Lithology

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Overview: Stratigraphy – Main Late Paleozoic-Mid Mesozoic Rift Phases Figure 4.4b shows the generalised stratigraphic chart for the Late Paleozoic-mid Mesozoic for the East Shetland Platform region. Following the Saalian Unconformity, Pangaea break-up and initiation of major rifting phases, contemporaneous with northward drift to more arid latitudes, occurred toward the end of the late Permian to the Early Jurassic.

• During late Permian, arid climate Rotliegend clastics, followed by Zechstein carbonates and Turbot and Shearwater evaporites were deposited in the Outer Moray Firth, and western and eastern edges of the ESP. Extension in the South Viking Graben occurred along the ~N-S boundary of the Fennoscandian Shield, accommodating kilometers of Rotliegend and Heron/Hegre syn-rift sediments. Intra-platform basins such as the Crawford-Skipper basin (Patruno and Reid, 2017a) have been recently identified as Permo-Triassic basins with Triassic-age syn-rift sediments. •

Mid Cimmerian Unconformity

During late Triassic to Early Jurassic tectonic quiescence and thermal subsidence, the southern North Sea area experienced shallow marine clastic sedimentation, in contrast to continental sedimentation in northern North Sea areas, focused in the areas of maximum extension.

• Thermal doming in the Forties area during the latest Early to earliest Middle Jurassic (Aalenian) (e.g. Graversen, 2006) is responsible for development of the major regional “Mid Cimmerian Unconformity” and possible volcanism in the Moray Firth and most of the southern ESP (Smith and Ritchie, 1993). • Thermal doming and associated weakening of already thinned crust may have influenced the location of extension during the main Upper Jurassic to mid-Early Cretaceous rift phase in the South Viking and Moray Firth grabens. This extension accommodated the deposition of marine shelf clastics (including the highly prospective Kimmeridge Clay) and deep basinal fans. In the East Shetland Basin, Late Jurassic fault-block rotation is imaged on seismic. Footwall uplift led to erosion (Davies et al. 1999; Kyrkjebø et al., 2004) and the ~139 Ma Base Cretaceous Unconformity (BCU, base of Cromer Knoll Group) that marked the end of rifting and the beginning of thermal subsidence. Permian, Triassic and Jurassic Plays Late Permian Auk sands, Rotliegend carbonates, thick Triassic continental clastics and Upper Jurassic deepwater sandstones are potential reservoir rocks. The Kimmeridge Clay is generally immature on the ESP. Mature Kimmeridge source kitchens include the deep depocentres of the East Shetland Basin, Viking Graben, Moray Firth and Central Graben to the south and southeast. Producing fields on the ESP margins (e.g. Bentley, Bressay, Kraken oil fields; Du and Key, 2015; Du and Hosseinzadeh, 2014) are charged via long distance, lateral migration of Kimmeridge oils from the Moray Firth and Viking source kitchens (Reid and Patruno, 2016). Pliensbachian and Oxfordian-Callovian marine mudstones are organically lean (Goff, 1983). Middle Jurassic organic-rich mudstones and coals are proven source rocks in the Inner Moray Firth and charge Upper Jurassic deep-water sandstones and Lower Cretaceous clastics. Where present, the Brent coals are an excellent dry gas source rock, with up to 10 m thick coal seams (Goff, 1983; Peters et al., 1989; Underhill, 2003). The earliest Permian Kupferschiefer mudstones have been geochemically assessed to be thermally mature in the Beryl Embayment and South Viking Graben, and are a potential source rock (Pedersen et al., 2004).

Saalian Unconformity

Lithology Figure 4.4:

b) Late Paleozoic-Mesozoic generalised stratigraphic chart of the East Shetland Platform (ESP) (continued next page). Timescale is after Ogg et al. (2016). See Figure 4.4a for references.

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Overview: Stratigraphy – Late Mesozoic to Present-Day Sag, Thermal Subsidence and Episodic Inversion Figure 4.4c shows the generalised stratigraphic chart for the late Mesozoic and Cenozoic for the East Shetland Platform. SW

• From the earliest Cretaceous to Late Cretaceous, thermal subsidence was interrupted by short-lived pulses of transpression and uplift, associated with reactivation of basement structures and terrane boundaries (e.g. Ritchie et al., 2008; Stoker et al., 2016). Dominantly marine mudstone and shales were deposited in the subsiding depocentres.

• During the Late Cretaceous, effects of the Alpine collisional event begin to manifest to the north of the East Shetland Platform area (Tuitt et al., 2010). • North of the AOI, opening of the North Atlantic during the Late Cretaceous-Paleocene (~53 Ma) marked a significant period of volcanism. A switch in the northwest European stress field from extension to compression was a two-fold result of North Atlantic opening and the early stages of the Alpine Orogen, leading to three major Tertiary compressive phases and unconformities; the Eocene (~55 – 38 Ma), Oligocene (~38 – 20 Ma) and Miocene to Recent (~20 – 0 Ma). All three events were associated with uplift of Fennoscandia and the UK/Shetland margin. The compressive phases manifested as a prolonged stratigraphic hiatus over parts of the East Shetland Platform, with sediment deposition sourced from uplift and erosion of the adjacent Orkney-Shetland Hinterland. The major Miocene inversion and unconformity can be correlated across the UK and Ireland. Tertiary Play A combination of thick Eocene and Oligocene-Miocene shelfal sandstones and Paleocene-Eocene deep-water sandstones provide the dominant producing reservoirs in the North Sea, sealed by intraformational mudstones and marls (Patruno and Reid, 2017a).

Lithology Figure 4.4:

c) Late Mesozoic-Cenozoic generalised stratigraphic chart of the East Shetland Platform (ESP). Timescale is after Ogg et al. (2016). See Figure 4.4a for references.

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Tectonic Evolution: Interpreted Structures The interpretation of regional structures in the basement of the East Shetland Platform region was undertaken using a range of datasets including gravity, magnetics, DEM, surface geology, georeferenced published structural maps, and hyperlinked published cross-sections/seismic images. Figure 4.5 shows all the basement faults (white) interpreted for the AOI, as well as the terrane boundaries (black). Structural evolution varies across the project area. Fault event maps are provided on the following pages for the stages of syn-rift development and main inversion events.

Caledonian Orogeny Structural Inheritance The main crustal-scale fault zones that control the structural evolution, terrane kinematics and degree of metamorphism in the area of interest are the Caledonian Great Glen Fault zone (GGF), the Walls Boundary Fault Zone (WBF), the Møre-Trondelag Fault Zone (MTFZ), Moine Thrust (MT), Wester Keolka Fault Zone (WKFZ), the Hardangerfjord Shear Zone (HFSZ) and the Tornquist Line (TL). The steep or sub-vertical GGF, WBF and MTFZ have been reactivated multiple times and controlled the collapse of post-Caledonian to Tertiary orogens, uplift and rift development (Watts et al. 2007 and references therein).

The complex Caledonian-aged first-order structural fabric of the ESP and surrounding area can be broadly described as follows: • Throughout the area, there is an ubiquitous steeply dipping ~NNW to ~ NNE-trending structure set, similar to the Walls Boundary Fault (WBF), future Viking Graben and West Norway coastline. Those structures were reworked during Devonian and Early Carboniferous transtension and locally ‘in reverse’ during Late Carboniferous transpression.

WBF

• Northwest and in proximity to the GGF and WBF, basement is characterised by a pervasive set of ‘Scandian’ ~ENE- and NE-trending deeply rooted structures, mostly SE-dipping. This set is archetypal of a retro-wedge thrust on the Laurentian margin resulting from the subduction of the western margin of Baltica underneath the Laurentian lithospheric plate during the Scandian Orogeny. The wider Baltican prowedge comprising continental and oceanic allochthonous units (e.g. Hacker et al., 2010) is interpreted toward the east of the Viking graben (Fossen et al., 2016) and extending south to the Scandinavian Caledonides Front and HFSZ. Within the ESP, a similarly trending set is observed that appears to act locally as transform to the NW-trending thrust structures described below. • A ‘Tornquistian’ ~NW-oriented, steep to NE-dipping, thrust system similar in orientation to the major NWSE- trending Tornquist Line extending outside the AOI as the Tornquist-Teysseire Shear Zone. Similar trending shear zones like the Elbe Odra Line, Thor Suture Zone and Trans European Suture Zone also exist outside the AOI to the southeast. Those shear zones are defined as diffuse plate boundaries related to the final closure of the Tornquist Sea arm of the Iapetus Ocean west of the Baltican Shield (e.g. Pegrum, 1984, Pharaoh, 1999, Lyngsie and Thybo, 2007). NW-trending deep magnetic anomalies and deep seismic NEdipping reflectors (McBride and England, 1999) suggest that structures of a similar trend form a primary fabric that extends down to middle to lower crustal depths under the ESP. These structures were reactivated during the Caledonian and Variscan collisions as transfer structures. This set is believed to have been reactivated in right-lateral motion during all of the main rifting events and to have exerted a fundamental control on the development of pull-apart basins, such as the South and North Viking graben, East Shetland Basin, Crawford-Skipper Basin and Witch Ground Graben.

GGF

Moray Firth basin

• An EW-trending fault set is also identified in the centre and south ESP and Moray Firth Basin that act as transfer zones. The age of these structures might be Caledonian, reworked during the Variscan (Coward, 1993; Schroot and de Haan, 2003). GGF Great Glen Fault WBF HBF MT WKFZ TL Frogtech Geoscience Post: PO Box 250, Deakin West ACT 2600, AUSTRALIA Office: Suite 17F, Level 1, 2 King Street, Deakin West ACT 2600, AUSTRALIA T: +61 02 6283 4800, E: info@frogtech.com.au W: frogtechgeoscience.com.au

Walls Boundary Fault Highland Boundary Fault Moine Thrust Wester Keolka Fault Zone Tornquist Line

Figure 4.5: Basement faults (white), terrane boundaries (black) region overlain on 100 km High-pass filter of Bouguer Gravity. The black dashed in the centre of the image defines the AOI.

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Late Caledonian to Late Variscan: Late Silurian-Early Permian (~425 – 285 Ma) The chart below (Figure 4.6) displays the main events and the lithological units filling the basins of the Mid North Sea High region from Late Caledonian to the Late Variscan, together with the main geodynamic events, inferred active structures, local stress orientation and plate reconstruction for key time slices (Figure 4.7). More detailed analysis of the tectonic response and active basement fault maps are presented in the following pages for this time period dominated by closure of the Iapetus and Rheic Oceans:

320 Ma Siberia

Collision of Siberia/Kazakhstania

• Late Caledonian to Mid Variscan: Early Devonian-Early Carboniferous ( ~420 – 330 Ma), and Area deformed during Variscan Orogeny

Fennoscandia

• Late Variscan: Late Carboniferous – mid Permian (~320 – 285 Ma). Laurentia

Paleotethys SW

350 Ma Gondwana

“Expulsion” of Fennoscandia

Siberia

Inversion and transpression on NNW structures. Fennoscandia Laurentia Sinistral strike-slip on ENE structures; possible inversion on NNE and NNW structures.

Rheic subduction zone

390 Ma Siberia

Sinistral strike-slip on ENE structures; Sinistral normal on NNE and NNW

Rheic subduction zone

Latest Silurian to early Permian generalised stratigraphic chart and events of the East Shetland Platform. See Figure 4.4 for references.

Figure 4.7: Gondwana

Area deformed during Caledonian Orogeny

Fennoscandia

Laurentia

Figure 4.6:

Gondwana

Frogtech Geoscience plate reconstruction models for key Paleozoic events.

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Early to Mid Variscan: Devonian – Mid Carboniferous (~420 – 330 Ma) At the end of the Scandian Orogeny, sinistral transpressional reactivation of major NE-trending Caledonian structures occurred coeval with orogenic collapse (Beach, 1985; McClay et al., 1986, Gee et al,. 2008). Deep pull-apart basins formed (e.g. Orcadian Basin) and were filled with coarse, non-marine clastic sediment (Old Red Sandstone) from the adjacent Caledonian uplands. This Early extension phase created a structural framework that must have influenced the formation and evolution of the Permian–Late Jurassic North Sea Rifts (Fossen et al., 2016).

Greenland

NVG FSB

In the AOI, the most significant structural reactivation occurred on the major Caledonian sinistral MøreTrondelag Fault Zone (MTFZ) and Great Glen (GGF) – Walls Boundary (WBF) Fault shear zones (Watts et al., 2007, Roberts et al.,1990) (Figure 4.8). In the northwestern corner of the study area the dominant NEtrending vertical or SE-dipping faults were reactivated in a left-lateral movement generating extension on NStrending structures. This structural architecture is similar to the Norwegian Basins to the NE along the Baltica margin, where Devonian to mid-Carboniferous sedimentation occurred within a complex series of anastomosing pull-apart sub-basins (Cartoon A, below). In the southern part of the Platform the regional ~NW-SE fault pattern is interpreted to have been reactivated in a dextral sense allowing the development of pull-apart basin (Cartoon B).

ESB

WSB WBF

SESB

During the middle part of the Variscan (Early Carboniferous), fragments of Gondwana from the southeast collided and caused a significant change in the structural dynamics of NW Europe (Figure 4.7). Fennoscandia acted as a rigid crustal block and was “squeezed” out to the NE via sinistral reactivation of major Caledonian structures in northern UK (e.g. Figure 4.7 and Coward et al., 1989). A slight counter-clockwise rotation of the maximum horizontal stress directions may have occurred then, with the maximum extensional direction becoming more E-W, mobilising the E-W-oriented structures as left-lateral strike slip allowing further ~E-W extension of basins bounded by ~NS structures, e.g. the Crawford-Skipper Basin.

LB WSP

ESPB

WFI

Fennoscandia

Following those structural evolution models, potential Lower and Upper Old Red Sandstone depocentres may exist on the platform itself, below the featureless Base Cretaceous Unconformity (BCU), and in the deepest parts of the Viking Graben. Examples of these depocentres may occur within the Unst basin (UB), the East Shetland Platform Basin (ESP), the West fair Isle Basin (WFSB).

CSB

During the orogenic collapse, numerous intrusives, mostly felsic to intermediate granites, were emplaced throughout AOI, including the well intersected Halibut Granite in the Greater Moray Firth, the Bressay Granite in the Crawford-Skipper Basin on the Eastern Shetland Platform and probably the Utsira High Granite on the east side of the South Viking Graben. These “Newer Granites” intruded along major SW-NE crustal structures and had a significant structural influence during subsequent tectonic events. For example, granites along the outer edge of the present-day ESP mark the footwall of the western boundary of the South and Central Viking graben where it is bounded by deep, steeply dipping faults. Caledonian strike-slip fault with left step developing in a pull-apart basin controlled by N-S fault

BE CSB DBB EFI EOB ESB ESPB FSB HP IMF

During Devonian-Carboniferous strike-slip deformation, a releasing bend in the strike-slip fault introduced an elliptical to sigmoidal depression, i.e.. pull-apart structure, that filled with sediment.

LB NVG S SESB SVG U WFI WGG WSB WSP

Lerwick Basin North Viking Graben Sandwick Basin Southeast Shetland Basin South Viking Graben Unst Basin West Fair Isle Witch Ground Graben West Shetland Basin West Shetland Platform

Hardangerfjord Shear Zone Great Glen Fault Møre-Trondelag Fault Zone Walls Boundary Fault Tornquist Line

DBB

EFI

EOB

GGF

Intruded areas

SVG

HP WGG

IMF

Relative Direction of Motion

Figure 4.8:

Fault Kinematics @

)

Dextral Reverse

Dextral Normal0

)

Sinistral

Sinistral Reverse

Transfer

T # Dextral T Normal Sinistral Normal Basement faults inferred to have been active during the Devonian – Mid Carboniferous. SEEBASE is displayed in the AOI, the DEM is displayed outside the AOI. Outlined basins in red are tectonically active basins or depocenters.

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Mid to Late Variscan Orogeny: Mid Carboniferous-Earliest Permian (~330 – 270 Ma) During the Late Carboniferous, collision between Siberia and Fennoscandia in the Ural Orogeny reversed the eastward movement of Fennoscandia, forcing it back to the west into the Variscan Orogen. Collision between Laurentia and Gondwana continued until the start of the Permian. This inversion is recorded by the Late Carboniferous to Early Permian right-lateral cataclasite deformation along the Walls Boundary and Great Glen Faults (Watts et al., 2007) and led to regional uplift, erosion of some Middle and Late Carboniferous formations, and to the Saalian Unconformity.

NVG FSB

Active Basement Structures

This movement caused dextral strike-slip reactivation of major NE-oriented Caledonian structures in the northern UK and the Norwegian-Greenland margin, and inversion of Early Carboniferous normal faults (Coward et al., 1989; Roberts et al., 1999; Wilks and Cuthbert, 1994; Hartz et al., 1997; Glennie, 1998). Widespread series of NE-trending folds and inversion structures are present in the Devonian beds of the Moray Firth, Orkney and Shetland (Coward et al., 1989). More recently, inverted structures have been identified under the Kraken oil field at the southeastern end of the East Shetland Platform basin (Reid and Patruno, 2017a) confirming that ~NS oriented intra-platform structures and structures along the eastern platform edge likely acted as reverse faults while SE-NW structures acted as transfer zones.

