Boletin_LXII_No_01_ene-jun2020

BOLETÍN DE LA ASOCIACIÓN MEXICANA DE GEÓLOGOS PETROLEROS, A.C. VOLUMEN LXII

N

NÚMERO 1

ENERO-JUNIO 2020

1300

El Madrono

1200

00 16

1400

1100

1300

La Anacahuita

1200

10 00 90

1100

0

El Huizachal 1100

‘

1000

1200

0

La Joya Verde Cementerio

1000

Aguas de Minas

100 0 1200

00 14

130

13 00

1400

12

00

1100 120 0

1300

Explanation Undifferentiated Cretaceous strata (Ku)

La Casita Formation (Jc)

Olvido Formation (Jo)

Novillo Formation (Jn)

La Joya Formation (Jlj)

Rhyollite (Ji)

La Boca Formation upper member (Jlbs)

La Boca Formation lower member (Jlbi)

Symbols

Geologic contact Inferred geological contact

Locality School

Coordinate system: UTM Topografic base map from INEGI: CiudadVioctoria F-14-A-20

INSTRUCCIONES A LOS AUTORES El Boletín de la Asociación Mexicana de Geólogos Petroleros es una publicación semestral dedicada a la divulgación de artículos sobre geología, especialmente aquellos relacionados con la aplicación de las geociencias a la industria petrolera.

Los trabajos factibles a publicarse se pueden dividir en tres tipos principales: Reportes de investigación originales, no publicados con anterioridad, que no excedan de 25 cuartillas.

Notas técnicas originales que no excedan de 10 cuartillas.

Notas técnicas de divulgación que no exceden de 10 cuartillas.

Los trabajos deberán enviarse a:

COMISIÓN DE ESTUDIOS TÉCNICOS Ernesto Cabalero García ecaballerog_2006@yahoo.com.mx Leonardo Enrique Aguilera Gómez leonardo.enrique.aguilera@pemex.com Dionisio Figueroa Rodríguez nicho072547@yahoo.com

En caso de que el trabajo sea aceptado, la Comisión de Estudios Técnicos turnará el mismo a la Comisión Editorial, quien se encargará de su preparación y adecuación para su publicación.

COMISIÓN EDITORIAL

Chamaly Revelez Ramírez Jorge Antonio Velasco Segura

Editor Técnico

Sharon Carolina Carrillo Zoto

Ilustración de portada: Figure 5: Generalized modified geologic map of Valle de Huizachal (Rubio-Cisneros and Lawton, 2011). Stars and polygons indicate the units studied in this work, and correlate to those formations explained in Figure 4. Map symbols: Jlbi—lower member of La Boca Formation; Jlbs—upper member of La Boca Formation; Ji—rhyolite domes; Jlj—La Joya Formation; Jn—Novillo Formation; Jo—Olvido Formation; Jc—La Casita Formation; Ku—undifferentiated Cretaceous strata.

BOLETÍN DE LA ASOCIACIÓN MEXICANA DE GEÓLOGOS PETROLEROS, A.C.

Directiva Nacional (Bienio 2018-2020) Dr. Faustino Monroy Santiago Presidente

Ing. María de Lourdes Clara Valdés Ing. Josías Samuel Estrada Macías Vicepresidenta Tesorero M. en G. Daniela Romero Rico Dra. Sandra Ortega Lucach Secretaria Protesorera Ing. Sergio Arturo Ruíz Chico Ing. Juan Medina de la Paz Coordinador de Ayuda Mutua Subcoordinador de Ayuda Mutua Ing. Chamaly Revelez Ramírez Biol. Judith Rosales Lomelí Ing. Jorge Antonio Velasco Segura Comisión de Membresía Comisión Editorial Ing. Román Sánchez Martínez Ing. Leonardo Enrique Aguilera Gómez Ing. Mitzi Berenice Benítez Canchola Ing. Ernesto Caballero García Comisión de Excursiones Ing. Dionisio Figueroa Rodríguez Ing. Jaime Patiño Ruíz Comisión de Estudios Técnicos Comisión Legislativa

Dr. Efraín Méndez Hernández Ing. Luis Francisco Fuentes Pacheco Comisión de Asuntos Internacionales Comisión evento del 70th aniversario

Boletín de la

Asociación Mexicana de Geólogos Petroleros volumen lxii

número

1

enero

-

junio

2020

CONTENIDO Editorial A collection of sandstones to discriminate types of sedimentary basins analyzing detrital modes: a case study in northeastern Mexico Igor Ishi Rubio-Cisneros and Yam Zul Ernesto Ocampo-Díaz Analysis of mineralogy and porosity on a carbonaceous mudstone of the Pimienta Formation, western margin of the Tampico Misantla Basin, Mexico Análisis de mineralogía y porosidad en mudstone carbonáceo de la formación Pimienta, en el margen centro-oeste de la Cuenca Tampico Misantla, México Carlos Vega-Ortiz , Bryony Richards, John D. McLennan, Raymond Levey, Néstor Martínez-Romero

Editorial Como sabemos el análisis petrográfico y mineralógico de rocas, bajo el microscopio petrográfico y electrónico de barrido, ha sido una disciplina que proporciona al geólogo petrolero evidencias directas de la calidad de rocas generadoras, almacenadoras y sellos, además de proporcionarnos datos de las características intrínsecas de estas rocas para interpretar sus orígenes. El geólogo que realiza un estudio petrográfico requiere del conocimiento profundo de varias ciencias, entre ellas, cristalografía, diagénesis, sedimentología, paleontología, geoquímica, etc., para poder interpretar lo que está observando. Varios de nuestros asociados tienen una amplia experiencia en esta disciplina, pero cada día es menos común entre los jóvenes profesionistas. Aquellos que dominan esta disciplina podrán atestiguar lo apasionante que es el estudio de las rocas a esta escala, que últimamente también se ha usado para detallar cuestiones tan especializadas como la determinación de características o atributos de fracturas naturales de los yacimientos más importantes de nuestro país.

