SEEBASE EAST SHETLAND PLATFORM
VII. HEAT FLOW
FROGTECH GEOSCIENCE
83
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.
a
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
HF
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
b
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
0
c
Gravity Magnetics
0
③ Depth (m)
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
PRODUCT CODE: UK703 © All Rights Reserved Frogtech Pty Ltd (Australia) no part of this document may be reproduced without the express written permission of Frogtech Geoscience.
Importance of a Basement Heat Flow Model
DEM
④
0
① ②
Moho Basement Composition Intrusive Felsic intermediate intrusive
Metasediment/extrusive Metamorphic
Intermediate Intrusive
High/ Ultra-High Pressure Unit
Metasediment
Serpentinite
Gneiss
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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.
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
PRODUCT CODE: UK703 © All Rights Reserved Frogtech Pty Ltd (Australia) no part of this document may be reproduced without the express written permission of Frogtech Geoscience.
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
b
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.
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
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
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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.
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
PRODUCT CODE: UK703 © All Rights Reserved Frogtech Pty Ltd (Australia) no part of this document may be reproduced without the express written permission of Frogtech Geoscience.
Calibration Dataset Compilation
No published data
Published Heat Flow Values (mW/m2) 45 60
No data area Figure 7.3:
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
Published heat flow data contours with DEM in the background. After Burley (1993).
86
VII. HEAT FLOW
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
PRODUCT CODE: UK703 © All Rights Reserved Frogtech Pty Ltd (Australia) no part of this document may be reproduced without the express written permission of Frogtech Geoscience.
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:
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
Examples of gravity response for some lithologies that occur in outcrop or are intersected in wells, on a 100km high-pass Bouguer grid.
87
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.
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
PRODUCT CODE: UK703 © All Rights Reserved Frogtech Pty Ltd (Australia) no part of this document may be reproduced without the express written permission of Frogtech Geoscience.
Component I: Radiogenic Heat Flow – Basement Composition
Gneiss E
E
E
E Intrusive
E
E
E
E
E
E
E
E Mafic
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
intrusive
BBBBMetamorphic BBBB Quartzite Ultramafic intrusive
East Grampian Batholith NE extension?
Figure 7.5:
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
MF
Interpreted basement composition
Grampian Highlands
88
VII. HEAT FLOW
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
PRODUCT CODE: UK703 © All Rights Reserved Frogtech Pty Ltd (Australia) no part of this document may be reproduced without the express written permission of Frogtech Geoscience.
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.
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
89
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.
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
PRODUCT CODE: UK703 © All Rights Reserved Frogtech Pty Ltd (Australia) no part of this document may be reproduced without the express written permission of Frogtech Geoscience.
Component I: Radiogenic Heat Flow – Radiogenic Heat Production
b
Dunite 1.1
Peridotite Diorite
1
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)
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
< 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.
90
VII. HEAT FLOW
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
PRODUCT CODE: UK703 © All Rights Reserved Frogtech Pty Ltd (Australia) no part of this document may be reproduced without the express written permission of Frogtech Geoscience.
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.
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
Fladen Ground Spur
Crawford Skipper Basin
South Viking Graben
91
VII. HEAT FLOW
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
PRODUCT CODE: UK703 © All Rights Reserved Frogtech Pty Ltd (Australia) no part of this document may be reproduced without the express written permission of Frogtech Geoscience.
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.
a
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
34
265
2
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.
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Figure 7.11: Radiogenic heat flow model of the crust.
Fladen Ground Spur
Crawford Skipper Basin
South Viking Graben
92
VII. HEAT FLOW
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
PRODUCT CODE: UK703 © All Rights Reserved Frogtech Pty Ltd (Australia) no part of this document may be reproduced without the express written permission of Frogtech Geoscience.
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.
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NVG
CSB DBB Fladen Ground Spur
PR170_betafactor_g.img Basement Stretching Factor ß
Value
113.464
2
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
93
VII. HEAT FLOW
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
PRODUCT CODE: UK703 © All Rights Reserved Frogtech Pty Ltd (Australia) no part of this document may be reproduced without the express written permission of Frogtech Geoscience.
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.
94
VII. HEAT FLOW
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
PRODUCT CODE: UK703 © All Rights Reserved Frogtech Pty Ltd (Australia) no part of this document may be reproduced without the express written permission of Frogtech Geoscience.
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
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Figure 7.15: Calculated model of present-day basement heat flow.
Crawford Skipper Basin Fladen Ground Spur South Viking Graben