NS constraining bend acts as “pin point” in continental strike-slip system and inverted pull–apart basins

Horst and Highs

Beryl Embayment Crawford-Skipper Basin Dutch Bank Basin East Fair Isle East Orkney Basin East Shetland Basin East Shetland Platform Basin Faroes-Shetland Basin Halibut Platform Inner Moray Firth Lerwick Basin North Viking Graben Sandwick Basin Southeast Shetland Basin South Viking Graben Unst Basin West Fair Isle Witch Ground Graben West Shetland Basin West Shetland Platform Tornquist Line

HFSZ GGF WBF

Hardangerfjord Shear Zone Great Glen Fault Walls Boundary Fault Proposed direction of motion of the intruded blocks

ESB

WSB

SESB

This inversion phase is likely to have led to uplift and erosion of the sediments deposited in the interpreted anastomosing pull-apart basins during the previous event (Cartoon A for northeastern corner of AOI; B for eastern). However, preservation of Devonian to Carboniferous sediments is possible in the deepest parts of the basins or sub basins outlined on Figure 4.8 on the previous page, but, except for the Crawford-Skipper Basin, there is insufficient data to assess confidently. This event is also likely to mark the initiation of large banks or intra-basinal highs (e.g. the East Shetland Basin, Crawford-Skipper, Beryl Embayment, Dutch Bank basins). BE CSB DBB EFI EOB ESB ESPB FSB HP IMF LB NVG S SESB SVG U WFI WGG WSB WSP TL

LB WSP

Kraken Oil Field

ESPB

WFI

Fennoscandia

CSB DBB

EFI

SVG

EOB

GGF

WGG IMF

Fault Kinematics

) )

Figure 4.9:

))

?Dextral Reverse Dextral Reverse

)

Reverse Sinistral Reverse

Transfer

Sinistral Reverse

Basement faults inferred to have been active during the Late Carboniferous – Early Permian.

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Main Rifting Phases: End Paleozoic-Mesozoic (~254 – 55 Ma) 80 Ma

The chart below displays the main events and lithological units filling the basins in the East Shetland Platform during the Mesozoic (Figure 4.10), together with the main geodynamic events and plate reconstruction for key time slices (Figure 4.11). More detailed analysis of the tectonic response and active basement fault maps are presented in the following pages for time period dominated by Pangea breakup and Central Atlantic and Bay of Biscay seafloor spreading:

Bay of Biscay Seafloor Spreading

• Late Permian-Early Jurassic extension (~254 – 180 Ma), • Jurassic-Early Cretaceous extension (~180 – 130 Ma), and • Late Cretaceous-Paleocene (~83 – 55 Ma).

Rifting

Normal faulting on NE-trending faults accommodated NW-trending transfer

Figure 4.10: Mesozoic generalised stratigraphic chart and events of the Rockall area. See Figure 4.4 for references.

160 Ma

? Central Atlantic Seafloor Spreading

220 Ma Central Atlantic Seafloor Spreading

Rifting

Figure 4.11: Frogtech Geoscience plate models for key Mesozoic events.

IV. TECTONIC EVENTS

Toward the end of the Permian, Pangaea began to break up, with inward-propagating fractures/rifts initially opening the Paleotethys Ocean to the south (See plate reconstruction, Figure 4.11; 220 Ma). Similar rifts propagated south from the Arctic Sea into the North Sea, and northeast from the Central Atlantic into the South Western Approaches and Celtic Sea. Extension in Pangaea operated in many different directions associated with widespread interior continental sedimentation, however near the proto-North Atlantic, extension was oriented ~WNW-ESE.

Greenland

NVG FSB

Importantly, the initial tectonic phase established the extensional architecture of the proto-NW Atlantic and Greater North Sea possibly reactivating Devonian-Early Carboniferous extensional structures. Subsequent extension in the Mesozoic largely involved reactivation of normal faults initiated or reactivated during the Permo – Triassic. In the North Sea subsequent extension phases were focused along the triple arm rift system formed by the Moray Firth Basin, Viking Graben (east of AOI) and Central Graben.

ESB

WSB

SESB

LB WSP

BE CSB DBB EFI EOB ESB ESPB FSB HP IMF LB NVG S SESB SVG U WFI WGG WSB WSP TL

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

Late Permian-Early Jurassic Extension (~254 – 180 Ma)

CSB DBB

EFI

SVG

EOB

HP WGG IMF

Fault Kinematics T

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Fennoscandia

ESPB

WFI

Dextral Normal

Normal

Transfer

Sinistral Normal

Basement faults inferred to have been active during the Permo-Triassic.

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Mid Jurassic-Early Cretaceous Extension (~180 – 130 Ma) Ongoing breakup of Pangaea caused further extension in the proto-North Atlantic during the Jurassic to Early Cretaceous (See plate reconstruction, Figure 4.11; 160 Ma). A major plate tectonic reconfiguration at ~180 Ma caused the extension direction to rotate to ~NNW-SSE, coeval with the onset of seafloor spreading in the Central Atlantic. By the end of this basin phase, extension culminated in the linkage of the Arctic and Atlantic rifts, creating a continuous series of linked basins including the Vøring, Møre, Faeroe-Shetland and Rockall basins. Extension was widespread and mainly focused on structures active during Permo – Triassic rifting, i.e. NE-trending Caledonian thrust faults and terrane boundaries.

Laurentia

NVG FSB S

In the AOI, Middle Jurassic to Early Cretaceous represents the main period of extension on the Triassicdefined boundaries of the ESP. Overall the Jurassic to Early Cretaceous is a period of renewed extension with a number of pulses (e.g. Erratt et al., 1999; Davies et al., 1999; Davies et al. 2001). Paleogeography maps show that extension in the Viking and Central Graben is focused along the ~N-S boundary of the Fennoscandian Shield (Rattey and Hayward, 1993). In the study area, the southern East Shetland Basin and Central Viking Graben depositional environment evolved from coastal plain (Bathonian to mid Oxfordian) to shelf (early Kimmeridgian), to continental basin (Kimmeridgian to mid-Volgian) and submarine high (basinal muds, mid-Volgian to late Ryazanian) as subsidence propagated westward over time (The Millennium Atlas, 2003). Figure 4.13 displays the proposed direction of motion of the competent blocks, here the Fladen Ground Spur. This direction implies for example that the Dutch Bank basin is an extensional basin during this rifting phase, and so is the south of the ESPB and CSB.

ESB

Fennoscandia

WSB

SESB

South of the AOI, recently rheologically weakened by development of the Early Jurassic Forties Volcanics Province, the Inner and Outer Moray Firth linked with the south Viking Graben, creating space along the Witch Ground Graben and isolating the Halibut Horst by a complex reactivation W–E, NE–SW and WNW–ESE to NW–SE structures during early Kimmeridgian-Ryazanian.

ESPB

WFI

BE CSB DBB EFI EOB ESB ESPB FSB HP IMF NVG S SESB SVG U WFI WGG WSB

Beryl Embayment Crawford-Skipper Basin Dutch Bank Basin East Fair Isle East Orkney Basin East Shetland Basin East Shetland Platform Basin Faroes-Shetland Basin Halibut Platform Inner Moray Firth North Viking Graben Sandwick Basin Southeast Shetland Basin South Viking Graben Unst Basin West Fair Isle Witch Ground Graben West Shetland Basin

Tornquist Line

CSB DBB

EFI

EOB

SVG

HP WGG IMF Halibut Horst

Forties volcanic center Aalenian doming area Middle Jurassic volcanics extent

Proposed direction of motion of the intruded blocks

Fault Kinematics T

Dextral Normal

Normal

Transfer

Sinistral Normal

Figure 4.13: Basement faults inferred to have been active during the Jurassic – Early Cretaceous.

IV. TECTONIC EVENTS Mid Cretaceous (~120 – 83 Ma)

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

Laurentia

The onset of closure of Paleotethys at ~120 Ma, coupled with ongoing opening of the Central Atlantic, caused Africa to undergo clockwise rotation with respect to Europe, opening a small ocean basin in the Bay of Biscay. At the same time, Central Atlantic seafloor spreading propagated north into the Labrador Sea (Figure 4.11). The gradual cessation of rifting to the west of Britain and Norway created an additional complexity to the structural evolution of the study area such that the regional strike-slip stress regime may switch locally from transpression to transtension.

FSB

A range of compressional and extensional events have been documented. In the Faroe-Shetland region, northeast of the AOI, an angular unconformity separates folded and eroded Turonian and older strata from younger Coniacian – Maastrichtian rocks in the West Shetland, North Rona, East Solan and West Solan basins, as well as the Foula Sub-basin (Booth et al., 1993; Dean et al., 1999; Goodchild et al., 1999; Grant et al., 1999; Stoker et al., 2016). In other West Shetland Shelf basins, much of the Cenomanian–Turonian section is absent, and the unconformity essentially separates Upper and Lower Cretaceous (Stoker et al., 2016). Several other intra-Lower and Upper Cretaceous unconformities occur in the West Shetland Shelf and Hebrides Shelf basins, ranging from Hauterivian to Campanian in age, and indicate the persistence of differential uplift and subsidence throughout the Cretaceous.

ESB Fennoscandia

WSB WBF

SESB

Late Cretaceous-Paleocene: Extension (~83 – 55 Ma) Initially extension was focused in the Rockall Trough – Faeroe-Shetland trend, including the West Shetland and Faroe-Shetland basins of the AOI. However during the final stages the locus of extension shifted northwest into the proto-North Atlantic rift, ultimately leading to seafloor spreading at ~55 Ma in the early Eocene (e.g. Lundin, 2002). The Upper Cretaceous sequence is more widely distributed and by the end of the Cretaceous period most of the major basement highs had been isolated or drowned (Dean et al., 1999; Stoker and Ziska, 2011).

Structures involved in extensional deformation are mostly NE-trending. Occurrence of large volcanic centres and dyke swarms along NS structures (British Igneous Province to the west of the AOI) and along NWtrending transfer faults (e.g. Anton Dohrn Transfer Zone) suggest a component of EW-oriented extension to the stress field by the end of the Cretaceous. In this scenario, EW structures would have been susceptible to sinistral displacement.

CSB EFI

During the Paleocene, voluminous magmatism occurred along the entire proto-North Atlantic margins, north of the AOI.

SVG

EOB

Basement Structures Only minor extension occurred in the northern North Sea during this period, focused around the Halibut Horst south of the area of interest (Figure 4.16) and east of the Platform (Thickest Paleocene deposited along N-S structure). During the Late Cretaceous the locus of extension shifted into the proto-North Atlantic rift. During the Paleocene, the Scottish Highlands and East Shetland Platform were uplifted. BE CSB EFI FSB IMF SESB SVG WGG WSB

Beryl Embayment Crawford-Skipper Basin East Fair Isle Faroes-Shetland Basin Inner Moray Firth Southeast Shetland Basin South Viking Graben Witch Ground Graben West Shetland Basin

GGF WBF

GGF

WGG IMF

Great Glen Fault Walls Boundary Fault

Fault Kinematics #

Figure 4.14:

Dextral

Dextral Normal T

Normal

Sinistral Normal Transfer

Basement faults inferred to have been active during the Late Cretaceous – Paleocene.

IV. TECTONIC EVENTS

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

Inversion Phases: Cenozoic (~56 Ma to Present) The chart below displays the main events and lithological units filling the basins in the Rockall region during the Cenozoic (Figure 4.15), together with the main geodynamic events and plate reconstructions for key time slices (Figure 4.16). More detailed analysis of the tectonic response and active basement fault maps are presented in the following pages for time period dominated by the North Atlantic seafloor spreading, Iceland Hot spot emergence, Pyrenean and Alpine orogenies:

20 Ma New Kolbeinsey Ridge

Extinct Aegir Ridge

• Eocene – Oligocene (~55 – 23.8 Ma), and • Miocene – Recent Structures (~23.8 – 0 Ma).

Alpine Orogeny

Local inversion on E- and ENEtrending structures; ~NS, NNW oriented transfer faults

50 Ma

Pyrenean Orogeny

Figure 4.15: Mesozoic generalised stratigraphic chart and events of the Mid North Sea High. See Figure 4.4 for references.

Figure 4.16: Frogtech Geoscience plate models for key Cenozoic events.

IV. TECTONIC EVENTS

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

End Eocene-Early Miocene (~55 – 23.8 Ma) Breakup and seafloor spreading began in the North Atlantic at ~53 Ma, and corresponded with a change from extension to compression in northwest Europe. The sources for compressional stress were most likely related to the early stages of the Alpine Orogeny. Compressional stresses were probably oriented ~N-S through much of northwest Europe. Around 36 Ma the Iceland Hot Spot emerged from beneath the eastern margin of Greenland with Iceland Plateau pulses starting to form around 23 Ma (White and Lovell, 1997). The onset of the Pyrenean Orogeny in southern Europe at ~38 Ma (Oligocene) caused a significant change in the compressional stress regime in northwest Europe, and slowed the rate of seafloor spreading in the North Atlantic. Regional uplift occurred in the UK, Shetland, Ireland, and western Scandinavia. Volume-balance of sediment derived from the Scottish Highlands and Orkney–Shetland ridge during the Cenozoic suggests that 1900 – 2400 m of post-Caledonian sediment could have been eroded during this interval, plus any covering of Chalk. (Wilkinson, 2016).

FSB S

WSB

Active Basement Structures Localised inversion on major NW-trending basement structures occurred in response to Eocene-Oligocene compression generating long wavelength, subtle anticlinal features such as the Wyville-Thomson Ridge or the Judd anticline West of the AOI (Figure 4.18) (Tuitt et al,. 2010). In the AOI compressional stresses were oriented ~NW-SE, and caused localised inversion of major basement structures. .

LB WSP

DBB EFI ESPB FSB LB U WFI WGG WSB WSP

WFI

Dutch Bank Basin East Fair Isle East Shetland Platform Basin Faroes-Shetland Basin Lerwick Basin Unst Basin West Fair Isle Witch Ground Graben West Shetland Basin West Shetland Platform

ESPB

DBB

EFI

Orientation of horizontal maximum compressive stress from global geodynamic kinematics

WGG

Fault Kinematics

)

Reverse

)

Sinistral Reverse

Figure 4.17: Basement faults inferred to have been active during the Eocene.

IV. TECTONIC EVENTS

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

Miocene-Recent (~23.8 – 0 Ma) The onset of the main phase of the Alpine Orogeny at the start of the Miocene caused a major plate rearrangement at ~20 Ma. Present-day well breakout azimuth data shows that compressional stresses rotated to EW or ~WNW-ESE (The Millennium Atlas, 2003), a stress state reflected today by the measured maximum horizontal compressive stress in northwest Europe (Müller et al., 1992; Reinecker et al., 2003). West of the AOI the Wyville-Thomson and Ymir Ridges experienced further uplift during the mid Miocene (Boldreel and Andersen 1998; Johnson et al., 2005; Stoker et al., 2005). In the region surrounding the AOI, extensive uplift occurred in the UK and Norway resulting in long wavelength inversion structures. In the Faroe-Shetland Basin, inversion of structures continued.

FSB S

WSB

WSP

FSB LB U WFI WSB WSP

WFI

Faroes-Shetland Basin Lerwick Basin Unst Basin West Fair Isle West Shetland Basin West Shetland Platform Average orientation of horizontal maximum compressive stress in wells

Fault Kinematics

)

Reverse

)

Sinistral Reverse

Figure 4.18: Basement faults inferred to have been active during the Miocene – Recent.

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

SEEBASE EAST SHETLAND PLATFORM

V. SEEBASE AND DERIVATIVES FROGTECH GEOSCIENCE

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

V. SEEBASE AND DERIVATIVES SEEBASE: Structurally Enhanced View of Economic BASEment

“Map View / 3D” Versus “Cross-Section View” Interpretation

Frogtech Geoscience believes that if we can produce a picture of the shape of the basin or container that is being explored early in an exploration program, then this will provide a very effective tool to help evaluate exploration concepts at the petroleum system and play level. Traditional techniques for generating depth to basement generally rely on one rock property such as velocity, magnetic susceptibility, or density. The SEEBASE method relies on all three plus rigorous structural interpretation.

A powerful aspect of the SEEBASE workflow is that the interpretation is performed in 2D map view and in 3D. This is a significant departure from conventional seismic-based basin analysis which is predominantly carried out in 2D cross-section view, especially when evaluating large areas. The Frogtech Geoscience method is very effective in defining spatial variation in basin architecture in extensional, compressional, and strike-slip settings.

What is SEEBASE? SEEBASE is a depth-to-basement model that represents the culmination of a number of calibration and integration steps: •

Integrated structural/kinematic interpretation

•

Geophysical modelling

•

Seismic and well calibration

•

Integration of tectonic events and responses

The more we apply the Frogtech Geoscience method, the more we realise the power of non-seismic datasets for predicting the location of “sweet spots” along margins and within continental regions. The Frogtech Geoscience method has proven to be very efficient and effective for locating 3D seismic surveys over such sweet spots.