En este Boletín de la AMGP, se incluyen dos trabajos técnicos: el primero es una investigación e integración de una base de datos petrográficos y de clasificación y madurez textural de areniscas de varias áreas, con el objetivo de buscar una firma que identifique y caracterice a un tipo de cuenca en específico. Además, propone un método, que junto con la estadística podría determinar, según los autores, la proveniencia y tipo de cuenca de los componentes que conforman estas rocas. Como ejemplo, los autores analizaron las areniscas de cuatro formaciones del noreste de México, concluyendo que los componentes provienen de cuencas de tipo rift, cuenca tras arco extensional y strike-slip. Este es un trabajo que principalmente tiene una aplicación directa en el estudio de plays terciarios productores del país.

El segundo trabajo de este Boletín, es un ejemplo de un análisis mineralógico y de porosidad en carbonatos de la Fm. Pimienta, de la Cuenca Tampico Misantla, enfocada a evaluar el potencial no convencional de esta formación. El objetivo principal, de acuerdo a los autores, es elaborar una descripción detallada a nivel micro y nano escala para comprender las implicaciones, de las propiedades petrofísicas y de la distribución de porosidad, en la capacidad de almacenamiento de hidrocarburos, el flujo de fluidos y la permeabilidad de esta formación. Con base a 12 muestras de siete pozos, usando microscopia de barrido y pruebas de difracción de rayos x, muestran un análisis detallado de la composición química, morfológica, tipo de materia orgánica y distribución de porosidad. Basado en los resultados de este estudio, los autores determinan que la Fm. Pimienta es análoga a las formaciones Eagle Ford y Bakken de Norte América, sin embargo, concluyen que, si bien el área analizada no tiene propiedades óptimas, es posible encontrar zonas estructurales en donde las propiedades geoquímicas y petrofísicas puedan conformar un yacimiento no convencional con potencial comercial. A pesar de esto, este trabajo es sin duda un aporte de datos importantes en el estudio de plays no convencionales en nuestro país y enriquece el conocimiento que tenemos de este tipo de yacimientos.

Atentamente Dr. Faustino Monroy S.

A collection of sandstones to discriminate types of sedimentary basins analyzing detrital modes: a case study in northeastern Mexico Igor Ishi Rubio-Cisneros (1) and Yam Zul Ernesto Ocampo-Díaz (1, 2) (1)

Grupo De Geología Exógeno y Del Sedimentario, freelance. igor_rubio@yahoo.com

(2)

Área de Ciencias de la Tierra, Facultad de Ingeniería, Universidad Autónoma de San Luis Potosí, Av. Manuel Nava No. 8, San Luis Potosí, San Luis Potosí. CP. 78290

Submitted to Boletín de la Asociación Mexicana de Geólogos Petroleros, February 2018.

ABSTRACT

This work develops a statistical bivariate analysis and a discriminant analysis on 1411 sandstones from nine types of sedimentary basins. The study rates the variations of framework components and differentiates the modal composition for each basin by partitioning the data into discriminant fields in a ternary plot. The discriminant analysis of the full sedimentary framework petrography indices improves the use of ternary subcompositions (Qm, Qp, P, K, Lv, Ls, Lm), and provides additional support for sandstone provenance analysis.

Petrographic indices in basins related to subduction represent the processes of major newly formed sourceareas like intra-oceanic arcs and continental-margin arcs. Indices in rift, foreland, and accretionary basins indicate sedimentary processes like particle sorting, textural maturity, and the exhumation/unroofing of the sedimentary carapace. In contrast, the indices strike-slip basins hold a mixture of the processes previously described, but with significant input from crystalline and metamorphic rocks.

The analyses with statistical and discriminant methods are for interpreting which basins occur where some sandstones deposited in the Sierra Madre Oriental northeastern Mexico. This exploratory method is for Upper Triassic–Lower Cretaceous sandstones selected from four different formations: El Alamar, La Boca, La Joya, and La Casita. The framework composition in sandstones discriminates among the types of basins in a ternary plot. The analysis indicates a rift, forearc, and strike-slip basins, respectively, to the age of the formations from Late Triassic, Early–Middle Jurassic, Middle Jurassic, and Late Jurassic to Early Cretaceous.

Keywords: sandstone petrography, discriminant analysis, provenance, tectonic setting, sedimentary basins, source-rock

RESUMEN

Este trabajo desarrolla un análisis estadístico bivariante y un análisis discriminante en 1411 areniscas de nueve tipos de cuencas sedimentarias. EL estudio evalúa las variaciones de los componentes principales y diferencia la composición modal para cada cuenca, mediante la partición de los datos en campos discriminantes en un diagrama ternario. El análisis discriminante del total de los índices en la petrografía de los componentes BOLETÍN DE LA ASOCIACIÓN MEXICANA DE GEÓLOGOS PETROLEROS

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principales sedimentarios mejora el uso de las subcomposiciones ternarias para la procedencia (Qm, Qp, P, K, Lv, Ls, Lm) y provee apoyo adicional en el análisis de procedencia de areniscas.

Los índices petrográficos en cuencas relacionadas con la subducción representan los procesos de áreas fuente recién formadas, como arcos intra-oceánicos y arcos de margen continental. Los índices en las cuencas de rift, foreland y acreción indican procesos sedimentarios como la clasificación de partículas, madurez textural y exhumación de la cubierta sedimentaria. Contrastantemente, los índices en las cuencas strike-slip tienen una mezcla de los procesos descritos anteriormente, pero con un aporte significativo de rocas cristalinas y metamórficas.