SEEBASE is a qualitative model of economic basement topography that is consistent with the structural evolution of the basin. SEEBASE defines basin architecture and forms the basis for the systematic evaluation of exploration strategies. SEEBASE is not static; with the acquisition of new data that allow more precise calibration, SEEBASE can be updated. SEEBASE provides a foundation for petroleum systems evaluation, including play element distribution and quality (source/reservoir/seal), fluid focusing, zones of structural complexity, trap distribution, trap type and integrity, paleogeography, oil vs. gas distribution, etc.

SEEBASE results in: • New Exploration and Acquisition Strategies • New Views in Old Basins • Efficient and Effective Exploration • More Rigorous Play and Prospect Risking

Depth msl (m) 3000 500 -2500 -5000 -7500 Figure 5.1: Oblique 3D View of the East Shetland Platform SEEBASE.

-9500

V. SEEBASE AND DERIVATIVES

The East Shetland Platform SEEBASE grid has been imaged using a colour-drape algorithm with northeast illumination (Figure 5.2). The current SEEBASE is a regional (approximately 1:2:000,000 scale) model of basement depth, but resolution increases to ~1:200,000 for more detailed features such as smaller depocentres. The SEEBASE depth-to-basement interpretation was based primarily on the interpretation of gravity and magnetic data, published cross-sections and interpreted seismic, and publicly available well data.

Legend AOI Coastline

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

SEEBASE: Structurally Enhanced View of Economic BASEment

Depth Contours msl (m) -9000 -8000

Depth msl (m) 3000 500

-7000 -6000

-2500

-5000 -4000

-5000

-3000 -2000 -1000

-7500 -9500

0 1000 2000 3000 Figure 5.2: a) SEEBASE image of the East Shetland Platform overlain on the Digital Elevation Model, with the SEEBASE AOI shown by the dashed black polygon; b) SEEBASE image overlain with depth contours (1000 m spacing; at left).

V. SEEBASE AND DERIVATIVES

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

SEEBASE: Basins and Terranes Basin outlines (translucent polygons in Figure 5.3a) are modified from those provided on the UK OGA website. The outlines of basin extents have been revised based on the East Shetland Platform SEEBASE. For newly interpreted depocentres within the AOI, names have been adopted from nearby geographical locations. For example, the depocentre to the east of the Shetland Islands is herein named the Lerwick Basin (LB) after the town of Lerwick, and also after the legacy seismic line LERWICK acquired by the British Institutions Reflection Profiling Syndicate (BIRPS) that crosses this basin. Basement terrane boundaries are shown as white lines on the East Shetland Platform SEEBASE in Figure 5.3b. Key features of the SEEBASE are highlighted on the following page.

Basement Terranes

Basin Outlines NVG

FSB

Caledonides Belt

ESB U Hebrides

Legend Coastline

Basins

SESB ESPB WFI LB CSB BE

EFI

DBB

EOB

WGG

IMF

Figure 5.3a: Basin outlines over the SEEBASE image of the East Shetland Platform AOI.

SVG

BE CSB DBB EFI EOB ESB ESPB FSB HP IMF LB NVG S SESB SVG U WFI WGG WSB WSP

Northern Highlands

Grampian Highlands

Caledonides Belt

Figure 5.3b: Basement terrane boundaries over the SEEBASE image of the East Shetland Platform AOI.

V. SEEBASE AND DERIVATIVES

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

3D SEEBASE: Terranes and Structure The oblique 3D image below, looking towards the NW, highlights key basement terrane and structural features of the East Shetland Platform SEEBASE. The SEEBASE AOI includes the West Shetland Platform, East Shetland Platform and the broader Orcadian Basin.

N A striking feature of the SEEBASE is that the western and northern parts of the AOI is characterised by NE-trending structures and basins, while the eastern and southern sides are characterised predominantly by NWtrending structures and basins. This contrast is interpreted in this study as resulting from the curved geometry of the Baltica indentor into Laurentia at lower crustal levels during the Caledonian, illustrated schematically by the dotted purple line (see Section III for further detail).

Several basins nucleated over major fault zones and basement terrane boundaries. Examples include the Unst Basin (U) over the MØre-Trondelag Fault Zone (MFTZ, with its southern boundary shown by the dashed black line); West Fair Isle Basin (WFI) over the Great Glen Fault; and the shallow Sandwick Basin (S) over the Walls Boundary Fault. These faults were active during the Devonian and reactivated during the Permo-Triassic, Late Jurassic and Tertiary.

ESB The dotted black line shows the basement expression of a splay off the Great Glen Fault that bounds the eastern margin of the West Fair Isle Basin (WFI) and the northern margin of the Lerwick Basin (LB). In the East Shetland Basin (ESB), the splay is overprinted by Jurassic structures associated with the opening of the Viking Graben.

East Shetland Platform EFI

EOB

Depth msl (m)

IMF

3000

DBB

Orcadian Basin

The Greater East Shetland Platform is interpreted to be cored by numerous intrusives of Caledonian age (see Basement Composition in Section III). Several larger blocks of competent intrusives have remained structurally elevated since the Caledonian Orogeny, such as the Bressay Granite (BG), Kraken High (KH) and Fladen Ground Spur (FGS, to the south).

WFH

500 -2500

GRAMPIAN HIGHLANDS TERRANE

FGS

CSB

WGG

-5000 A basement terrane boundary coincides with the sudden deepening of the SEEBASE from the Fladen Ground Spur (FGS) into the South Viking Graben (SVG). The deepening along this boundary is due to a rheological contrast between the highly-intruded, competent, Grampian Highlands Terrane and the less competent Caledonides Belt Terrane, with the latter becoming the focus for Mesozoic extension.

-7500 -9500 Figure 5.4: Oblique 3D image of the East Shetland Platform SEEBASE. Thin grey lines show the coastline. Thick white lines show basement terrane boundaries. Abbreviations: CR – Caithness Ridge; CSB – Crawford-Skipper Basin; DBB – Dutch Bank Basin; EFI – East Fair Isle Basin; EOB – East Orkney Basin; FGS – Fladen Ground Spur; IMF – Inner Moray Firth; KH – Kraken High; LB – Lerwick Basin; MFTZ – More-Trondelag Fault Zone; S – Sandwick Basin; SVG – South Viking Graben; U – Unst Basin; WFI – West Fair Isle Basin; WGG – Witch Ground Graben. Frogtech Geoscience Post: PO Box 250, Deakin West ACT 2600, AUSTRALIA Office: Suite 17F, Level 1, 2 King Street, Deakin West ACT 2600, AUSTRALIA T: +61 02 6283 4800, E: info@frogtech.com.au W: frogtechgeoscience.com.au

SVG

V. SEEBASE AND DERIVATIVES

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

3D SEEBASE: Basins The oblique 3D image below, looking towards the NE, highlights key basin features of the East Shetland Platform SEEBASE.

Pull-apart basins formed within the Devonian “Orcadian” Basin during the Late Caledonian Orogeny, as a result of transtensional collapse of the orogen. Earlier thrusts were reactivated as sinistral strike-slip and transtensional faults on the Greater East Shetland Platform (e.g. Fossen, 2010).

Shallow depocentres overlie some of the smaller intrusives on the East Shetland Platform, interpreted from potential field data and constrained by various calibration data.

ESB U

West Shetland Platform

Shallow depocentre of the Lerwick Basin likely comprising a thin preserved Devonian and/or Permo-Triassic package (e.g. Fossen, 2010). Basement is cored by numerous granitic intrusives.

KH LB

Devonian basins include the Dutch Bank Basin (DBB), Crawford-Skipper Basin (CSB), East Shetland Basin (ESB), Unst Basin (U), Sandwick Basin (S), East Orkney Basin (EOB) and Inner Moray Firth (IMF) (Hitchen and Ritchie, 1987; Zanella and Coward, 2003; Fossen, 2010; Patruno and Reid, 2017a).

WFI GRAMPIAN HIGHLANDS TERRANE

East Shetland Platform

Extension of limit of the Crawford-Skipper Basin of Patruno and Reid (2017a) based on SEEBASE interpretation.

CSB

DBB

FGS

EFI

Orcadian Basin

Depth msl (m) 3000 500 -2500

WFH

Asymmetric half-graben, structural highs and tilted blocks within the Halibut Platform (HP), East Orkney Basin (EOB), and Witch Ground Graben (WGG) were interpreted from high-pass filters of Bouguer gravity data. The basins deepen to the north and are bound to the north and northeast by steep faults (e.g. Beach, 1984; Andrews et al., 1990).

EOB WGG CR

-5000 -7500

-9500 Figure 5.5: Oblique 3D image of the East Shetland Platform SEEBASE. Thin grey lines show the coastline. Thick white lines show basement terrane boundaries. Abbreviations: CR – Caithness Ridge; CSB – Crawford-Skipper Basin; DBB – Dutch Bank Basin; EFI – East Fair Isle Basin; EOB – East Orkney Basin; FGS – Fladen Ground Spur; IMF – Inner Moray Firth; KH – Kraken High; LB – Lerwick Basin; S – Sandwick Basin; SVG – South Viking Graben; U – Unst Basin; WFI – West Fair Isle Basin; WGG – Witch Ground Graben. Frogtech Geoscience Post: PO Box 250, Deakin West ACT 2600, AUSTRALIA Office: Suite 17F, Level 1, 2 King Street, Deakin West ACT 2600, AUSTRALIA T: +61 02 6283 4800, E: info@frogtech.com.au W: frogtechgeoscience.com.au

IMF

SVG

The Caithness Ridge (CR) and West Fladen High (WFH) separate the Devonian and Permo-Triassic basins to the north from the mainly Mesozoic Witch Ground Graben (WGG) to the south (e.g. Klemperer and Hobbs, 1991; Richardson et al., 2005).

V. SEEBASE AND DERIVATIVES

The total sediment thickness (Figures 5.6a and 5.6b) is calculated as the difference between the SEEBASE and the bathymetry/digital elevation model (DEM) grids. The sedimentary section is between 5,000 – 7,000 m thick in:

North Viking Graben

• the West Fair Isle Basin (labelled WFI) and East Fair Isle Basin (EFI); • the East Shetland Basin (labelled ESB) also comprises a thick sedimentary package up to 7,000 m thick;

ESB

• the Crawford-Skipper Basin (labelled CSB) on the flank of the Viking Graben; and • the Witch Ground Graben in the southern part of the AOI. Sediment thickness is over 8000 m in: • the easternmost part of the AOI within the North and South Viking Graben, as well as the Beryl Embayment; and • the Faroes-Shetland Basin (labelled FSB) west of the Shetland Islands, with thick Mesozoic-Cenozoic sediments over preserved Permo-Triassic (and possibly Devonian) stratigraphy.

B WFI

Legend Sediment Thickness (m) Thickness Contour (m)

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

Sediment Thickness

1 - 500 501 - 1000

500

1001 - 1500

1000

1501 - 2000

1500

2001 - 2500

2000

2501 - 3000

2500

3001 - 3500

3000

3501 - 4000

3500

4001 - 4500

4000

4501 - 5000

4500

5001 - 5500

5000 5500 6000

Beryl Embayment

EFI CSB

5501 - 6000 6001 - 6500

Witch Ground Graben

South Viking Graben

6501 - 7000

6500 7000 7500 8000 8500 9000

7001 - 7500 7501 - 8000 8001 - 8500 8501 - 9000 9001 - 9500

Figure 5.6: a) Image of sediment thickness for the East Shetland Platform AOI; b) thickness contours at 500 m intervals for sediment thickness.

V. SEEBASE AND DERIVATIVES

Depth to Moho (Figure 5.7a and 5.7b) for this project was modelled from low-pass filtering of gravity data calibrated with published seismic refraction data, seismic reflection data and gravity modelling data (published in Holliger and Klemperer, 1989; McGeary, 1989; Christiansson et al., 2000; Odinsen et al., 2000; Lyngsie and Thybo, 2007; Artemieva and Thybo, 2013; Funck et al., 2016; Petersen and Funck, 2016).

Moho is deepest beneath the Shetland Islands (~34 km). Moho is also interpreted to be ~35-36 km deep in the southwestern part of the AOI, as a result of thickened crust of the Grampian Highlands during the Caledonian Orogeny. The Moho is interpreted to shallow towards the Caledonides Belt Terrane (CB) (~24 km) and the FaroesShetland Terrane (~25 km), and reaches shallower depths outside the AOI within the North Sea and North Atlantic Ocean. The Moho is interpreted to have a horizontal offset (during sinistral movement in the Late Caledonian) and also vertical offset (deep seismic reflection profile in McGeary, 1989) across the Walls Boundary Fault (labelled WBF in Figure 5.6a), which transects the Shetland Islands. The interpretation of Moho for this study is consistent with Lowpass (LP) 200 km and Low-pass (LP) 300 km Bouguer gravity data, and also consistent with published Moho depths from seismic and receiver function data, e.g. Funck et al. (2016) in the Faroes-Shetland Terrane, and Artemieva and Thybo (2013) in the Viking Graben. The resultant Moho grid may have an error in depth of up to 25% due to the resolution of the grid. Areas of interest should be revisited and updated at higher resolution. Moho depth and its relationship with basement depth and crustal thickness are shown in the gravity model profiles in Section VI (refer also to the Discussion in Section VIII).

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

Depth to Moho

Legend

Grampian Highlands

Moho Depth (msl) Depth Contour msl (m) -36000 -35000 -34000 -33000 -32000

-24999 - -24000 -25999 - -25000 -26999 - -26000 -27999 - -27000 -28999 - -28000 -29999 - -29000 -30999 - -30000

-31000

-31999 - -31000

-30000

-32999 - -32000

-29000

-33999 - -33000

-28000

-34999 - -34000

-27000

-35999 - -35000

Witch Ground Graben

-26000 -25000 -24000

Figure 5.7: a) Image of Moho depth for the East Shetland Platform AOI, shown with basement terrane boundaries in white; b) depth contours at 1000 m intervals for Moho depth.

V. SEEBASE AND DERIVATIVES

Basement thickness (Figures 5.8a and 5.8b) is calculated as the difference between Moho and SEEBASE and ranges from ~14,500 – 37,000 m in the AOI.

Viking Graben

Basement is thinnest (yellow to red contours) in the Faroes-Shetland Basin (FSB) and Viking Graben, associated with thinning during Permian and Mesozoic rifting.

East Shetland Basin

Thicker basement (green to blue) is found below: •

the Shetland Islands, and parts of the East Shetland Platform and West Shetland Platform; and

•

the Caithness Ridge and West Fladen High, which are pervasive structural highs as a result of the Ordovician-Devonian Caledonian Orogeny. East Shetland Platform

Legend

Thickness Contour (m)

Basement Thickness (m) 15001 - 16000

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

Basement Thickness

16000

16001 - 17000

17000

17001 - 18000

18000

18001 - 19000

19000

19001 - 20000

20000

20001 - 21000

21000

21001 - 22000

22000 23000

22001 - 23000 23001 - 24000 24001 - 25000

24000 25001 - 26000

25000 26000

27001 - 28000

27000

28001 - 29000

28000

29001 - 30000

29000

30001 - 31000

30000

31001 - 32000

31000

32001 - 33000

32000

33001 - 34000

33000

34001 - 35000

34000 35000

West Fladen High

26001 - 27000

Caithness Ridge

35001 - 36000 36001 - 37000

36000 37000

Figure 5.8: a) Image of basement thickness for the East Shetland Platform AOI; b) thickness contours at 1000 m intervals for basement thickness.

V. SEEBASE AND DERIVATIVES

Crustal thickness (Figures 5.9a and 5.9b) is calculated as the difference between the modelled Moho and Bathymetry/DEM. Crustal thickness for the East Shetland Platform AOI ranges from 23,000 – 37,000 m.

Due to the flatness of the DEM offshore, crustal thickness varies gradually and closely reflects the topography of the Moho.

Legend

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

Crustal Thickness

Crustal Thickness (m) Thickness Contour (m)

23001 - 24000 24001 - 25000

24000

25001 - 26000

25000

26001 - 27000

26000

27001 - 28000

27000 28000 29000 30000 31000

28001 - 29000 29001 - 30000 30001 - 31000 31001 - 32000 32001 - 33000

32000 33000 34000 35000 36000

33001 - 34000 34001 - 35000 35001 - 36000 36001 - 37000

37000

Figure 5.9: a) Image of crustal thickness for the East Shetland Platform AOI; b) depth contours at 1000 m for crustal thickness.

V. SEEBASE AND DERIVATIVES

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

Crustal Extension Present-day crustal thickness compared to the original thickness of the crust can be expressed as a Beta Factor (β) where (β)~1 => unstretched original crust. Higher numbers reflect greater degrees of crustal extension and thinning. The image in Figure 5.10a shows a map of calculated crustal extension for the East Shetland Platform AOI, assuming an average original crustal thickness of 32 km. Crustal thickness varies between 30 – 36 km on the platform (e.g. Odinsen et al., 2000). The lithospheric stretching factor is calculated as part of the heat flow workflow in order to assess variation due to local extension (refer to Section VII for Heat Flow).

NVG East Shetland Basin

Areas in dark blue with lower ß values have undergone little to no crustal stretching (such as the East Shetland Platform and West Shetland Platform). In contrast, areas with higher ß values (light blue to red) have undergone crustal extension over time. Red and orange coloured areas illustrate the most significant amount of crustal extension in the eastern section of the East Shetland Basin before it steps into the North Viking Graben (NVG in Figure 5.10a), as well as in the Faroe-Shetland Basin (FSB). The Beryl Embayment and western flank of the South Viking Graben are also calculated to have undergone more crustal extension compared to the East Shetland Platform. Although the Greater East Shetland Platform has undergone little extension overall, strain partitioning has occurred in less competent zones around more competent structural highs. For example, the Dutch Bank Basin (DBB) and Crawford-Skipper Basin (CSB) have higher beta factor values than the Fladen Ground Spur, interpreted to be cored by a numerous intrusives within a magmatic complex.

East Shetland Platform

Note that while the yellow to orange areas represent the greatest degree of stretching in the AOI, the ß values in fact only reach a maximum of 2.2. Overall, the values suggest that other than the eastern part of the East Shetland Basin, Viking Graben and Faroes-Shetland Basin, the crust in the AOI has not undergone significant stretching. The maximum beta factor for the centre of the Viking Graben (outside of the AOI) was calculated at 3.2 (Holliger and Klemperer, 1989). A beta factor of 2.2 calculated here for the western flank of the Viking Graben is consistent with stretching concentrated towards the centre of the graben.

Beta Factor Contours

0.8 - 1

0.9

1.01 - 1.1

1.0

1.11 - 1.2

1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8

Beryl Embayment

Basement Stretching Factor (β)

1.21 - 1.3

CSB

DBB Fladen Ground Spur

1.31 - 1.4 1.41 - 1.5 1.51 - 1.6 1.61 - 1.7 1.71 - 1.8 1.81 - 1.9

South Viking Graben

1.91 - 2

1.9 2.01 - 2.1 2.0 2.1

2.11 - 2.2

Figure 5.10: a) Stretching factor map calculated using an initial basement thickness of 32 km; b) 0.1 contour intervals for beta factor.

V. SEEBASE AND DERIVATIVES

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

Crustal Architecture: Cross-Sections Several cross-sections were constructed to illustrate the geometry of the crust in the AOI (line locations in Figure 5.11a). Two of the crustal sections are presented in Figures 5.11a and 5.11b. Two other lines were selected for further gravity modelling (presented in Section VI). The crustal sections display the DEM, SEEBASE, and Moho. The schematic Top Devonian horizon was constrained in several locations by available well data. Note that several wells on the lines have conflicting data (including total depth and the age of stratigraphy at total depth). The sections show that although the East Shetland Platform is structurally higher than the Viking Graben (North Sea) to the east, basement topography is varied and does not form a broad peneplain surface as conventionally perceived. The cross sections also highlight the tilted fault block geometry of basement and half-graben geometry of early Paleozoic basins. Many of these half-graben contain Permo-Triassic sediments as well (see Section VI), with potentially thinner Devonian sediments than depicted in some localities. Unusually thick Paleozoic packages above structural highs (based on well data) may indicate inversion, such as on the Fladen Ground Spur in Figure 5.11b. Magnetic model bodies (clear polygons) are dominantly within the upper crust and are interpreted as Paleozoic intrusives.

Top basement appears to be non-magnetic in many areas. Non-magnetic basement can be explained as metasedimentary units in the upper levels levels of basement of the Northern Highlands and Grampian Highlands orogenic belt terranes. Metasediments can have relatively low percentages of magnetic minerals depending on the original depositional environment and conditions of metamorphism. Most of the magnetic model bodies in Figure 5.11a lie below the top of basement and cannot be used to directly constrain the SEEBASE surface, which was predominantly interpreted from gravity data. Gravity modelling (Section VI) was undertaken to test the validity of depth-to-basement below Paleozoic basins of the East Shetland Platform as well as the regional crustal architecture. Legend DEM (grid) Interpreted Top Devonian

Schematic Basement Fault Interpreted Basement Terrane Boundary

SEEBASE surface (grid)

Magnetic Depth Model Body

Moho (grid)

Total Depth of Wells Within 2 km of Line

Crustal Section A West Fair Isle W

East Fair Isle

Devonian Well

Dutch Bank Basin

Fladen Ground Spur SEEBASE

Gravity Model Line 1 (Section VI)

Crawford-Skipper Basin

Viking Graben E

DEM Interpreted Top Devonian

Devonian Well Caledonides Belt

Magnetic Depth Models

Northern Highlands

Grampian Highlands

Mantle

Crustal Section A (Fig 5.11b)

C Crustal Section B W

Fladen Ground Spur

Witch Ground Graben

Inner Moray Firth

DEM

Devonian Well

Viking Graben E

SEEBASE Devonian Well

Interpreted Top Devonian

Caledonides Belt

Grampian Highlands

Mantle Figure 5.11: a) SEEBASE image over the DEM showing the location of the two gravity model lines (presented in Section VI) and two crustal cross-sections. Terrane boundaries are in yellow; b) Crustal Section A; and c) Crustal Section B showing crustal architecture with the SEEBASE surface (red, from grid), Moho (brown, from grid), DEM (blue, from grid), and a schematic Top Devonian stratigraphic horizon (blue). Dotted black lines are interpreted terrane boundaries, solid black lines are schematic faults. Magnetic model bodies (clear polygons) dominantly lie within the basement and not at the top of basement. Note that the SEEBASE grid does not extend past the AOI, although the lines do. Frogtech Geoscience Post: PO Box 250, Deakin West ACT 2600, AUSTRALIA Office: Suite 17F, Level 1, 2 King Street, Deakin West ACT 2600, AUSTRALIA T: +61 02 6283 4800, E: info@frogtech.com.au W: frogtechgeoscience.com.au

V. SEEBASE AND DERIVATIVES

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

Interpretation Confidence The map at right (Figure 5.12) aims to communicate the interpreters’ confidence in the interpretation of depth to basement for the SEEBASE model by evaluating the various datasets based on criteria described below. During the interpretation process the interpreter evaluates the various datasets available for the project, and assigns confidence values for each dataset (see Figure 2.18 – Data Confidence). For the East Shetland Platform SEEBASE project, the datasets assessed were:

• Mapped basement outcrop (at selected scale); • Published seismic lines, cross-sections, and maps;

• Magnetic coverage; • Gravity coverage; and • Available well data.

In addition, the interpreter evaluates their confidence in the ability of the available data to accurately reflect the geology. This Interpretation Confidence is based on:

100

• How well the datasets image the depth (and geometry) of basement, and

• Whether the local geology is conducive to being imaged by the available datasets (especially the gravity and magnetic data). Frogtech Geoscience is currently considering methods to enhance the Interpretation Confidence and Data Confidence to better represent geological features, continuity and uncertainty. Feedback is appreciated.

Interpretation Confidence

Potential depocentre from gravity data which partly reflects basement composition, and structural / tectonic model, but no constraining data.

Basement depth deduced from gravity data, but gravity may partly reflect basement and/or mantle depth. Conflicting calibration data.

Potential field data is indicative of basement, but multiple cross-sections provide inconsistent depth. Gravity partly reflects basement composition.

Increasing basement depth on margin, good calibration from well data consistent with magnetics and gravity; or deep basin interpreted from gravity and magnetics based on adjacent analogue, multiple cross-sections/seismic provide inconsistent calibration data.

Known basement highs and lows consistent with gravity and magnetic data and reliable crosssections. Minimum basement depth constrained by wells intersecting overlying stratigraphy.

90 100

Basement interpreted adjacent to outcropping geology. Basement depth and geometry from potential field is consistent with available cross-sections and well data.

Basement outcrop. Figure 5.12: SEEBASE Interpretation Confidence map for the East Shetland Platform AOI. The confidence value is determined based on the availability of datasets at each point and the accuracy and reliability of these datasets to determine basement depth.

SEEBASE EAST SHETLAND PLATFORM

VI. GRAVITY MODELLING FROGTECH GEOSCIENCE

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

VI. GRAVITY MODELLING

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

Introduction Two crustal-scale gravity models (line locations in Figures 6.1a and 6.1b) were developed to test the structural interpretation of the SEEBASE depth-to-basement model and Moho across the East Shetland Platform, which lies at the junction of the Laurentia and Baltica megaterranes. Gravity modelling allows possible crustal geometries inferred from potential field data to be tested (see Sections IV and V). In particular, the gravity models evaluated: i)

the structure of the Moho across major faults and terrane boundaries (such as the Walls Boundary Fault, labelled WBF in Figure 6.1a; and the Møre-Trondelag Fault Zone, MTFZ),

ii)

the effect of basement composition on gravity in the East Shetland Platform, and

F-S

iii) the potential depth of Paleozoic basins in the AOI. Better understanding of the potential depth and extent of Devonian basins in this region is crucial to assessing a potentially viable Devonian petroleum system, with a proven analog in the southern part of the AOI (Moray Firth).

Line 1

Modelling Workflow The 2.5D forward models were performed using ModelVision software by Tensor Research, which computes the gravity field effect of all source bodies along a model line as a function of their 3D location and physical properties (location, size/volume, orientation and type/density).

East Shetland Platform

The gravity model lines cross several Paleozoic basins and intersect major terrane boundaries. Stratigraphic horizons from adjacent published cross-sections and/or seismic were used to constrain the depth and distribution of major stratigraphic packages where possible.

Modelling Assumptions and Limitations The SEEBASE depth-to-basement was tested in the gravity models. The preferred models presented in this study are based on geologically-driven density contrasts and basement thickness. Note that the models are not a unique solution, but present one possibility that is consistent with the available data.

B SEEBASE AOI

Line 1

Basement Terranes F-S

Faroes-Shetland

Grampian Highlands

Hebrides

Northern Highlands

Caledonides Belt

Figure 6.1: a) Gravity model lines 1 and 2 shown as thick white lines over the Free Air gravity image, with basement terrane boundaries in yellow; b) Model lines over the DEM image. The dashed grey line shows the approximate location of the southern boundary of the Møre-Trondelag Fault Zone (MTFZ).

VI. GRAVITY MODELLING

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

Sedimentary and Crustal Densities Stratigraphic Section Densities of different stratigraphic packages used in this study (Table 6.1) were adapted from Zervos (1987) and Holliger and Klemperer (1989). The stratigraphic section was divided into four major packages for the purpose of gravity modelling: i)

Geological Unit

Lithology

Water

Density 1.03

Stratigraphy

Cenozoic (Paleocene to Recent) – passive margin sediments overlying Mesozoic stratigraphy, with increasing thickness towards the Viking Graben and Faroes-Shetland Basin,

Cenozoic

Paleocene to Recent mudstones, sandstones and clastics

2.10

Mesozoic (Jurassic to Cretaceous) – rift sediments overlying Paleozoic basins, with increasing thickness towards the Viking Graben and Faroes-Shetland Basin,

Mesozoic

Jurassic to Cretaceous clays, sandstones, mudstones and shales

2.35

iii) Undifferentiated Carboniferous and Permo-Triassic – rift sediments deposited in half-graben above early Paleozoic rocks, and

Carboniferous and Permo-Triassic

Mixed clastics, mudstones, sandstones

2.50

iv) Devonian – Late Caledonian sedimentary package modelled as a separate unit, as it is an important potential source rock and reservoir in the Greater East Shetland Platform and Orcadian Basin region.

Devonian

Sandstones, mudstones, siltstones

2.60

ii)

Basement Terranes

Refer to Section III for further detail on terrane composition, and Section IV for tectono-stratigraphy.

Upper and Lower Crust

Faroes-Shetland

Caledonian basement (granitoids, gneiss, metasediment) that has been highly attenuated

2.70

A two-layered crust (excluding sediment) has been used for the gravity models in this study. The upper crust comprises multiple terranes (see Section III for further detail), and varies in composition and hence density across the AOI.

Hebrides

Granulites and gneisses

2.73

Northern Highlands

Caledonian basement, metamorphics and intrusives

2.72

Caledonides Belt

Caledonian basement that has been highly attenuated

2.74

Grampian Highlands

Caledonian basement, metamorphics and intrusives

2.72

The Grampian Highlands Terrane, which comprises a large part of the SEEBASE AOI, was given a density of 2.72 g/cc in the gravity models. These values are consistent with published densities of 2.715 g/cc in the Central North Sea region (Lyngsie and Thybo, 2007), and 2.72 g/cc and 2.73 g/cc in the Grampian Highlands (Dimitropoulos, 1981; Christiansson et al., 2000). Densities for the other terranes vary relative to the Grampian Highlands depending on terrane type. The lower crust has been assigned a uniform density of 2.9 g/cc in the gravity models following Lyngsie and Thybo (2007). The mantle has been assigned a density of 3.3 g/cc, which is within range of published values (cf. Zervos, 1987; Lyngsie and Thybo, 2007).

Lower Crust and Mantle Lower crust

2.9

Mantle

3.3

Table 6.1:

Density values assigned to geological units for gravity modelling.

VI. GRAVITY MODELLING

Line 1 Gravity model Line 1 crosses the Faroes-Shetland Basin (FSB), several Paleozoic basins on the Shetland Platform, and the western margin of the North Viking Graben (NVG; line location in Figure 6.2a). Depths of basement and key stratigraphic horizons were constrained by limited well data in several locations. Line 1 was selected to assess: i) the presence of Devonian sediments in the Faroes-Shetland Basin (FSB), East Shetland Basin (ESB), Unst Basin (U), and Viking Graben; ii) depth-to-basement (interpreted SEEBASE); and iii) depth-to-Moho. The line also crosses numerous potential field anomalies interpreted as intrusive complexes (see Basement Composition in Section III), and tests whether variations in basement composition play a major role in the gravity signal.

Model 1 (Figure 6.2b) satisfactorily explains the gravity anomalies in the Faroes-Shetland Basin and Viking Graben. Here, the gravity signal is controlled predominantly by thick Mesozoic-Cenozoic sedimentary packages, as well as by depth variation of the lower crust and Moho. However, the centre of the line which comprises Devonian and Permo-Triassic basins over shallower basement, is characterised by several distinct misfits between observed Free-Air gravity (black) and modelled gravity (red). These misfits could not be closed by varying the depth, thickness or density of the model layers within the constraints of wells and seismic and within the geologically reasonable range. Model 2 in Figure 6.2c shows that the misfits in Model 1 are mainly related to density variations in the upper crust. The series of lower and higher density bodies in Model 2 are interpreted as granites and mafic intrusions respectively, consistent with outcropping geology. Low density tabular model bodies representing granites have been added below the West Shetland and Sandwick basins. The higher density bodies beneath the Unst Basin coincide with positive magnetic anomalies, consistent with a mafic composition.

The model includes an abrupt vertical displacement in Moho depth across the Walls Boundary Fault (WBF). The WBF is a major strike-slip fault interpreted as part of the boundary between the Hebrides and Northern Highlands terranes (Walker et al., 2016; and references therein). A deep seismic reflection profile (BIRPS line UNST of McGeary, 1989) imaged a potential displacement in reflectors representing Moho across the WBF. Two models were tested for Line 1. Model 1 (Figure 6.2b) shows terranes with individual densities in the upper crust, and the response of modelled gravity with no intrusive complexes. Model 2 (Figure 6.2c) shows additional variation from basement composition, illustrating heterogeneity within the terranes, and the contribution this variation has on the gravity response.

B Km 0

FSB

ESB

NVG

Line 1

East Shetland Basin

Unst Basin

Sandwick Basin

Faroes-Shetland Basin West Shetland Basin

Observed Free-Air

North Viking Graben

SEEBASE Surface FaroesShetland 2.70

2.74 Caledonides Hebrides

2.73

2.72

MTFZ

WBF

Belt

Grampian Highlands

2.90

Model 1

3.30

Modelled Gravity

Km Faroes-Shetland Basin West Shetland Basin Sandwick Basin 0 SEEBASE Surface 10

Purple line shows inferred extent of Baltica lower crust below the Grampian Highlands

Modelled Gravity

East Shetland Platform

Positive gravity anomaly in the Unst Basin modelled by high density tabular body in Model 2

FaroesShetland 2.70

2.66 2.66

Hebrides

2.73

2.65

WBF

Observed Free-Air

LAURENTIA

East Shetland Basin

Unst Basin 2.35 2.6

2.79

2.6 2.9

MTFZ

2.8

2.72

2.5

2.68 2.64 2.56

Grampian Highlands

2.8

2.6

BALTICA

North Viking Graben 3/04a-M12

Model 2 allows for 0.5 – 1 km of Devonian sediments in the Unst Basin, and 1 – 2 km preserved within halfgraben in the East Shetland Basin. The models illustrate that it is possible for the West Shetland and FaroesShetland basins to include Devonian sediments as well. However, there is little constraint and the density of the Devonian sediments (2.6 g/cc) is only slightly lower than the basement units (2.72. to 2.73 background) in many areas, therefore a range of thickness values is possible.