• VOL. LXII No. 1 • ENERO-JUNIO 2020 •

Los análisis con métodos estadísticos y discriminantes son para interpretar cuáles cuencas existen para un depósito de areniscas en la Sierra Madre Oriental, noreste de México. Este método exploratorio es para las areniscas del Triásico Superior-Cretácico Inferior, seleccionadas de cuatro formaciones diferentes: El Alamar, La Boca, La Joya, and La Casita. La composición de las areniscas se discrimina entre los tipos de cuencas en el diagrama ternario. El análisis discrimina cuencas de tipo rift, cuenca tras arco extensional y strike-slip, respectivamente, a la edad de las formaciones del Triásico Tardío, Jurásico Temprano-Medio, Jurásico Medio y Jurásico Tardío al Cretácico Temprano.

Palabras clave: petrografía sedimentaria, análisis discriminante, procedencia, ambiente tectónico, cuencas sedimentarias, áreas fuente.

1. INTRODUCTION–PROVENANCE This work uses petrographic data sets from previous works for solving multivariate and discriminant analyses.

The goal is to correlate the compositional characteristics in sandstones among the types of basins where sediment deposits, for either recent or ancient settings. Also, this work restricts to link between types of basins and framework composition in clastic sediments.

The modal composition of siliciclastic sediments reflects factors such as weathering, climate, relief, sourcerock composition, and tectonic setting (Johnsson, 1993). Quantitative studies of lithic fragments are the best way to distinguish source rocks (Garzanti, 2010, personal communication). A compositional variability in sandstones can be shared in different types of depocenters by plate-tectonic interaction (e.g., Busby and Ingersoll, 1995). Sandstone composition may prove exhumation and unroofing of rocks, or the formation of new source areas by magmatic processes (Ocampo-Díaz and Rubio-Cisneros, 2013).

Tectonics studies the erosion of orogenic source areas using petrographic and mineralogical data (Crook, 1974; Dickinson et al., 1983; Critelli et al., 1995; Garzanti et al., 2004; 2010). Modal composition and petrofacies in sandstones are sensitive to changes in source-areas for explaining depositional sequences (Ingersoll, 1983; Arribas et al., 2003).

Some publications have contributed to studies using sandstone composition for classifying tectonic settings in ternary diagrams. (e.g., Dickinson and Suczek, 1979; Ingersoll and Suczek, 1979). Meanwhile, other works study some constituents in sandstones and their type of basin in tectonic settings (e.g., Ingersoll and Suczek, 1979; Marsaglia and Ingersoll, 1992; Garzanti et al., 2003). Other authors calculate an empirical and statistical Sedimentary Recycling Index (SeReIn) for detecting compositional variability in detrital modes, an indicator of facies (e.g., storm deposits versus sand flat), grain size (e.g., tidal channels versus mixed flat), 6

or intra-formational recycling called “cannibalism” versus formation of unconformities (Ocampo-Díaz and Rubio-Cisneros, 2013). However, provenance analyses have not reached a consensus to correlate the types of known sedimentary basins with the composition of detrital constituents in sandstones.

Although this work does is not intended for petroleum purposes or geological-economic implications, it explains petrographic characteristics of formations comparable to reservoirs of siliciclastic rocks, with applications to sedimentary basins in the oil and gas industry.

The tectonic setting classification in this work is according to the QmFL diagram from Dickinson et al. (1983). The characteristics of basins and their classification belong to Ingersoll and Busby (1995) and Miall (1996). To avoid confusion about the terminology of basins and tectonics, we adopted those described in Busby and Ingersoll (1995) and Busby and Azor (2011).

1.1 INTRODUCCIÓN - PROCEDENCIA Este trabajo utiliza conjuntos de datos petrográficos en trabajos previos para calcular análisis multivariados y discriminantes. El objetivo es correlacionar las características de composición en las areniscas entre los tipos de cuencas donde se depositan sedimentos, ya sea para ambientes recientes o antiguos. También, este trabajo se restringe a vincular los tipos de cuencas y la composición de sedimentos clásticos.

La composición modal de los sedimentos siliciclásticos refleja los factores como el intemperismo, clima, relieve, la composición de la roca fuente y el ambiente tectónico (Johnsson, 1993). Los estudios cuantitativos de fragmentos líticos son la mejor manera para distinguir las rocas fuente (Garzanti, 2010, comunicación personal). La variabilidad composicional en las areniscas puede compartirse en diferentes tipos de cuencas mediante la interacción de las placas tectónicas (p.ej., Busby e Ingersoll, 1995). La composición de areniscas puede comprobar la exhumación y erosión de rocas, o la formación de áreas fuente por procesos magmáticos (Ocampo-Díaz y Rubio-Cisneros, 2013).

La tectónica estudia la erosión de las áreas fuente orogénicas utilizando datos petrográficos y mineralógicos (p.ej., Crook, 1974; Dickinson et al., 1983; Critelli et al., 1995; Garzanti et al., 2004; 2010). La composición modal y las petrofacies en areniscas son sensibles a los cambios en las áreas fuente para explicar las secuencias del depósito (Ingersoll, 1983; Arribas et al., 2003).

Algunas publicaciones han contribuido al estudio usando la composición de las areniscas para clasificar los ambientes tectónicos en diagramas ternarios (p.ej., Dickinson y Suczek, 1979; Ingersoll y Suczek, 1979). Mientras tanto, otros trabajos estudian algunos componentes en las areniscas y su tipo de cuenca en ambientes tectónicos (p.ej., Ingersoll y Suczek, 1979; Marsaglia e Ingersoll, 1992; Garzanti et al., 2003). Otros autores calcularon un índice de reciclaje sedimentario (SeReIn) empírico y estadístico para detectar variabilidad en las composiciones de las modas detríticas, un indicador de facies (p.ej., depósitos de tormenta contra planicie de lodos), tamaño de grano (p.ej., canales de marea contra planicies mareales), o reciclaje intraformacional llamado “canibalismo” contra la formación

de discordancias (Ocampo-Díaz y Rubio-Cisneros, 2013). Sin embargo, los análisis de procedencia no han alcanzado un consenso para correlacionar el tipo de cuencas sedimentarias conocidas con la composición de los constituyentes detríticos en areniscas.