3/01-1

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

Gravity Model Profiles

2.1 2.35

2.74 Caledonides Belt

2.90

Model 2

3.30 40 Figure 6.2: a) High-pass 70 km image of Bouguer Gravity showing location of gravity model Line 1, with basement terrane boundaries in yellow. The dotted black polygon shows the SEEBASE AOI; b) Model 1 for Line 1 showing misfit between observed and modelled gravity; c) Model 2 for Line 1 showing closure of misfit with variation in basement composition. Blue and purple polygons are modelled bodies of lower and higher density than basement respectively. Vertical red lines show well locations and depth. Thick black lines show interpreted basement terrane boundaries. Abbreviations: MTFZ - Møre-Trondelag Fault Zone; WBF – Walls Boundary Fault. Frogtech Geoscience Post: PO Box 250, Deakin West ACT 2600, AUSTRALIA Office: Suite 17F, Level 1, 2 King Street, Deakin West ACT 2600, AUSTRALIA T: +61 02 6283 4800, E: info@frogtech.com.au W: frogtechgeoscience.com.au

VI. GRAVITY MODELLING

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

Gravity Model Profiles Line 2 Gravity model Line 2 (line location in Figure 6.3a) crosses the southern part of the Shetland Islands, the East Shetland Platform, and the western margin of the Viking Graben (VG). Line 2 was selected to assess: i) the presence of Devonian sediments; and ii) depth-to-basement (interpreted SEEBASE) below the East Shetland Platform; and iii) depth-to-Moho. Similar to Line 1 on the previous page, two models were tested for Line 2. Model 1 observes the response of gravity without intrusive complexes modelled in the upper crust (i.e. terranes have homogeneous densities). Mismatches between observed and modelled gravity occur in the centre of the line below the Shetland Islands, Lerwick region (L in Figure 6.3a) and eastern part of the East Shetland Platform. Changing basement depth, stratigraphic thicknesses and densities within a geologically reasonable degree, did not significantly affect the gravity response in the areas of gravity mismatch. Changing the depth of the lower crust or Moho resulted in overly long wavelength responses in modelled gravity. Introducing tabular bodies in the upper crust (Model 2 Figure 6.3c) achieved the best fit between the modelled gravity and observed Free Air. This requires a major contribution from basement composition.

The gravity models below do not fully represent the geological complexity beneath the Lerwick area and East Shetland Platform. The upper basement levels of the Grampian Highlands is characterised by nappe stacks (shown schematically by the thin dashed black lines in Figure 6.3c). An alternative contribution to the gravity response in this region lies within the lower crustal domains (refer to Section VIII for Discussion). Multiple (Caledonian and older) cross-cutting thrusts inferred from deep seismic profiles (Beach, 1984; McGeary, 1989; McBride and England, 1999; Lyngsie and Thybo, 2007) can be interpreted to mark the transitional zone of Baltica crust beneath Laurentia at depth.

Observed Free-Air East Shetland Basin

Modelled Gravity

Km 0

The SEEBASE in the Lerwick region was interpreted to be shallow (~2500 m), with thin Devonian and/or Permo-Triassic sediments. The Bouguer image in Figure 6.3a shows that the Lerwick region is characterised by several circular negative gravity anomalies. A low density Tertiary package overlying the PaleozoicMesozoic sediments contributes to the overall observed gravity low here. In Model 1 (Figure 6.3b), the modelled gravity response was too high. The upper crust, low density model bodies introduced in Model 2 allow a good fit to observed gravity (Figure 6.3c). The low density model bodies can therefore be explained as granitic intrusions in the upper crust. The clustering of low density model bodies in this location coincides with the potential extent of Baltican lower crust below the East Shetland Platform as far westward as the Lerwick area (inferred schematically by purple dashed line in Figure 6.3b).

Shetland Islands

West Fair Isle Basin

East Shetland Platform

Lerwick

Viking Graben

SEEBASE Surface 2.72 Northern Highlands

East Shetland Platform

WFI

2.74 Caledonides

2.72

Belt

Model 1 W

LAURENTIA

Modelled Gravity Km 0

West Fair Isle Basin 2.6

2.72 20

2.5

2.35

Shetland Islands

Northern Highlands

East Shetland Basin

East Shetland Platform

Lerwick 2.67 2.67

WBF

BALTICA

Observed Free-Air

2.79

2.81

10 Purple line shows inferred extent of Baltica lower crust below the Grampian Highlands

Grampian Highlands

Line 2

Circular negative gravity anomalies in the Lerwick area modelled by as low density intrusives in Model 2

WBF

2.58 2.65

2.65

SEEBASE Surface

Grampian Highlands

2.72

2.65

2.6 2.84

E Viking Graben

3/29-1 3/29-2

2.35

2.1

2.74 Caledonides Belt

Possible Baltican Lower Crust 2.90

Model 2

3.30 40 Figure 6.3: a) High-pass 70 km image of Bouguer Gravity showing location of gravity model Line 1, with basement terrane boundaries in yellow. The dotted black polygon shows the SEEBASE AOI; b) Model 1 for Line 1 showing misfit between observed and modelled gravity; c) Model 2 for Line 1 showing closure of misfit with variation in basement composition. Blue and purple polygons are modelled bodies of lower and higher density than basement respectively. Vertical red lines show well locations and depth. Thick black lines show interpreted basement terrane boundaries. Abbreviations: MTFZ - Møre-Trondelag Fault Zone; WBF – Walls Boundary Fault. Frogtech Geoscience Post: PO Box 250, Deakin West ACT 2600, AUSTRALIA Office: Suite 17F, Level 1, 2 King Street, Deakin West ACT 2600, AUSTRALIA T: +61 02 6283 4800, E: info@frogtech.com.au W: frogtechgeoscience.com.au

SEEBASE EAST SHETLAND PLATFORM

VII. HEAT FLOW

FROGTECH GEOSCIENCE

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

VII. HEAT FLOW

BG e

The present-day basement heat flow model is derived from an analysis of basement: its composition, structure and history. The model is generated through an integrated, iterative interpretation and analysis of a wide range of publicly available geophysical and geological datasets. The result is a geologically based, spatially continuous, heat flow model.

Basin

Heat flow is an elemental boundary condition in numerical models of basin and petroleum systems. Yet, predicting heat flow away from well control is a complicated process. Where data are scarce, heat flow is frequently, and incorrectly, assumed to be constant over the area. Where data are abundant, a timeconsuming, qualitative assessment of heat flow measurements is required. Analyses and modelling of poorly constrained input heat flow data can lead to predictions of prospectivity that are overly-optimistic, incorrect, or may even fail to identify truly prospective areas. This risk is particularly great for areas with limited data control such as frontier basin areas, or deep, under-explored plays in mature basins.

Depth (m)

RTP d

① ②④

The key objective of the present-day basement heat flow model is to develop an understanding of regional variations in basement heat flow by independently assessing changes in the radiogenic heat contribution from basement, mantle input and the impact of local igneous and tectonic events. It has been built and calibrated with Frogtech Geoscience’s proprietary workflow and tools, utilising a set of databases (basement lithology, heat production and global heat flow) that have been assessed for quality and reliability, and assigned a confidence ‘grading’.

Importance of Basement for Heat Flow and Temperature in Basins

Heat flow across the basement surface exerts a direct control on the temperature profile in the sedimentary package of the overlying basin. For a given thermal conductivity the temperature gradient at the base of the basin is proportional to the basement heat flow.

③

Figure 7.1: a) Computed basinal geotherms down to 7 km-depth for three different basement heat flow (HF) values at the base of sediment. Basin thermal properties are identical for all geotherms and constant with depth for demonstration purposes: thermal conductivity = 2 W/mK; heat production = 1.5 mW/m3; surface temperature = 20◦ C. b) Example of stacked, present-day, radiogenic and mantle heat flow calculated along a crustal-scale profile cross-section, presented in c). A profile of interpreted heat production (green line) is also shown. c) Coloured polygons along the SEEBASE profile represent basement composition interpreted from surface geology, gravity (red profile), and magnetic (blue profile) data. Circled numbers represent coincident profile locations for b) and c). d) and e) are maps of magnetic (RTP; d) and gravity (BG = Bouguer; e) datasets used to interpret basement structure and composition (black outlines on images) along the cross-section (yellow line) shown in b) and c). Section Created using Section Sketch by Frogtech Geoscience. Sketch Frogtech Geoscience Post: PO Box 250, Deakin West ACT 2600, AUSTRALIA Office: Suite 17F, Level 1, 2 King Street, Deakin West ACT 2600, AUSTRALIA T: +61 02 6283 4800, E: info@frogtech.com.au W: frogtechgeoscience.com.au

Total Heat Flow = HF

④

For example, Figure 7.1a displays computed geotherms down to 7 km depth in a simplified basin with constant thermal properties for different values of basement heat flow. Figure 7.1b shows the stacked radiogenic and mantle heat flow computed using Frogtech Geoscience’s workflow, and a profile (green line) of heat production assessed from available heat flow data, displaying a good match to the computed radiogenic heat flow. Figure 7.1c displays the crustal-scale architecture highlighted by the profiles of SEEBASE, Moho and DEM, and shows the basement composition at top basement. The line location is shown in Figure 7.1d and e, and the gravity and magnetic profiles are shown at top of Figure 7.1c. In this example, low heat flow values ① occur where basement is deepest, continental crust is extremely thinned and there is a negligible impact from radiogenic heat flow. Basement depth (Figure 7.1c) at locations ②,③ and ④ is similar but there are significant differences in crustal thickness and heat flow values (Figure 7.1b) at those points. Similarity in heat flow values at locations ② and ④ is the result of a higher contribution of mantle heat flow at ② and higher radiogenic heat flow at ④, mainly due to a thicker continental crust. Location ③ shows about the same basement thickness as ④ but has a much higher radiogenic basement heat flow due to the interpreted presence of felsic intrusives which have a higher radiogenic heat production. Figures 7.1a-c demonstrate the importance of understanding the basement composition as well as the crustal architecture (basement depth and crustal thickness) in order to confidently assess heat flow at the base of the basin.

③

Temperature (◦C)

①

Radiogenic Heat Flow

②

Mantle Heat Flow

Heat Production

Gravity Magnetics

③ Depth (m)

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

Importance of a Basement Heat Flow Model

DEM

④

① ②

Moho Basement Composition Intrusive Felsic intermediate intrusive

Metasediment/extrusive Metamorphic

Intermediate Intrusive

High/ Ultra-High Pressure Unit

Metasediment

Serpentinite

Gneiss

VII. HEAT FLOW

Frogtech Geoscience has developed an integrated, geologically-driven approach to generate a model of present-day basement heat flow that encompasses the entire area of interest, including zones where thermal data coverage is sparse or non-existent. The workflow (Figure 5) results in a full set of interpretive layers that are integrated and calibrated with available heat flow data to produce a geodynamically consistent thermal model. To do so, basement heat flow is broken down into its different components which are processed independently.

Heat Flow Components For non-oceanic basement, at a regional scale, contributions to basement heat flow come from two different components. In general, those components are: 1. Radiogenic heat flow (based on basement composition, age and radiogenic basement thickness), Component I; 2. Mantle heat flow, stable or transient, from recent local tectono-thermal events – Component II; Continental basement heat flow can be expressed as the sum of these two components. Heat flow in oceanic crust is mainly mantle-derived due to the negligible radiogenic heat production of mafic lithologies. A separate workflow has been developed for oceanic basement based on crustal age. The sketch in Figure 7.2a displays an example of the relative proportion of heat flow resulting from crust and mantle, compared to the total heat flow in an extensional setting. Continental Basement Heat Flow = Components I + II

a LOC

Heat Flow

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

Basement Heat Flow Components and Frogtech Geoscience Approach

Component II: Mantle Heat Flow At any location mantle heat flow can be either stable (i.e. under cratonic blocks) or transient. Most measurable present-day manifestations of transient heat flow can be attributed to two phenomena: recent igneous activity (hot spot, volcanic arc, intrusions) and/or recent lithospheric-scale extension. In order to assess the magnitude and location of heat flow anomalies for the East Shetland Platform, Frogtech Geoscience compiled information on recent tectono-thermal events for the region (See section IV - Tectonic Events) Frogtech Geoscience has developed separate workflows for transient heat flow related to various types of recent igneous activity, including volcanoes, magmatic arcs/subduction zones, localised intrusions, and hotspots. In the case of recent extension or rifting, the background heat flow anomaly is modelled using a set of Frogtech Geoscience algorithms, modified mantle heat flow variation through time from Waples (2001), and peak time delay from Morgan (1983).

The workflow used to calculate the two components of the heat flow and to build the present-day heat flow model for continental basement is presented in Figure 7.2b.

Continental Mantle -- Component II

Transient

The basement composition is a key input to calculate radiogenic heat flow. Interpretation of basement composition is based on potential field data and available calibration datasets (surface geology, wells, and rock sample database; see Figure 2c as an example). Radiogenic heat production values are assigned to the interpreted basement lithologies using in-situ measured values where available, or a statisticallyrepresentative value based on the 17,000+ values in Frogtech Geoscience’s worldwide database. The radiogenic thickness is assumed to be the upper crust and represents a portion of the basement thickness. The radiogenic component is then calculated from the radiogenic thickness and the heat production values for basement lithologies.

The “stable” or stationary background/sub-crustal heat flow can be assessed either through analysis of the regional variation of measured heat flow or based on the estimated lithospheric thickness. In reality, the term “stable” applies to old undisturbed cratonic continental crust whose underlying lithospheric mantle is close to its thermal equilibrium.

Continental crust -- Component I

Oceanic Heat Flow

Component I: Radiogenic Heat Flow

Stable

Continental Heat Flow Model Components

Frogtech Geoscience Workflow Steps 1. Basement composition interpretation

Component I (Radiogenic)

2. Calculation of heat production values based on composition 3. Attributing heat production to basement composition 4. Assessment of thickness of radiogenic crust 5. Calculation of the radiogenic heat flow

Component II (Mantle) Figure 7.2:

a) Example of the spatial distribution of the basement heat flow components along a recently extended passive margin. Black arrows represent the relative proportion of heat flow coming from below the crust (mantle) and measured at the surface of the basement. b) Workflow steps for Frogtech Geoscience’s continental basement heat flow model. LOC: limit ocean-continent.

Components I + II

6. Review of recent tectonic events 7. Assessment of mantle heat flow, stable or from recent tectonothermal events 8. Present-day heat flow

VII. HEAT FLOW

The public domain International Heat Flow Commission (http://www.geophysik.rwth-aachen.de/IHFC/) database does not contain any heat flow assessment data in the area of interest. Some commercial databases exist (e.g. WISDOM database from the now dissolved Robertson Research International Ltd) but were not available to Frogtech Geoscience during the course of the study. The contour map of present-day heat flow of Figure 7.3 after Burley (1993) is based upon earlier work of Carstens and Finstad (1981), Oxburgh and Andrews-Speed (1981), Eggen (1984), and Andrews-Speed et al. (1984). The contours do not cover the shallow platform due to the scarcity of wells. Where data is available, the highest heat flow occurs on the east side of the platform with values between 60 and 75 mW/m2. Burley (1993) argues that lower values on the Viking Graben flank is influenced by Cenozoic aquifers transferring fluids and heat to the basin margins, but the shallow depth to radiogenic basement in the platform areas might be a controlling parameter. As it is impossible for Frogtech Geoscience to assess the quality of the data used to generate those contours without accessing the original dataset, the heat flow values on Figure 7.3 will be used as a first-order guide only, to compare patterns observed in our model on the eastern part of the East Shetland Platform.

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

Calibration Dataset Compilation

No published data

Published Heat Flow Values (mW/m2) 45 60

No data area Figure 7.3:

Published heat flow data contours with DEM in the background. After Burley (1993).

VII. HEAT FLOW

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

Component I: Radiogenic Heat Flow – Basement Composition

Brassey Granite

Methodology Basement composition is a key contributor to radiogenic heat flow (Figure 7.5). The composition of basement is interpreted from potential field data calibrated with surface geology and is an integral part of the workflow used in generating the SEEBASE depth-to-basement product. In interpreting basement composition for the AOI, the following criteria have been adopted based on the correlation from outcropping basement geology and corresponding potential field responses together with observations from previous authors (e.g. Beamish et al., 2016 and references therein): Gravity: • Felsic to intermediate felsic intrusives from the “Newer Granite” suite and equivalents are generally expressed as gravity lows. For example, the East Grampian Batholith, onshore UK or the Ronas Hill Granite in West Shetland are characterised by the sharp contrasts between the main granitic batholith, mafic intrusives and medium-grade metamorphic protoliths. However, some batholiths appears as positive anomalies on the eastern part of the platform, such as the Brassey Granite, perhaps due to the presence of intermediate intrusives.

Ronas Hill Granite

Metamorphosed shear zone?

Gneisses

• The high grade ”Lewisian” Gneisses in the Northern Highlands appear as a high-amplitude positive anomaly, as are the metamorphosed Caledonian shear zones. • Sharp positive responses similar to the Ordovician Gabbro in the Grampian Highlands Terrane have been interpreted as mafic rocks. Magnetics (see Section III – basement Interpretation, Figure 3.9): • In the Grampian Highlands Terrane medium-grade metamorphics have generally a low magnetic response.

Felsic to Intermediatefelsic intrusives

• Gneisses (such ”Lewisian“ Gneisses) and, to a lesser extent, regional-scale shear zones are positively magnetised. • Felsic to intermediate felsics tend to be weakly magnetised. • Positive anomalies from mafic intrusives can be also be interpreted using magnetic modelling (Figure 2.17). • Some deep regional features are present (see figure 3.9). The interpretation of granitic intrusions proposed in this study might be conservative, i.e. more intrusives might exist that have not been mapped on the East Shetland Platform on Figure 7.4.

East Grampian Batholith, NE extension?

Grampian Highlands

Ordovician Gabbros ? Figure 7.4:

Examples of gravity response for some lithologies that occur in outcrop or are intersected in wells, on a 100km high-pass Bouguer grid.

VII. HEAT FLOW Brassey Granite

Interpreted Lithology Basement composition in the East Shetland Platform Region is interpreted to be predominantly metamorphics to gneiss with abundant granitic intrusions. As it is difficult from potential field data only to decipher between felsic and intermediate-felsic intrusives, all interpreted granites are symbolised as intrusives on Figure 7.5.

Obducted Ophiolite (not considered to be heat producing basement in the following steps)

The intrusives to the southwest, e.g. in and to the north of Moray Firth (MF), are similar in orientation to the felsic to intermediate-felsic Grampian Batholith onshore Scotland, and are probably contemporaneous with, and hence part of, the Devonian ”Newer Granites” suite. Some of those intrusives flooring the Moray Firth could be high heat producing granites. In the vicinity of the inferred mafic intrusives, likely gabbros, some plutons could be older and have been emplaced during Ordovician between the Grampian I and Grampian II Orogenies, as seen onshore. In the northern part of the AOI, the interpreted intrusions are similar in orientation to the main SEdipping thrust trends of the Scandian Orogeny. Melting and channeling of granites may have occurred along the main crustal-scale thrusts during and at the end of the Orogeny, following slab breakoff and subsequent regional strike-slip faulting (e.g. Bird et al., 2013). Originating from a dominantly felsic gneissic and metamorphic protolith, they are likely felsic to intermediate felsic in composition. On the eastern part of the platform, batholiths are interpreted to have an orientation similar to the boundary faults of the west Norwegian margin. They may have been emplaced during the subduction of Baltica under Laurentia.

Basement Composition (Compostion) Basement Composition Composition

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

Component I: Radiogenic Heat Flow – Basement Composition

Gneiss E

E Intrusive

E Mafic

intrusive

BBBBMetamorphic BBBB Quartzite Ultramafic intrusive

East Grampian Batholith NE extension?

Figure 7.5:

Interpreted basement composition

Grampian Highlands

VII. HEAT FLOW

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Component I: Radiogenic Heat Flow – Radiogenic Heat Production 75th Percentile (High-End values) Median Value 25th Percentile (Low-End values)

Component I of the basement heat flow is the radiogenic heat flow generated by decay of unstable isotopes in the radiogenic part of the crust. Radiogenic heat flow is dependent on the heat production (Figure 7.6), controlled by basement lithology, terrane type and age, and on the thickness of the radiogenic portion of the crust. The basement in the East Shetland Platform is predominantly metasediments to paragneiss in composition with abundant felsic to intermediate intrusives.

Heat Production Statistics on Basement Lithology

Figure 7.6:

Simplified statistical representation of Frogtech Geoscience’s heat production database for the basement lithology interpreted in the AOI.

The median-curve trend shows that the granitic rocks have the highest heat production whereas intermediate and mafic lithologies produce less heat, though in reality heat production within the same lithology may vary by an order of magnitude or more. This trend is similar to those observed and discussed by Slagstad (2008), Vilà et al. (2010) or Willmot Noller et al. (2015) and is related mainly to processes controlling the formation and evolution of the magmas for igneous rocks or sedimentation for sedimentary rocks.

Heat Production (µW/m3) 0.8 - 1 1 - 1.5 1.5 - 2 2 - 2.5

REPLACE

a Figure 7.7:

Frogtech Geoscience has compiled laboratory measurements of radiogenic isotope concentration from more than 17,000 samples around the world and calculated the heat production based on the rock sample density. The graph in Figure 7.6 is a simplified statistical representation of the calculated heat production for the basement lithologies listed, classified by decreasing median value. The graph displays a box plot without error bars for clarity. The bottom of the boxes is the 25th percentile, linked by the blue curve, the top of the boxes is the 75th percentile joined by the red curve. The median values are represented by the black curve.

End Member Heat Production Models

2.5 - 3

The availability of percentiles allows different heat production models to be built. The low-end value model (25th percentile) and high-end value model (75th percentile) are presented in Figure 7.7 and highlight the potential range of heat production values.

3 - 3.5 3.5- 4

b End-member heat production models based on the values presented in Figure 7.6. a) Low-end values based on 25th percentile curve ; b) High-end values based on 75th percentile curve. Legend applies to both images.

VII. HEAT FLOW

Methodology Each mapped basement lithology is assigned a radiogenic heat production value for use in the radiogenic heat flow calculations. These values are close to the mean values for the corresponding lithology or lithology groups within the Frogtech Geoscience database. Within the database several common lithologies within the project area, including metamorphic and gneiss lithologies, have a very large range of values due to the potential range in primary rock composition. After testing with the other heat flow components, the preferred model (Figure 7.9) is that calculated using median values (Figure 7.6). Figure 7.8a shows the decay of normalised heat production for different basement lithologies through time. Computed curves predict that for rocks older than 1600 Ma, the decrease in heat production exceeds 20% of the initial value. This is about the same decrease in value that is observed between the 25th percentile and the median curve (Figure 7.6). So, where outcropping basement is older than Mesoproterozoic (>1600 Ma; Figure 7.8b), the 25th percentile curve was used in modelling heat production.

East Shetland Platform AOI Heat Production (µW/m3) 0.8 - 1 1 - 1.5 1.5 - 2 2 - 2.5 2.5 - 3 3 - 3.5

Heat Production Estimates

3.5- 4

In the course of the Geothermal Energy Program during the 1970s and 1980s, a few intrusions constituting the East Grampians Batholith of Scotland were assessed. Four intrusions (Cairngorm, Mt Battock, Ballater and Bennachie) were drilled to 300 m depths and heat production values were measured. Compared to other UK granites, the East Grampians intrusions have the highest heat production values (5.0 – 7.3 μW/m3), but the heat flow values are only moderately elevated (~30% higher, in the range 59 – 76 μW/m2) with respect to the average value for the UK. An intrusive suite with similar orientation and that may be a northeasterly continuation of the onshore East Grampians batholith has been interpreted within and to the north of the Moray Firth Basin. Those intrusions could have higher heat production values than have been modelled during this study (Figure 7.9).

a Normalised Heat Production

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

Component I: Radiogenic Heat Flow – Radiogenic Heat Production

Dunite 1.1

Peridotite Diorite

Gabbro/basalt Continental crust

0.9

Granodiorite Granite

0.8 0.7 0.6 0.5 0.0E+00 0

0.51.0E+09 1

1.5 2.0E+09 2

2.5 3.0E+09 3

Age (Ga) Figure 7.8:

a) Decay of heat production through time for different basement lithologies according to the relative proportions of major isotopes they contain. b) Maximum age map for basement terranes.

Basement Terranes Maximum Age (Ma)

< 1600 Ma > 1600 Ma

Possible high heat production granites (not modelled)

Figure 7.9:

Preferred heat production model produced following the methodology explained in the text. The scale is the same as that used for the 25th and 75th percentile calculations in Figure 7.7.

VII. HEAT FLOW

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Component I: Radiogenic Heat Flow – Radiogenic Thickness of the Crust Basement thickness is used to derive the radiogenic thickness of the crust. The thickness of the radiogenic portion is calculated as a percentage of the continental crust (Figure 7.10) following the approach described below.

East Shetland Platform AOI

An empirical relationship between measured heat production and heat flow exists in large tectonic units sharing a long common geological history. The relationship can be expressed in the form: Q = A * H + Q0 (Roy et al., 1968), where Q is the measured heat flow, A is the measured heat production, H is the slope of the line representing the radiogenic thickness, and Q0 represents heat flow through the base of the radiogenic layer, in this case the mantle heat flow. Published studies give an indication of radiogenic thickness. For example, Hasterok and Chapman (2011) used H ≈ 10 km and Q0 = 25 mW/m2 for the Proterozoic provinces in North America. McLaren et al. (2003) used similar values, i.e. H ≈ 10 km and Q0 = 25 mW/m2, for the Proterozoic in central Australia. Published values of radiogenic thickness represent between 40% and 60% of the total thickness of continental crust. In regions of unstretched or uniformly stretched crust in this study, the radiogenic upper crust is assumed to equal 40% of the total basement thickness. This value approximates models of vertical distribution and measurements indicating that radiogenic heat flow mainly comes from the upper crust (Percival and Card, 1983; Arshavskava et al., 1987; Ashwal et al., 1987).

NVG East Shetland Basin

Radiogenic Thickness

East Shetland Platform

In the course of the Geothermal Energy Program during the 1970s and 1980s, a few intrusions constituting the onshore East Grampians Batholith of Scotland southwest of the AOI were assessed. Gravity modelling indicates that the granite extends to a depth of ~13 km (e.g. Busby, 2010). This depth estimate can be used to define the minimum radiogenic thickness of the crust and is consistent with 40% of the total basement thickness mentioned above considering a ~ 35km-thick crust. The thickest radiogenic portion of the crust is located beneath the Shetland islands, West and East Shetland platforms and Fladen Ground Spur. The thinnest radiogenic layer is found where crustal thinning occurred, i.e. East Shetland, Crawford-Skipper and Faroe-Shetland basins, together with the basins along the western flank of the Viking Graben.

Beryl Embayment Dutch Bank Basin

Thickness of Radiogenic Layer (m) 15000

11000

6000

Figure 7.10: Radiogenic thickness of the crust.

Fladen Ground Spur

Crawford Skipper Basin

South Viking Graben

VII. HEAT FLOW

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Component I: Radiogenic Heat Flow – Radiogenic Heat Flow Model The preferred radiogenic heat flow model (Figure 7.11) has been generated by multiplying the radiogenic heat production model with the radiogenic thickness (reproduced below as Figures 12a and 12b). Regionally, the highest radiogenic heat flow corresponds to the East Shetland Platform, Fladen Ground Spur and Moray Firth Basin areas. They are underlain by thicker radiogenic crust with inferred granite intrusions. In contrast, the lower values of radiogenic heat flow in the Faroe-Shetland and Viking Graben and reflect thinner radiogenic crust with the Faroe-Shetland being floored by older, lower heat production basement. At local scale the regional trend is punctuated by the presence of radiogenic intermediate felsic intrusives or minimally radiogenic mafic lithologies.

East Shetland Platform AOI NVG East Shetland Basin

b East Shetland Platform

Beryl Embayment Dutch Bank Basin Heat Production (µW/m3) 0.8 - 1

Radiogenic Thickness (m)

Radiogenic Heat Flow (mW/m2)

14,000

265

Moray Firth

1 - 1.5 1.5 - 2 2 - 2.5 2.5 - 3 3 - 3.5 3.5- 4

Figure 7.12: Radiogenic heat-flow model inputs. a) Radiogenic thickness,: b) Heat production.

Figure 7.11: Radiogenic heat flow model of the crust.

Fladen Ground Spur

Crawford Skipper Basin

South Viking Graben

VII. HEAT FLOW

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Component II: Mantle Heat Flow – Transient Mantle Heat Flow Transient Heat Flow Methodology Component II is a post-tectonic “transient” cooling function based on the age and type of the most recent tectono-thermal event. Each basement composition polygon within the project area is attributed with the age of the most recent major tectono-thermal event to affect it. The tectono-thermal events compiled for this study are available in section IV- Tectonic Events.

East Shetland Platform AOI

East Shetland Basin

Stretched Lithosphere Transient Heat Flow Approach Conductive heat flow in recently extended continental lithosphere (crust and sub-continental mantle) is generally higher than in the surrounding unstretched lithosphere. Thinning of the lithosphere results in uplift of the lithosphere-asthenosphere boundary taken here to be the 1330 °C isotherm. During or shortly after onset of extension, isotherm uplift causes an increase of conductive heat flow in the lithosphere, Q0. After extension stops, the lithosphere regains thermal equilibrium. During this phase Q0 decreases until it achieves a steady state consistent with the new lithospheric structure. The maximum value of Q0 is dependent on the final steady-state heat flow value and lithospheric stretching factor. Different models have been proposed to estimate heat flow variation during rifting or continental extension through time (e.g. MacKenzie, 1978; Jarvis and McKenzie, 1980; Wernicke, 1985; Waples, 2001). These models differ on the thermal state (steady vs transient), shear modes (pure vs simple) and stretching rate (constant rate vs instantaneous), but they all use crustal “tectonic” (d) or lithospheric stretching factor (β) and time since rift onset (t0) as direct or indirect inputs. They also differ on the heat flow values they are modeling, surface values (Q) versus background values (Q0).

Beryl Embayment East Shetland Platform

Waples (2001) proposed a modification of the MacKenzie (1978) model that permits calculation of the evolution of background heat flow Q0(t) through time if the basement stretching factor (d ; Figure 7.13) and the time of extension onset (t0) are both known. This model also takes into account the delay (td) in the arrival of the thermal effect (heat pulse) to the surface developed by Morgan (1983). The model proposed by Waples (2001) has been used here. The maximum basement thickness value over the Platform is ~32km and is considered to be the “unstretched” thickness of areas that have not been significantly impacted by post Permian extension phases. Areas in dark blue on Figure 7.13 with lower ß values have undergone little to no crustal stretching (such as the East and West Shetland platforms). In contrast, areas with higher ß values (pink) have undergone crustal extension over time. The most significant amount of crustal extension is in the eastern part of the East Shetland Basin before it steps into the North Viking Graben (NVG in Figure 7.13), as well as in the FaroeShetland Basin (FSB). The Beryl Embayment and western flank of the South Viking Graben are also calculated to have undergone more crustal extension than the East Shetland Platform. While the Greater East Shetland Platform has overall undergone little extension, strain partitioning has occurred in less competent zones around more competent structural highs. For example, the Dutch Bank Basin (DBB) and Crawford-Skipper Basin (CSB) identified as potential Devonian depocenters have higher beta factor values than the Fladen Ground Spur, interpreted to be cored by a large intrusive complex. Areas with ß values lower than 1 represent areas that have been inverted such as the Shetland Islands. The ß factor values only reach a maximum of ~2.1 in the AOI. Overall, the values suggest that other than the eastern part of the East Shetland Basin, Viking Graben and Faroes-Shetland Basin, the crust in the AOI has not undergone significant stretching. The maximum beta factor for the centre of the Viking Graben (outside of the AOI) was calculated at 3.2 by Holliger and Klemperer (1989) and at 2.7 by Lippard and Liu (1989) to the North Viking Graben. A beta factor of 2.2 calculated here for the western flank of the Viking Graben is consistent with stretching concentrated towards the centre of the graben.

NVG

CSB DBB Fladen Ground Spur

PR170_betafactor_g.img Basement Stretching Factor ß

Value

113.464

1.15 1

0.955123

0.84

East Shetland Platform AOI

Figure 7.13: Stretching factor map calculated using an initial basement thickness of 32 km.

South Viking Graben

VII. HEAT FLOW

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Component II: Mantle Heat Flow Present-Day Transient Heat Flow due to Lithosphere-Scale Stretching The tectonic events that resulted in lithosphere stretching induced, transient, thermal anomalies for the study area are summarised in section IV – Tectonic Events. The project area has been subdivided for the transient heat flow calculation in order to account for variations in the extensional history.

East Shetland Platform AOI

East Shetland Basin

Deconstructing the present-day finite stretching factor into its extension phase components is difficult in the East Shetland Platform area for the following reasons: • The present study and recent publications (e.g. Patruno and Reid, 2016a) confirmed the existence of thick Devonian to Mid-Carboniferous depocenters in the Greater East Shetland Platform adding another phase of stretching to published subsidence analysis. Despite Carboniferous inversion which may have reduced the impact of this extensional phase, it remains a non-negligible part of the finite stretching factor. In the North Viking Graben, Lippard and Liu (1989) attribute a  value of 1.3-1.5 to the Permian and Triassic rift and = 1.7-1.8 for the Mid- to late Jurassic rift. If we include a Devonian extension phase prior to the Permian with up to 1.5 km of sediments we will be close to = 1.7-1.8 for the pre-Jurassic rifts. In turn, it implies that the Jurassic component represents about half of the finite stretching factor. A stretching factor equal to half the present-day factor has been used in the calculations. The other parameters used for the calculation are shown on Table 7.1. • As highlighted by Turner and Scrutton (1993) available observed subsidence data in the Faroe-Shetland basin (FSB) are not sufficient to constrain precisely the relative magnitude of the different Mesozoic extensional events: (1) Late Jurassic (160 – 146 Ma), (2) Early Cretaceous (141 – 120 Ma) and (3) Late Cretaceous (90 – 70 Ma). Moreover the subsidence rate on the flanks slowed drastically during the Paleocene meaning that the transient effect of the extension was at least partly overprinted by magmatic events occurring to the north of the AOI. In terms of transient heat flow, this most recent thermal event is more significant than the lithosphere thinning induced heat flow and is calculated following a different workflow explained below.