Aunque este trabajo no está destinado a fines petroleros o implicaciones geológico-económicas, explica las características petrográficas de formaciones comparables con yacimientos de rocas siliciclásticas, de aplicación a cuencas sedimentarias en la industria del gas y petróleo.

La clasificación del ambiente tectónico en este trabajo está de acuerdo con el diagrama QmFL de Dickinson et al. (1983). Las características de cuencas y su clasificación pertenecen a Ingersoll y Busby (1995) y Miall (1996). Para evitar la confusión en la terminología de cuencas y tectónica, hemos adoptado los descritos en Busby e Ingersoll (1995) y Busby y Azor (2011).

2. MATERIALS AND METHODS The parameters used in here are some of the classic sandstone constituents such as i) Qt (total quartz), Qm (monocrystalline quartz), and Qp (polycrystalline quartz) that indicate transport and textural maturity; ii) Lm (metamorphic lithic fragments), Lv (volcanic lithic grains), K-feldspar, and P (plagioclase) – for the nature and composition of source rocks; and iii) Ls (sedimentary lithic grains) –relates to recycling. The petrographic parameters and their corresponding tectonic settings in this work were gathered from various publications, as described in Table 1. All the values represent the main tectonic settings, including those proposed as provenance-types from Dickinson and Suczek (1979) and Dickinson et al. (1983). All basins fall into two main categories of tectonics settings, extensional basins and non-subductable; for example, foreland basins, surrounding arc massive basins (e.g., forearc basins), and strike-slip basins (s.s., Busby and Ingersoll, 1995). The selected compositional parameters have restrictions based on the Sedimentary Recycling Index (SeReIn; Ocampo-Díaz and Rubio-Cisneros, 2013).

Category

Definition

Qt

total quartz

Qm

monocrystalline quartz

Qp

polycrystalline quartz

F

potassium feldespar

P

plagioclase feldespar

Lv

volcanic lithic

Lm

metamorphic lithic fragment

Ls

sedimentary lithic fragment

Table 1. Principal detrital group components following the SeReIn method proposed by Rubio-Cisneros and Ocampo-Díaz (2014) and Ocampo-Díaz and Rubio-Cisneros (2013).

All data is set to center log-ratio transformation (clrlog) and isometric log-ratio transformation (ilr-log). All indices adopt the ln- prefix to scale the compositional information in a simplex of Euclidean characteristics.

The methods to convert compositional data applying log-ratio transformations “clr-log” and isometric logratio transformations “ilr-log” are in Aitchison (1986, 1992), Barceló et al. (1996), and Ohta and Arai (2007).

We used the methods from Swan and Sandilands, (1995), Agrawal (1999) and Agrawal et al. (2004) for testing predictor variables in a canonical discriminant analysis. Wilk’s lambda method helped us to select the variables to reduce the dataset.

We run the “Statistica-Statsoft/PC+” statistical software to analyze the discrimination of samples.

Every canonical discriminant score arranges in cases with a normal distribution. The analyses for discrimination examine each group of samples at a time, calculating all possible combinations of groups.

The discriminant analysis determines which variables classify among the dataset to group together in basins. Then, all orthogonal components are recalculated on a composition to solve the discriminant scores and fit the data in a ternary diagram using a closure operation (Ohta and Arai, 2007). Each tectonic group of samples comes after solving its discriminant score D, to then classify a sample into a given compositional tectonic setting.

Samples not corresponding to any compositional field are discarded from the analysis and do not qualify for any specific type of basin. We consider that at each discrete point in the stratigraphy, the composition of the basin is not limited by time, undertaking instead lateral variability in the stratigraphy driven by autogenic processes. Therefore, samples not corresponding to any compositional field at every time in the evolution of the crust fit neither basin nor the “hybrid” category.

3. PREVIOUS WORKS AND RESULTS: COMPOSITIONAL ANALYSIS (QFL) FOR BASIN DISCRIMINATION We compiled 1411 samples of source rock composition from sandstones reported on nine basins: (I) Accretionary n=85 (Critelli et al., 2007 n=83; Garzanti et al., 1996 n=2), (II) Backarc n=164 (Critelli et al., 2002 n=34; Li et al., 2004 n=130), (III) Collisional n=52 (Critelli and Reed, 1999), (IV) Continental-margin arc n=189 (Critelli et al., 1997 n=4; Ingersoll and Eastmoond, 2007 n=135; Lee and Lee, 2000 n=23; Michaelsen and Henderson, 2000 n=27), (V) Forearc n=443 (Critelli and Nielsen, 2000 n=38; Garzanti et al., 1996 n=3; Krueger, 1990 n=28; Kutterolf et al., 2008 n=337; Marsaglia et al., 1992 n=17; Rubin, 2002 n=20), (VI) Foreland n=74 (Critelli and Le Pera, 1995 n=10; Garzanti et al., 1996 n=5; Hossain et al., 2010 n=33; Lawton et al., 2009 n=26), (VII) Intra-arc n=49 (Colquhoun et al., 1999 n=23; Schwarzer et al., 2003 n=26), (VIII) Rift n=104 (Arribas et al., 2003 n=11; Garzanti et al., 1996 n=1; Gonzalez-Acebrón et al., 2007 n=26; Mader and Neubauer, 2004 n=24; Marsaglia et al., 2007 n=42), (IX) Strike-slip n=248 (Critelli and Ingersoll, 1995 n=59; Critelli et al., 1995 n=73; Critelli et al., 1997 n=9; Rumelhart and Ingersoll, 1997 n=71; Spencer et al., 2011 n=36).

All samples from the nine sedimentary basins relate to four main tectonic settings, according to their detrital modes (Table 2). For more details about the types of basins and tectonic settings, see Table 3 for Basin Classification from Ingersoll and Busby (1995).

BOLETÍN DE LA ASOCIACIÓN MEXICANA DE GEÓLOGOS PETROLEROS

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Continental block Confidence

Confidence

Geometric

-99.000%

+99.000%

Mean

2.81636

2.41411

3.21861

45

0.52293

0.27232

ln-Qm

45

2.63298

ln-Ls

45

0.92472

ln-Lm

45

ln-Lv ln-F ln-P

Valid N

Mean

ln-Qt

45

ln-Qp

Standard

Median

Variance

2.45786

3.14302

1.0045

1.00226

1.37852

0.77353

0.16614

0.24102

0.3899

0.62442

1.59682

2.30443

2.96152

2.44926

2.63640

0.6701

0.81862

-0.02674

0.44473

1.40471

0.51958

0.34756

1.4303

1.19597

2.70331

1.16902

0.55589

1.78216

0.62474

0.34756

2.3339

1.52772

2.30221

45

1.84918

0.40364

3.29472

0.62609

0.34756

12.9728

3.60178

6.01710

45

5.06340

2.71172

7.41509

1.73755

1.99849

34.3344

5.85956

-0.33356

45

14.46375

4.55028

24.37722

2.83190

2.49433

610.1323

24.70086

2.63502

Confidence

Confidence

Geometric

-99.000%

+99.000%

Mean

Median

Variance

Deviation

Kurtosis

• VOL. LXII No. 1 • ENERO-JUNIO 2020 •

Magmatic arc Standard

Valid N

Mean

ln-Qt

566

2.38897

2.21573

2.56222

1.72380

2.60565

2.54308

1.594704

42.23142

ln-Qp

566

0.80531

0.68143

0.92918

0.26233

0.26871

1.30022

1.140273

3.46785

ln-Qm

566

2.86831

2.72748

3.00914

2.21164

3.13227

1.68046

1.296326

-0.20926

ln-Ls

566

0.81423

0.65477

0.97369

0.21561

0.17658

2.15454

1.467836

11.59491

ln-Lm

566

2.49100

2.21097

2.77103

0.80796

1.66154

6.64435

2.577664

-0.48740

ln-Lv

566

4.77919

4.41452

5.14385

2.41062

4.76758

11.26759

3.356723

-1.09979

ln-F

566

1.20605

1.10602

1.30608

0.69099

1.25599

0.84785

0.920786

2.96314

ln-P

566

4.04062

3.80201

4.27924

3.44611

3.53986

4.82443

2.196459

0.24979

Confidence

Confidence

Geometric

-99.000%

+99.000%

Mean

Median

Variance

Deviation

Kurtosis

Recycle orogen Standard

Valid N

Mean

ln-Qt

710

1.93490

1.87498

1.99482

1.77235

2.03506

0.38212

0.618157

0.33816

ln-Qp

710

0.71505

0.66533

0.76476

0.39413

0.72896

0.26309

0.512922

-0.31226

ln-Qm

710

1.58375

1.52865

1.63885

1.43177

1.66120

0.32313

0.568447

0.19469

ln-Ls

710

4.28426

3.85217

4.71636

2.22935

2.70695

19.87192

4.457793

5.53477

ln-Lm

710

3.79742

3.43913

4.15572

1.38865

2.95457

13.66372

3.696447

0.37896

ln-Lv

710

2.84090

2.49696

3.18484

1.04006

1.51478

12.59084

3.548357

5.67102

ln-F

710

0.72131

0.66198

0.78064

0.47514

0.56702

0.37466

0.612094

2.42696

ln-P

710

1.08285

0.99217

1.17353

0.65366

0.80588

0.87522

0.935533

0.42336

Confidence

Confidence

Geometric

-99.000%

+99.000%

Mean

Median

Variance

Deviation

Kurtosis

Mixed provenance Standard

Valid N

Mean

ln-Qt

97

1.94771

1.785587

2.10984

1.795891

2.209222

0.369172

0.607595

-0.12812

ln-Qp

97

0.50969

0.410779

0.60859

0.329874

0.475052

0.137392

0.370664

0.85394

ln-Qm

97

1.70483

1.642170

1.76748

1.688252

1.675885

0.055136

0.234811

0.31816

ln-Ls

97

2.74315

2.143078

3.34322

1.798389

2.070361

5.057304

2.248845

1.88456

ln-Lm

97

1.68370

1.126865

2.24053

0.825333

0.955756

4.354765

2.086807

3.15732

ln-Lv

97

1.04560

0.693536

1.39767

0.439245

0.779449

1.740863

1.319418

11.11062

ln-F

97

1.22288

1.046644

1.39911

1.024485

1.061379

0.436210

0.660462

-0.30456

ln-P

97

1.69233

1.511729

1.87293

1.497183

1.832098

0.458089

0.676822

0.10824

Table 2. Basic statistics for different provenance settings, according to Dickinson and Suckzek (1979).

10

Deviation

Kurtosis

Compositional types of basin Tectonic setting equivalent to Busby and Ingersoll (Rubio-Cisneros and (1995) Ocampo-Díaz, this work)

Basin classification (c.f., Busby and Ingersoll, 1995)

Accretionary –type basins

Acretionary prims, Trenches, Trenches –Slope

Convergent setting

basins Back-arc basins

Convergent settings

Back-arc

Collisional basins

Convergent settings

Piggyback Basins, Trenches, Trench-Slope Basins

Forearc basins

Convergent settings

Forearc

Foreland basins

Convergent settings

Retroarc Foreland Basins, Peripheral Foreland Basins, Foreland Intermontane Basins (Broken Forelands)

Intra-arc basins

Convergent settings

Intrerarc Basins:

Continental-margin arc

Convergent settings

Interarc Basins

Rift basins

Divergent settings

Terrestrial Rift Valley, Proto-Oceanic Rift Troughs

Intraplate settings

Continental Rises and Terraces, Continental Embankments, Intracratonic Basins, Active Ocean Basins, Passive margins (Oceanic Islands, Aseismic Ridges and Plateaus, Dormant Ocean Basins, Intraplate Basin)

Strike-Slip

Transform settings (and

Transtensional Basins, Transpressional Basins,

transcurrent fault-related basins) Transrotational Basins: Table 3. Basins and their equivalent tectonic setting. (modified after Dickinson, 1974b, 1976a, and Ingersoll, 1988, in Ingersoll and Busby, 1995).