East Shetland Platform

Beryl Embayment

Present-Day Transient Heat Flow due to Recent Magmatism Vitorello and Pollack (1980) and Jessop (1990) recognised that the heat flow in recently thermally disturbed areas can be fitted with a curve with follwing this equation Q = Qo + Qo*exp(-0.0039t) where Q is the heat flow value, Qo is the initial background heat flow, generally close to 30 mW/m2, both in mW/m2, and t the time in million years. Values for t=60 Ma are close to 40 mW/m2.

Fladen Ground Spur

Figure 7.14 displays the resulting transient heat flow map combining the effect of magmatism and stretching.

South Viking Graben

Transient Heat Flow Model Constants

Name

Viking Graben

Original Crustal Thickness

32 km

Onset of Rifting

160 Ma

Thermal Diffusivity

8.0 x 10-7 m2/s

Thermal Conductivity

3.3 W/mK

Table 7.1: Values for Transient Heat Flow due to Lithosphere Stretching Model calculation. Frogtech Geoscience Post: PO Box 250, Deakin West ACT 2600, AUSTRALIA Office: Suite 17F, Level 1, 2 King Street, Deakin West ACT 2600, AUSTRALIA T: +61 02 6283 4800, E: info@frogtech.com.au W: frogtechgeoscience.com.au

CSB

DBB

Transient Heat Flow (mW/m2) 40 36 34

Figure 7.14: Calculated model of Present-day transient heat flow due to stretching.

VII. HEAT FLOW

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Present-Day Heat Flow Pattern The present-day heat flow model is shown in Figure 7.15 .The radiogenic heat flow (Figure 7.11) and transient heat flow (Figure 7.14) have been added to give the total heat flow within continental crust. East Shetland Platform AOI

• The computed heat flow values on the East Shetland Platform are between 60 mW/m2 to 77 mW/m2, consistent with the published contours presented on Figure 7.3. On the Platform, the highest present-day heat flow values occur where the continental crust is thicker and intruded by granites or where thinning has been less (Dutch Bank Basin). Maximum values occur where the crust has been thickened as in the Shetland Islands.

NVG East Shetland Basin

• For identical reasons the Moray Firth area shows elevated values. It should be noted that those values might be underestimated if some of the granites prove to be highly radiogenic. • The present-day heat flow remains high in the rifted regions east of the Platform (East Shetland Basin, Beryl Embayment, Crawford Skipper Basin and South Viking Graben) where the computed low radiogenic heat flow is partly compensated by the transient positive thermal anomaly inherited from the Mesozoic rifts. • The low values in our model (~45 mW/m2) are associated with the Faroe-Shetland Basin, interpreted here to have a thinned, low heat production basement as opposed to the thick, intruded Platform. The method used to calculate the transient heat flow may underestimate the actual mantle heat flow. Using a higher initial value for Qo would have result in higher mantle heat flow and hence higher total heat flow. The use of a higher Qo can be justified as it would represent the positive transient heat flow generated from the older, successive extensional events, but the chosen value would be accompanied by a large margin of uncertainty. The possibility of additional, uninterpreted granites would also locally increase the basement heat flow.

East Shetland Platform

Beryl Embayment

• Locally the lowest values are found where either Proterozoic gneisses or mafic intrusives have been interpreted.

Implications for Petroleum Exploration The interpreted occurrence of Early Devonian granites is a critical factor to take into account when performing basin modelling and considering the maturation level and timing of the viable petroleum systems on the East Shetland Platform and surrounds. Effectively, those intrusions warrant high crustal heat flow values and in turn higher than average temperature gradients in the overlying sediments. Hence, even weakly thinned areas such as the Crawford-Skipper Basin, Beryl Embayment or Moray Firth Basin, suspected or proven to host Middle Devonian source rocks, might not need a thick sediment blanket to reach peak maturation.

Dutch Bank Basin

As granites are rheologically competent, they will tend not to deform during compressional events, such as the Late Carboniferous inversion and ensure better preservation of the overlying sediments. For example Middle Devonian and Early Carboniferous source rocks are more likely to be preserved if deposited above those intrusions. If large structures exist in those granites then they are likely to form highs and banks that will resist erosion and tectonic deformation extension. Boundary faults around those highs will define pathways for the hydrocarbons to migrate. The granite might also serves as secondary permeable reservoirs ,if fractured, providing the fractured area is properly sealed by impermeable sediments.

Moray Firth

Basement Heat Flow (mW/m2) 77 40

Figure 7.15: Calculated model of present-day basement heat flow.

Crawford Skipper Basin Fladen Ground Spur South Viking Graben

SEEBASE EAST SHETLAND PLATFORM

VIII. DISCUSSIONS FROGTECH GEOSCIENCE

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

VIII. DISCUSSION

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

Key Results The SEEBASE depth-to-basement model (Figure 8.1) illustrates that far from being a peneplained structural high, the Greater East Shetland Platform is characterised by varied basement topography. Basement structure fundamentally influences the development of the overlying Paleozoic to Recent basins. This section discusses key results of this study and provides new insights on the potential prospectivity of the region.

FaroesShetland

• A re-interpretation of basement terranes has provided an improved understanding of the regional tectonic evolution for the East Shetland Platform AOI. • The first-order architecture of basement is controlled by: i) megaterrane and terrane boundaries at the basement surface; and ii) megaterrane boundaries in the lower crust.

Hebrides

• These factors exert an influence on the trend of major faults and shear zones, and the geometry and orientation of basins on the Shetland Platform.

Hebrides

• Gravity modelling results illustrate there is contribution from basement depth and composition towards the gravity response of the Shetland Platform. In comparison, the gravity response of the Faroes-Shetland Basin and Viking Graben on either side of the platform are predominantly controlled by sediment thickness and Moho depth. • The SEEBASE shows a continuous view of basement that is otherwise poorly imaged on legacy seismic, and provides better definition, as well as extending the limits, of known intra-platform depocentres with potentially viable Paleozoic petroleum systems. • Integration of terrane and basement composition is used to model regional present-day basement heat flow, providing additional input into future basin and/or thermal modelling studies.

Insights from the Nature of Basement

Northern Highlands

East Shetland Platform KH

Grampian Highlands

Detailed understanding of basement is critical to understanding the evolution of the overlying basins, their different depths and unique burial and/or exhumation histories. A review of published data and reinterpretation of basement terranes was undertaken in the East Shetland Platform AOI and surrounding areas. Integration of the terrane geology (based on potential field data) with tectonic analysis and plate reconstruction models has substantially improved the understanding of their amalgamation history, regional tectonic evolution and crustal architecture. • The interaction of Baltica and Laurentia has influenced the orientation of basins on either side of the Walls Boundary Fault (WBF) / Great Glen Fault (GGF) in the AOI. The lower crust of Baltica (white dashed line in Figure 8.1) is interpreted here to underthrust the Grampian Highlands Terrane (of Laurentia), extending as far westwards as the Lerwick Basin (labelled L). The curved geometry of the Baltican lower crust in this region explains the broadly NW-SE trending basins on the eastern and southern side of the AOI, and the broadly NE-SW trending basins on the western and northern side. • Competent blocks comprising intrusives have been interpreted based on comparable gravity and magnetic response on the Shetland Platform, including well-known structural highs such as the Bressay Granite (BG), Kraken High (KH), West Fladen High (WFH) and Fladen Ground Spur (FGS). These rheologically stronger blocks partition deformation around them, resulting in the deepening and infilling of surrounding depocentres.

FGS WFH

Northwest-oriented basins in the south and east and northeast-oriented basins in the north and west are attributed to the crustal architecture of Baltica and Laurentia at varying levels in the crust

Figure 8.1: SEEBASE image overlain with terrane boundaries in solid grey lines. The dashed white line represents the interpreted extent of Baltica lower crust below the Grampian Highlands Terrane.

VIII. DISCUSSION

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

Basement Topography and Geology Insights from Heat Flow Modelling Present-day basement heat flow was calculated using Frogtech Geoscience’s integrated workflow that considers basement composition and basement thickness variation together with transient mantle effects of the two most recent major Mesozoic rifting events.

NVG East Shetland Basin

• The computed heat flow values on the East Shetland Platform are overall high, comprised between 60 mW/m2 to 77 mW/m2 consistent with published contours with the highest values occurring where the continental crust is thicker and intruded by granites or where thinning has been less (Dutch Bank Basin). Presence of those most likely Early Devonian granites is a critical parameters to take into account when considering the maturation level and timing of the viable petroleum systems on the East Shetland Platform and surrounds. Effectively, those intrusions warrant a permanent high crustal radiogenic heat flow values and in turn higher than average temperature gradient in the overlying sediments. Hence, even weakly thinned areas such as the Crawford Skipped Basin, Beryl Embayment or Moray Firth basin suspected or proven to host Middle Devonian source rocks might not need a thick sediment blanket to reach peak maturation. • In this study, a first attempt has been made to map the different intrusions or batholiths undoubtedly coring the so-called “Platform” using moderate to low resolution potential data, acknowledging that it is probably a conservative first pass. As they control the rheology of the Platform and exert a primary control on its thermal regime, future efforts should be focused on a better understanding of the granites emplacement processes, their relation with the Caledonian structures and stress regime and the local geological parameters that may control their likely varying types. This will contribute to a better assessment their possible location, heat production capacity and zone of influence in the areas where viable petroleum system are identified.

East Shetland Platform

• A compilation and processing of all the temperature data from wells in the AOI would certainly help to identify anomalously high thermal gradients that might confirm the presence of underlying granite or test scenarios regarding their heat production values such as the possible presence of high heat productive granites similar to one known in the East Grampian Batholith in Scotland in the Moray Firth basin.

Beryl Embayment

• Constraining intrusives density and geometry, i.e. in map view their view outlines and depth extent might be possible through the use of coupled geologically-pertinent 3D magnetic and gravity modelling together with the acquisition of higher resolution potential field data. This will contribute to improve our estimation of the radiogenic thickness likely variable through the area.

Dutch Bank Basin

• Deformation-localising competent granites flooring the east part of the Platform are believed to be responsible for the local steepness of the central and South Viking Graben bounding faults, allowing for kilometers of Mesozoic, and older, sediments to be deposited despite a rather low horizontal deformation and finite stretching factor. Sediments deposited close to the granitic section of footwall of those faults might experience higher temperature due to additional contribution of lateral heat flow from the nearby productive granite walls. This proximity will favour locally hydrocarbon maturation and/or cracking. Same applies around granitic highs and banks.

Fladen Ground Spur South Viking Graben

• The low heat flow values computed in the Faroe Shetland Basin is mostly due to (1) the relative absence of interpreted granites and (2) the occurrence of a complex succession of recent extensional, Coastline compressional/uplift and magmatic events that limits the use of analytical methods to calculate mantle heat flow. Thus heat flow in this area is to be considered with caution. Regardless, during the Permo- Inferred Basement Triassic extension higher heat flow are expected to occur due the crustal thinning that might be sufficient Heat Flow (W/m2) to trigger maturation of the previously deposition Devonian and Carboniferous source rocks. High : 0.077

Moray Firth

Figure 8.2: Low : 0.040 Frogtech Geoscience Post: PO Box 250, Deakin West ACT 2600, AUSTRALIA Office: Suite 17F, Level 1, 2 King Street, Deakin West ACT 2600, AUSTRALIA T: +61 02 6283 4800, E: info@frogtech.com.au W: frogtechgeoscience.com.au

Crawford Skipper Basin

Inferred present-day basement heat flow computed during this study.

VIII. DISCUSSION

Insights from Gravity Modelling Gravity models were developed to assess crustal architecture across the East Shetland Platform at the junction of the Laurentia and Baltica megaterranes (Figure 8.3a). This area is situated within the collisional front between the Laurentian and Baltica megaterranes. The model for Line 1 also includes the FaroesShetland Basin on the eastern North Atlantic margin, where it crosses the Hebrides (H) and Faroes-Shetland (F-S) terranes. Line 2 models the West Fair Isle Basin (WFI) and Lerwick Basin (L), which have been previously identified as Devonian depocentres (e.g. Marshall and Hewett, 2003; Fossen, 2010).

• The lower crust and Moho undulate gently. The more varied topography of the lower crust for Line 1 can be attributed to its proximity to major Devonian NE-trending strike-slip faults, and potentially to its coincidence with the edge of the Baltica lower crust below the Grampian Highlands Terrane (GH).

• Basement depth (SEEBASE surface) is highly varied on the East Shetland Platform. The models allow for a geologically-reasonable alternative basement interpretation with fault block and graben geometries in the study area.

• Basement composition plays a major role in producing the observed gravity response over the East Shetland Platform. In comparison, the gravity response over the Faroes-Shetland Basin and Viking Graben are predominantly influenced by: i) thick Mesozoic-Cenozoic successions; and ii) Moho depth and topography.

• A vertical displacement in Moho of about 2-3 km at the Walls Boundary Fault (WBF) provides a good match between observed and modelled gravity, consistent with observations in McGeary (1989).

B F-S Line 1

East Shetland Platform

NH WFI

• The gravity models allow for the preservation of Paleozoic depocentres within the East Shetland Basin, Unst Basin and West Fair Isle Basin.

Modelled Gravity

Km Faroes-Shetland Basin 0 10

• Circular negative gravity anomalies that characterise the offshore Lerwick region and East Shetland Platform are a result of: i) shallow Paleozoic basins overlain by low density Tertiary sequences; and ii) density intrusions in the basement, supported by gravity model results.

Sandwick Basin

West Shetland Basin

LAURENTIA 2.6

2.66 2.66

Hebrides

2.73

2.65

2.79

2.6

MTFZ

WBF

2.72

Model 2 W West Fair Isle Basin 2.6

2.72 20

2.5

2.35

Shetland Islands

2.64 2.56

2.6

2.1 2.35

2.74 Caledonides

2.8

Belt

Grampian Highlands

Northern Highlands

East Shetland Basin

East Shetland Platform

Lerwick 2.67 2.67

WBF

BALTICA

Observed Free-Air

2.79

2.81

Figure 8.3a: High-pass 70 km image of Bouguer Gravity with location of gravity models, Line 1 and Line 2, overlain with basement terrane boundaries (yellow). SEEBASE AOI is the dotted black polygon; CB = Caledonides Belt; GH = Grampian Highlands Terrane: NH = Northern Highlands Terrane; F-S = Faroe-Shetland Terrane; H = Hebrides Terrane.

2.68

LAURENTIA

Km 0

2.5

3.30

Modelled Gravity

North Viking Graben

2.90

2.8

2.9

BALTICA

East Shetland Basin

Unst Basin 2.35

SEEBASE Surface

FaroesShetland 2.70

Observed Free-Air

3/01-1

• Up to 5000 m of sediments have been interpreted in several depocentres on the platform, with potentially 1-2 km of preserved Devonian sediments.

• Various mismatches between observed Free-Air gravity and modelled gravity in both model lines can be resolved by introducing higher and lower density tabular bodies accordingly, simulating intrusives.

3/04a-M12

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

Basement Topography and Geology

2.58 2.65

2.65

SEEBASE Surface

Grampian Highlands

2.72

2.65

2.6 2.84

E Viking Graben

3/29-1 3/29-2

2.35

2.1

Caledonides 2.74 Caledonides Belt Belt

Possible Baltican Lower Crust 2.90

Model 2

40 Figure 8.3b,c:

3.30

b) Line 1 and c) Line 2 showing a good match between observed Free-Air gravity (black signal) and modelled gravity (red signal). Blue and purple polygons are modelled bodies of a lower and higher density than basement respectively. The vertical red lines show well locations and depth. Thick black lines show interpreted basement terrane boundaries. Abbreviations: MTFZ - Møre-Trondelag Fault Zone; WBF – Walls Boundary Fault.

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VIII. DISCUSSION

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

Paleozoic Petroleum Systems in the Shetland Platform The SEEBASE depth-to-basement model (Section V) and tectono-stratigraphic analysis (Section IV) can be used to predict the location of potential petroleum systems on a first-pass regional scale, based on structural and basin analogues. In particular, the SEEBASE depicts the basement surface, which has previously been poorly imaged on legacy seismic datasets. Paleozoic sediments have also been difficult to image below bright Tertiary and Mesozoic reflectors in seismic. • Numerous studies have suggested the existence (or continuation) of a viable Devonian petroleum system on the East Shetland Platform to the north of the highly productive Moray Firth Basin (MF; Figure 8.4), i.e. within the broader Orcadian Basin. Prospective locations include the East Orkney Basin (EOB), Dutch Bank Basin (DBB) and Crawford-Skipper Basin (CSB) (e.g. Duncan and Buxton, 1995; Richardson et al., 2005; Trice, 2014; Patruno and Reid, 2016a; 2017a).

N Hydrocarbons may have spilled from the West Fair Isle and Unst basins due to proximity to a terrane boundary and major fault zones, as well as Mesozoic and/or Cenozoic tilting and uplift.