We describe the characteristics of each type of basin and its relationship to any known tectonic setting.

The bivariate and multivariate statistical analysis helped by organizing the data in basins regarding their composition. Then, we calculated the mean compositional parameters by sorting the composition of sand and sandstones in each type of basin, discriminating against any other sand or sandstone among the basins.

3.1 FRAMEWORK MODES: COMPOSITIONAL DIAGRAMS AND PROVENANCE TYPES

Our database consists of 1411 individual samples from sandstone point counts reported in 26 different references on peer-reviewed journals. The data was used to calculate compositional framework modes.

3.2 A COMPILATION OF TYPES OF BASINS AND THEIR COMPOSITION

To identify the basin from which provenance originates, we considered their tectonic occurrence, compositional features, and associations to the plots from Dickinson et al. (1983; see Table 3).

• 3.2.1 ACCRETIONARY–TYPE BASINS

Basins developed by accretionary orogen tectonics hold ocean plate stratigraphy, a fundamental structure for recognizing recent or ancient accretionary wedges (Condie, 2007; Kusky et al., 2010; Santosh et al., 2010). Accretionary basins contain sediments eroded from sequences related to subduction or duplexing.

Sandstones contain a suite of polycrystalline quartz, with high content of lithic fragments from high-middle grade metamorphic rocks (e.g., gneisses and schists), or middle-low grade metamorphic rocks (psammitesphyllites), rock fragments from mélange, high quartz content from the olistostrome accretionary complex, chert, basic volcanic lithics, calcareous arenites, including bioclasts, and fine-grained detritus in a matrix of mafic mudstones from continental regions. These quartzo-feldspathic sands contain detritus indicative of BOLETÍN DE LA ASOCIACIÓN MEXICANA DE GEÓLOGOS PETROLEROS

11

quartzose to mixed recycled orogenic sources, formed on a passive continental margin (Figure 1c). Accretionary basins are defined by three main discriminant compositional parameters: (i) ln-Qm: 2.356, (ii) ln-Ls: 5.038, and (iii) ln-P: 1.732 (Figure 2; Table 4).

(A)

(B)

Qm

al B Co

en

Magmatic arc

Lt Accretionary basins

F

(D)

Qm

Lt

Backarc basins

(E)

Qm

Collisional basins Qm

o

n

l

• VOL. LXII No. 1 • ENERO-JUNIO 2020 •

Back-arc Continentalmargin arc Rift Strike-slip

rog

nt i n

e nt

mixed

do cle cy Re

l oc k

Explanation

Forearc Accretionary Collisional Foreland Intra-arc

F

(C)

Qm

F

(F)

Lt

Forearc basins

F

(G)

Qm

Lt

Foreland basins

F

(H)

Qm

Intra-arc basins Qm

T

Mixed

T

T

F

(I)

Lt

Continental-margin arc basins Qm

F

(J)

Lt

Rift basins

F

(K)

Qm

T

F

Lt

Lt

Strike-slip basins Qm

T

Lt

F

Lt

F

Lt

Figure 1: QmFLt diagram from Dickinson et al. (1983), with 1411 samples taken from 26 different references. The data distributes in nine diagrams for better visualization of each type of sedimentary basin.

12

3.5

1.6

3.0

1.4 1.2

ln-Qp

ln-Qt

2.5 2.0 1.5

0.4 Mean Mean±0.95 Conf. Interval

0.0 Mean standard errors calculated -0.5

0.8 0.6

1.0 0.5

1.0

using pooled variance

0.2 0.0 -0.2

4.0

10

3.5

8

3.0

2.0

6

ln-Ls

ln-Qm

2.5

1.5

8

ln-Lm

6 4 2 0 -2

-2 -4 16 14 12 10

ln-Lv

0.0 10

2 0

1.0 0.5

4

8 6 4 2 0

-2

3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0

9 8 7 6 5

ln-P

ln-F

-4

4 3 2 1 0

Explanationn Forearc Back-arc Accretionary ContinentalCollisional margin arc Foreland Rift Intra-arc Strike-slip Figure 2: Box-plot diagram with values from the log-ratio transformations of the main compositional indices of sandstones, employing compositional data from nine basins.

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• VOL. LXII No. 1 • ENERO-JUNIO 2020 •

• 3.2.2 BACK-ARC BASINS

Back-arc basins belong to convergent settings (Ingersoll and Busby, 1995), but with either a contractional or extensional behavior (Stern, 2010).

Back-arc basins locate next to volcanic arcs. They are oceanic basins behind intra-oceanic magmatic arcs (including intra-arc basins between active and remnant arcs), or continental basins behind continental-margin magmatic arcs without foreland fold-thrust belts. The stratigraphy records variability in the composition of sandstones, according to the nature of the arc and pre-established rocks (e.g., Kusky et al. 2010). Some back-arc basins related to oceanic arc systems with sedimentation at high latitudes or in equatorial regions. The basins may contain high biological productivity that may contribute considerably to large proportions of biogenic components in sandstones (Marsaglia, 1995). The framework composition of back-arc basins suggests input from components from newly formed source-areas (e.g., volcanic lithic grains; Figure 1d). Statistically, the main discriminant composition for back-arc basins comes from three primary parameters: (i) ln-Lv: 4.841, (ii) ln-Ls: 3.891, (iii) ln-Qt: 1.794 (Figure 2; Table 4).

• 3.2.3 COLLISIONAL BASINS

Basins related to crustal collisions occupy convergent settings (Ingersoll and Busby, 1995). The geometry of the basin comes while suturing a tectonic collision and following frontal subduction at the final stage of accretion (Dewey, 1977). Collisional basins form before developing foreland basins; in both of its fronts are collision orogens with double vergence (e.g., Santosh et al., 2010). Detritus from axial nappes recycle with basement rocks and uplifted cover units from external thrust belts. The basin develops by merging the recycling of accreted molasses and alluvial plain deposits.

Each structural level determines the sediment composition in collisional basins, primarily while unroofing the metamorphic complex (Figure 1e).

Detritus from each structural level correspond to either oceanic or continental crusts, alternating in sources from the protolith at the basement and shallow units of the nappe pile (Garzanti et al., 2004). Detritus 14

from axial nappes incorporate fragments from the shallow, intermediate, and deep structural levels that provide sedimentoclastic lithics, metamorphiclastic quartzolithic, and quartzofeldspathic compositions, respectively — these three lithic fragments vary in their grade of metamorphism.

In contrast, detritus from the shallower structural level consist of calcareous, pellitic, and psammitic rock fragments. Quartz and feldspar contents are generally low, but recycling of arenaceous turbidites produces quartzofeldspathic compositions. Metamorphic detritus consists of low rank metasedimentary lithic grains. Sandstone compositions from the intermediate structural level at the accretion complex include fragments from retrograde highpressure metamorphic nappes, intermediate quartz, subordinate feldspars, and abundant metamorphic lithic grains. Metasedimentary rocks commonly supply recrystallized to foliated metacarbonate fragments. Detritus from deep structural levels and the external thrust belts are quartzofeldspathic in composition, with very high-rank metamorphic rock fragments from old basement sources. All constituents are mixed in various proportions with medium-rank to non-metamorphic lithic detritus, such as metamorphiclastic to sedimentaclastic compositions, mixing with fragments from axial nappes. Detritus from sedimentary thrust-sheets are lithic sands dominated by carbonate grains unless carbonate strata are absent from the sedimentary section in the thrust belt (Maruyama et al., 2010).

Collision-orogenic sediments are complex to determine. However, some significant considerations for improving the provenance analysis are determining (a) the metamorphic history of the axial nappe stack, (b) occurrence of old crystalline relics within both axial nappes and external belts, (c) recycling of accreted molasses, glacial, and alluvial deposits, (d) mixing of detritus from unrelated sources during long-distance transport, and (e) selective destruction of unstable components during transport and diagenesis (Garzanti et al., 2007). Discriminant values detect the framework complexity of collisional basins according to three prime compositional parameters: (i) ln-Lm: 6.635, (ii) ln-Qm: 2.00, and (iii) ln-P: 1.023 (Figure 2; Table 4).

• 3.2.4 FOREARC BASINS

The basin subdivision for convergent settings includes forearc basins. The forearc is the lithosphere that lies between the trench and the volcanicmagmatic arc. They also occur in arc-trench gaps.

The stratigraphy of forearc basins represents the split of an arc, lying immediately above arc volcanism.

Forearc basins preserve a juvenile arc lithosphere and its arc sequences. Sometimes forearc basins may be associated with an accretionary prism. Each structural zone in a forearc contrasts according to its erosion rates, newly formed source areas, and detrital transport (Figure 1f). The zoning consists of a stable inner forearc with thin rock sequences, in contrast to a deformed outer zone. Forearc basins with sedimentation under high biological productivity may present considerably larger proportions of biogenic components in sandstones (Marsaglia, 1995). Detrital compositions in forearc basins constrain six main discriminant parameters: (i) ln-Lm: 1.654, (ii) ln-Qt: 1.537, (iii) ln-Qm: 1.146, (iv) ln-Lv: 1.419, (v) ln-P: 1.07, and (vi) ln-Ls: 0.965 (Figure 2; Table 4).

• 3.2.5 FORELAND BASINS

Foreland basins develop mainly in convergent settings. These sedimentary basins lie between the front of a mountain and the adjacent craton in the extents of the shield zone (Allen et al., 1986; Ingersoll and Busby, 1995). Foreland basis includes Retroarc Foreland Basins and Peripheral Foreland Basins.

Retroarc foreland basins are elongate trough located on the continental side of continental-margin arctrench systems. These systems form between narrow contractional orogenic belts related to an active arc and a stable craton, with progressive compression and collision during subduction (DeCelles and Giles, 1996). Peripheral Foreland Basins lie above rifted continental margins pulled into subduction zones during a crustal collision. Mini foreland basins may set next to uplifted blocks at strike-slip basins caused by the flexural loading of the marginal crust.

The modal composition of foreland fold-thrust belts, especially during initiation, is significantly influenced by the former tectonic regime. Also, the evolution of foreland basins depends on preexisting basin configuration and the patterns in subsidence at the

foredeep induced by predecessor rift and back-arc basins. The provenance of foreland sediments is from source areas forming the fold-thrust belt (Dickinson, 1974a, 1976). The uplift of the depocenter along thrust faults and climate are the two major geologic processes affecting the basin (Lawton, 1986). Clastic sediments in the foreland basin are mixtures of detritus from both axial nappes and external belts, which have plutoniclastic, metamorphiclastic and sedimentaclastic compositions, grouping into several of the provenance types. The provenance of many ancient retroarc foreland basins is predominantly from recycling the sedimentary cover (Jordan, 1995; DeCelles and Giles, 1996). The massive sedimentary fill in foreland basins contains quartz-rich, feldspar-poor detritus eroded from cratons and basement blocks; however, the bulk of the fragments is less quartz-rich. Upsection lithic fragments become more abundant than the underlying profile. Source-areas of sediments are orogenic, either from, old continental margin deposits exposed in sedimentary and metasedimentary nappes, or from early sediments deformed in the fold-thrust belt (Figure 1g). Sources that supply the least in foreland basins are subduction complexes or magmatic arcs, mainly because the upland zone topographically shields such sources, or because longitudinal sediment dispersion covers the sources along the strike of the suturing orogenic belt (Schwab, 1986). Then, the conditions of sedimentary environments in proximity to source-rocks basins impact heterogeneity of sandstone composition (e.g., Damanti, 1993). Foreland basins relate to five principal compositional values: (i) ln-Ls: 3.903, (ii) lnQm: 2.785, (iii) ln-Qt: 2.482; (iv) ln-Lm: 1.426, and (v) ln-P: 1.095 (Figure 2; Table 4).