S U

basement depth of at least 4,000 – 4,500 m; structural geometry featuring tilted fault blocks and half-graben; and NW-SE trending depocentres controlled by NW-SE trending sigmoidal faults, cross-cut and/or linked by major ~N-S structures. • These basins are identified to contain a viable Devonian petroleum system (Duncan and Buxton, 1995; Richardson et al., 2005; Patruno and Reid, 2017a). Based on similar structural characteristics, localised depocentres in the East Fair Isle Basin (EFI), interpreted to be deeper than the abovementioned basins, may also contain a similar potential source rock. More well and seismic calibration data is required to better constrain depth-to-basement here, as the gravity signal in this area may have a significant contribution from basement composition.

ESB

WFI

• The SEEBASE depth-to-basement model indicates that the East Orkney, Dutch Bank and Crawford-Skipper basins are broadly characterised by similar basement structure and topography, including:

a) b) c)

ESPB

EOB MF

EFI DBB

CSB

• While the West Fair Isle and Unst basins are interpreted to have thick sediments present (> 5000 m), they are situated directly above or alongside terrane boundaries that act as major faults. There is potential for the development of even deeper depocentres than interpreted as well as multiple trapping mechanisms. However there is also the geological risk of breach of traps as a result of reactivation along these very structures. • The SEEBASE interpretation has extended the potential northwest extent of the Crawford-Skipper Basin to the north of the Dutch Bank Basin (Figure 8.4). The full Paleozoic-Mesozoic stratigraphic succession in the depocentre identified by Patruno and Reid (2017a) may also exist in the northwest continuation of the basin (refer to Figure 8.5 on the following page). Their identified depocentre is within the deepest section of the basin (based on the SEEBASE). Depocentres in the northwestern part of the basin may not contain the same maximum thickness of sediments. • The Lerwick Basin and East Shetland Platform Basin may also contain Devonian sediments (identified as Devonian depocentres in Fossen, 2010). However the sediments are too thin and too shallow, with no effective seal, to be considered as prospective source rocks. Exploration activities in the Northern North Sea and Witch Ground Graben are considered to be mature due to exploration successes in plays relying on the primary Jurassic Kimmeridge Clay source rock. An alternative deeper source rock is the Devonian lacustrine sediments within the broader Orcadian Basin. The Shetland Platform area remains a frontier region for exploration, with many undrilled opportunities on basement structural highs and depocentres interpreted in the SEEBASE depth-to-basement model. Frogtech Geoscience Post: PO Box 250, Deakin West ACT 2600, AUSTRALIA Office: Suite 17F, Level 1, 2 King Street, Deakin West ACT 2600, AUSTRALIA T: +61 02 6283 4800, E: info@frogtech.com.au W: frogtechgeoscience.com.au

Depth msl (m) 3000 The East Fair Isle Basin is part of a series of NW-oriented basins with over 4000 m of sediment. Viable Paleozoic petroleum systems may be present in localised depocentres. Its continuity along trend of known discoveries is also shown in Figure 8.5.

500 -2500 -5000 Patruno and Reid (2017a) identified a PaleozoicMesozoic depocentre of the Crawford-Skipper Basin. The SEEBASE depicts the potential areal extent of the basin (refer also to Figure 8.5). It is deepest on the flank of the Viking Graben.

-7500 -9500

Figure 8.4: Oblique 3D SEEBASE image showing terrane boundaries (dotted yellow lines) and basin outlines (white). Abbreviations: CSB – Crawford-Skipper Basin; DBB – Dutch Bank Basin; EFI – East Fair Isle Basin; EOB – East Orkney Basin; ESB – East Shetland Basin; ESPB – East Shetland Platform Basin; L – Lerwick Basin; MF – Moray Firth Basin; S – Sandwick Basin; U – Unst Basin; WFI – West Fair Isle Basin

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Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

Frontier Regions for Exploration Recent studies by Patruno and Reid (2016a, 2017a) present results of recently acquired and interpreted seismic imaging. A key feature of their interpretation depicts Devonian syn-rift wedges in between block-faulted basement on the flanks of the Viking Graben, as well as the location of Mesozoic-Cenozoic discoveries above (and potentially partially sourced by) these thick Devonian depocentres. The SEEBASE is an effective tool to extend preliminary assessments of both the basins and petroleum systems on the East Shetland Platform. Note the position of the fields relative to the basement surface is distorted by the 3D view.

N Based on extrapolation of structures in the SEEBASE, the East Fair Isle Basin that is partly controlled by the Tornquist Line may contain similar Devonian and PermoCarboniferous depocentres above collapsed basement highs linked to the Hood Field (see annotation for the Hood Field below). Seismic interpretations also indicate the presence of a deep Permian Zechstein seal above the Devonian (Richardson et al., 2005).

Potential Devonian depocentres are possible further westwards within the East Shetland Platform Basin (ESPB).

Interpretation of similar depocentres in Frogtech Geoscience’s revised outline of the Crawford-Skipper Basin (CSB) containing potentially thick Devonian syn-rift wedge

ESPB Kraken

EFI CSB

Mariner

DBB The Dutch Bank Basin is interpreted in this study to have evolved in a similar tectonic style to the East Fair Isle Basin (see above), controlled by the Tornquist Line and bound by N-S structures. Devonian potential source intervals have been previously identified here, making it a frontier basin for exploration.

Devonian depocentre identified from seismic structurally below the Kraken Field (Patruno and Reid, 2017a). The SEEBASE shows that this Devonian depocentre is located in the deepest part of the East Shetland Platform Basin (ESPB). An en-echelon counterpart is predicted to its south in the SEEBASE, which may contain a thick Devonian succession as well.

The Mariner Field is located above topographically uneven basement shaped by Devonian transtensional faults (Patruno and Reid, 2017a). The SEEBASE shows the structure of the basement ridge below the Mariner field continuing towards the northwest and southeast.

FGS

Crawford

Geometry of Paleozoic stratigraphy below the Hood Field is shown in Patruno and Reid (2017a; location of Hood Field from publication). We interpret the geometry as collapse of the Devonian basement high during strike-slip movement along the Tornquist Line, followed by rapid infilling of a thick Permo-Carboniferous sequence in the resulting accommodation space. Part of the Devonian basement is still preserved to the southeast.

Dotted yellow polygon shows approximate location of Crawford-Skipper Basin of Patruno and Reid (2017a). The white polygon shows the larger extent of the basin based on the SEEBASE model.

Hood

Depth msl (m) 3000 500 -2500

WGG

SVG

Up to 1.5 sec TWT of Devonian sediments identified from seismic (Patruno and Reid, 2017a) in this depocentre. The SEEBASE model illustrates that the depocentre is within a saddle to the south of the Fladen Ground Spur (FGS), linking the South Viking Graben (SVG) and the Witch Ground Graben (WGG). The saddle is bound to the east by a terrane boundary, which also influences the N-S location of Mesozoic-Cenozoic oilfields on the flank of the South Viking Graben.

-5000 -7500 -9500

Figure 8.5: Oblique 3D view of the southwest part of the SEEBASE showing basin outlines (white) and fields (green polygons are oilfields; blue are condensates; gas fields not visible in current extent). Abbreviations: CSB – Crawford-Skipper Basin; DBB – Dutch Bank Basin; EFI – East Fair Isle Basin; EOB – East Orkney Basin; ESPB – East Shetland Platform Basin. Note that the location of fields may appear offset as they are ‘floating’ above the SEEBASE in oblique view. Basin outlines are truncated at the AOI. Frogtech Geoscience Post: PO Box 250, Deakin West ACT 2600, AUSTRALIA Office: Suite 17F, Level 1, 2 King Street, Deakin West ACT 2600, AUSTRALIA T: +61 02 6283 4800, E: info@frogtech.com.au W: frogtechgeoscience.com.au

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VIII. DISCUSSION

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

Risks Geological Risks Various risks are expected to exist over a geologically complicated area such as in the East Shetland Platform and surrounding areas. Key geological risks with a wide influence across the region are summarised below. The SEEBASE model allows prediction of depocentres that may contain potential petroleum systems in several key areas, however risks associated with hydrocarbon migration and thermal maturity need to be properly assessed.

Geological features smaller than ~20 km cannot be resolved in satellite gravity data, whereas well data shows significant variation in stratigraphic depth.

• Many basins are located along major fault structures and terrane boundaries. Transtensional basement faults were reactivated as inversion structures during the Carboniferous, Cretaceous and Cenozoic. Basement and basin inversion are likely to have taken place in the intra-platform depocentres on the East Shetland Platform, with major implications for breach of trap or hydrocarbon accumulations. • Deep basins bounded by fault blocks are more likely to preserve the thick Devonian wedges that contain potential source intervals, such as the East Fair Isle and Dutch Bank basins. However proximity to multiple structures increases the risk of breach, while their greater depths may also lead to over-maturity of Devonian source rock. • Shallower intra-platform depocentres also have the potential to preserve thick Devonian wedges, such as within the deepest parts of the Crawford-Skipper Basin and East Shetland Platform Basin.

Sparse distribution of publicly available well information for the centre of the East Shetland Platform. Proprietary datasets may exist.

• These intra-platform depocentres are subject to both risks and benefits with increasing distance from the flanks of the Viking Graben: •

Increasing distance from the flanks of the Viking Graben increases the risk of thinner sedimentary packages and a smaller overburden, meaning immature Devonian source intervals.

•

However depocentres within the East Shetland Platform Basin and Fladen Ground Spur are cored by competent intruded basement blocks. Therefore, increasing distance from the Viking Graben reduces the risk of (structural) trap breach, as Mesozoic deformation is weaker on the platform.

Constraints on Interpretation This study is based on open-file data and data provided by the UK OGA. Other sources of calibration data would provide additional information and constraints to update the SEEBASE depth-to-basement model. • Well data is unequally distributed across the AOI, with most of the wells situated within the Viking Graben and Moray Firth. • In areas lacking well data, the depth of basins (i.e. sediment thickness) in the East Shetland Platform AOI was interpreted predominantly from potential field data and adjacent analogues. Depth of interpreted basement in published cross-sections varies significantly in any given area. In addition, basement pick and depth conversion from published legacy seismic varies between several authors.

Interpreted deep basement, however gravity response may partly be due to basement composition

• Some negative gravity anomalies interpreted as depocentres may at least partly reflect variations in basement composition, as well as basement depth. • Geological features smaller than ~20km cannot be resolved in the satellite gravity data. Therefore smallscale variations in basement depth suggested by well data cannot be interpreted from the available data. For example, on the western flank of the Viking Graben (210 and 211) and East Shetland Platform (quadrants 2 and 3) in the East Shetland Basin, well data shows significant variation in stratigraphic depth. It is possible that this will be reflected in the basement depth, but this interpretation cannot be assessed based on the available gravity data

Figure 8.6: High-pass 70 km of Bouguer gravity image showing distribution of publicly available well information (white dots) and UKCS quadrants (white polygons).

SEEBASE EAST SHETLAND PLATFORM

IX. CONCLUSIONS FROGTECH GEOSCIENCE

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IX. CONCLUSIONS

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

Conclusions The 21CXRM East Shetland Platform Project Phase 1 – SEEBASE Study provides a regional-scale geophysical and geological analysis of basement and basin evolution utilising public domain and UK OGA-supplied datasets. The study benefits from Frogtech Geoscience’s state-of-the-art processing of public domain gravity and magnetic data, including the generation of Frogtech Geoscience proprietary enhancements and a suite of standard filters. The study’s main objective was to develop a regional understanding of structural evolution, basement depth and geometry, and issues affecting petroleum prospectivity, in support of the 31st Licensing Round of the UK Oil and Gas Authority.

The resulting SEEBASE model of the depth and geometry of the basement surface (Figure 9.1) provides the first regional interpretation of basement across the Greater East Shetland Platform and provides significant new insights into the distribution of basins and depocentres. The SEEBASE structural model illustrates the potential for unrecognised depocentres in the under-explored East Shetland Platform region. It reduces the risk and challenges of interpretation in seismic and allows for targeted oil and gas exploration on the UK Continental Shelf. The structural model and tectonic overview of basement and basin evolution produced in this study provides the geological context for predicting the occurrence and distribution of potential petroleum system play elements. The study provides both a platform for rapid, cost-effective evaluation of petroleum prospectivity in the East Shetland Platform region, and a tool for focusing future exploration and data acquisition strategies to reduce exploration risks.

Depth msl (m) 3000 500 -2500 -5000 -7500 -9500

Figure 9.1: Oblique 3D view of the East Shetland Platform SEEBASE.

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IX. CONCLUSIONS

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

Conclusions The new SEEBASE depth-to-basement model developed for the Greater East Shetland Platform is an integrated result of multiple datasets using Frogtech Geoscience’s workflow. It provides a geologically-constrained, spatiallycontinuous illustration of basement surface for the East Shetland Platform AOI, and overcomes the issues of poor basement imaging on 2D seismic data. In conjunction with basement geology and structural analysis, the SEEBASE represents a significant step forward in understanding basement evolution and development of overlying basin systems in the project area.

The spatially continuous perspective of the SEEBASE basement topography highlights the trend of structural traps and depocentres on the East Shetland Platform, and provides a cost-effective foundation for analysis of petroleum systems and targeting prospects. Many of the geological features shown have not been drilled or tested and present significant new opportunities for exploration.

• The Greater East Shetland Platform is not a broad structural high as conventionally depicted. The SEEBASE shows the distribution of basement highs (fault blocks, horsts, anticlines) and Early Paleozoic en-echelon and pull-apart basins, and half-graben of varying depths on the platform.

• The location and geometry of basins on the platform are controlled by the complex interaction of the lower crust of the Baltica and Laurentian megaterranes, particularly below the Grampian Highlands, and also by basement terrane boundaries and major faults at the basement surface. • While the basement terranes are broadly organised in a NE-SW to N-S trend in the Area of Interest, the internal structural grain of the terranes has strongly influenced basin geometries. The NE-SW trend of basins in the western part of the AOI, reflects the structural fabric of the underlying Hebrides and Northern Highlands terranes, while the NW-SE structural grain of the Grampian Highlands Terrane has clearly influenced the trend of basins in the southern part of the AOI. Towards the northern part of the AOI, basin geometry is controlled by the N-S and NW-SE edge of the Baltica lower crust.

East Shetland Platform

ESPB KH

EFI

DBB

• Crustal-scale gravity models demonstrate that the gravity response of the East Shetland Platform is controlled by variation in basement composition, while the gravity response over the flanks of the Faroes-Shetland Basin and Viking Graben is influenced by Mesozoic-Cenozoic sediment thickness and Moho depth and topography.

CSB FGS

• Fault maps derived from structural and kinematic analysis of the basement’s response to successive regional tectonic events show the interpreted distribution of active faulting during each event and define the zones of rheological variation that controlled the formation of intra-platform basin depocentres and highs. For example: • Shallow basins (such as the East Shetland Platform Basin (ESPB) and Lerwick Basin (L); Figure 9.2) and sub-aerial highs (such as the Fladen Ground Spur (FGS) and Kraken High (KH)) are cored by numerous intrusives. The intrusives increased the rheological competence of the crust and resulted in the persistence of shallow basement in these parts of the East Shetland Platform.

WGG

• Deeper intra-platformal basins such as the Crawford-Skipper Basin (CSB) and Dutch Bank Basin (DBB) developed in rheologically less competent zones.

Figure 9.2: Oblique 3D close up view showing part of the SEEBASE, with basin outlines (white polygons) and field locations (green and blue)

• Due to the interpreted presence of numerous heat productive granites within and along the Platform edges , modelled heat flow values are relatively high. Those competent intrusions also support the preservation of basins during tectonic inversion events. • The SEEBASE, underpinned by structural analysis, can be used to identify prospective areas in frontier basins, based on similarities in tectonic style to known analogues. Examples include: • Interpreted Devonian syn-rift wedges in intra-platform depocentres, within the revised extent of basins such as the Crawford-Skipper Basin (CSB), and East Shetland Platform Basin (ESPB).

Recommendations

• Thick Devonian or Permo-Triassic depocentres formed by transtension and re-working of basement structures, such as within the East Fair Isle Basin (EFI) and Dutch Bank Basin (DBB). These structures are along-strike of discoveries in the Witch Ground Graben (WGG), and which are in proximity to or affected directly by the Tornquist Line.

The SEEBASE developed in this study is a regional, 1: 2M scale model. More detailed assessments of individual basins can be achieved with the integration of higher resolution geophysical datasets and/or additional calibration data. Frogtech Geoscience recommends the integration of proprietary data such as well formation tops and better quality seismic, which will significantly improve the SEEBASE model, as well as the interpretation of the thermal history of the different basins.

• Potential preservation of intra-platformal Early Paleozoic depocentres within sigmoidal fault zones during inversion events, such as within the East Fair Isle Basin, Crawford-Skipper Basin and East Orkney Basin.

Frogtech Geoscience’s unique services, geological expertise and data management capabilities can be contracted by the client for further enhancements and improvements to the SEEBASE Study of the East Shetland Platform.

SEEBASE EAST SHETLAND PLATFORM

X. REFERENCES FROGTECH GEOSCIENCE

Platform Project Phase 1 – SEEBASE Study. Frogtech Pty Ltd, Canberra, Australia.

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