• 3.2.6 INTRA-ARC BASINS

Intra-arc basins form along the margins of island chains overlying the basement of young continental crust (de Ronde et al., 2003). They form at the juncture of thin back-arc basins, forearc basin crust, and thick crust beneath the volcanic–magmatic arc.

The basin is deep with deposits of volcaniclastic sediments originated from an arc. In general, the sedimentary fill comes from a destructive plate margin with an active magmatic center, where hinge faults or fracture zones intervene. Sandstones vary in composition, depending on the magmatism through

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subduction. Also, intra-oceanic arcs are genetically related to other structures occurring in the Island arc model and herein treated as systems like a trench, forearc, arc, and back-arc regions.

• VOL. LXII No. 1 • ENERO-JUNIO 2020 •

Sediment input is dominantly volcanogenic and derives mainly from the active volcanic arc. Sands are volcaniclastic and plutoniclastic in composition.

Source-areas correspond to magmatic arcs that lie along continental margins where the crust is thicker than in intra-oceanic arcs, or where the arc massif remains partly submerged (Figure 1h). Other sources emerge from breaking an arc massif by parallel transforms or faults, exposing plutons and their wall rocks that supply arkosic detritus to the forearc region (Marsaglia and Ingersoll, 1992).

Detrital modes in sandstones indicate the composition of petrogenetic igneous processes, such as magma generation in intra-arc basins. Partial melting in the Earth’s upper mantle can modify the composition of primary magmas. Thus, magma composition, crystal fractionation, and air-fall deposits account for the variation of detrital modes in intra-arc basins.

Magmatism at intra-oceanic arc systems increases in alkalinity away from the trench; consequently, the composition of in clastic successions varies as the magmatic center migrates away from the sedimentary basin (Wilson, 1989, 2007; Gill, 2010). The petrography of sandstones from intra-oceanic arcs correlates to the magma series from calc-alkaline to potassic alkaline suites with high-MgO. Sandstone in intra-arc basins is highly porphyritic, generally containing major ferromagnesian minerals. Plagioclase is usually the most abundant phenocryst. Groundmasses can vary from glassy to microcrystalline, but generally, contains the same minerals as in phenocryst phases. Rock fragments also include xenocrysts. Intra-arc basins include five main discriminant compositional values: (i) ln-Lv: 7.203, (ii) ln-P: 5.43, (iii) ln-Qt: 1.22, (iv) ln-Qp: 0.95, and (v) ln-Qm: 0.857 (Figure 2; Table 4).

• 3.2.7 CONTINENTAL-MAGMATIC ARC BASINS

Basins at continental-margin arcs present magmatism related to the subduction of an oceanic plate beneath a continent (Wilson, 1989). Partial melting and fractional crystallization in the mantle and processes in the 16

crust of contamination and assimilation of magmas contribute to lately formed minerals to the basin (Figure 1i).

Ocampo-Díaz and Rubio-Cisneros (2013) documented a modal composition of sands from continental-margin arcs related to a change in source-area and sedimentary recycling. The results from the discriminant statistical analysis solve five compositional values corresponding to: (i) ln-P: 3.02, (ii) ln-Qm: 2.798, (iii) ln-Lv: 2.285, (iv) ln-Qt: 2.00, and (v) ln-Lm: 1.231 (Figure 2; Table 4). The suite of composition correlates to nascent arc activity, with similar values in newly active continental margins.

• 3.2.8 RIFT BASINS

Rift basins are depocenters over continental crust.

They are commonly associated with bimodal volcanism and incipient oceanic basins floored by new oceanic crust. Young rifted continental margins flank the basin. Tectonic activity is principal factor controlling sedimentation in rift basins (Prosser, 1993). Faults in these systems reactivate during stages of lithosphere extension. The erosion and dispersion of sediments in the basin determine the composition of sandstones (Arribas et al., 2003; González-Acebrón et al., 2007; Marsaglia et al., 2007). Although a variety of characteristics distinguish rift basins (Garzanti et al., 2003), the provenance for its sedimentary sequence defines the periods of extension, sedimentary renovation, climatic conditions, and the composition of the basement blocks, which eventually are buried by the sedimentary cover and subsidence (Johnsson, 1993).

Besides allogenic factors, autogenic processes affect sediment composition too, but to a lesser extent.

The first stage of deposition in a rift and pull-apart basins starts with the erosion of the pre-existing sedimentary cover and in fewer proportions basement rocks (volcanic rifted-margin provenance to rift-shoulder provenance; cf., Garzanti et al., 2001).

The subsequent stages involve deep erosion of the basement and its recycling with previous sequences (cf., Ocampo-Díaz and Rubio-Cisneros, 2013).

Volcaniclastic sands at rifted-margins are feldspatholithic, but with bimodal lithics either from basalt or rhyolite. Sands contain abundant granophyre grains and low rates of plagioclase/total feldspar (P/F) eroded from syn-rift hypersolvus alkali granites in a younger crust (Garzanti et al., 2001). The classification of rift-shoulder provenance has compositions of quartzolithic sedimentaclastic, arkosic, and basementaclastic. The provenance of detrital suites in rift-shoulder is continental blocks. Sandstones decrease in quartz content when deeper the erosion, as in the rift stages of undissected to dissected. However, sand from arc terranes containing paleovolcanic to metabasite lithics comes from recycled orogens. Sedimentary detritus from undissected rift shoulders comprise recycled quartz and carbonate lithic fragments. Arkosic sands from exposed basement rocks on dissected rift shoulders contain an excess of quartz contrary to the fact of an “ideal arkose”. For instance, clastic suites of rifted-margin commonly include quartzfeldspathic sands, but also quartzolithic and feldspatholithic volcano-plutonic sands. Rift clastics can share sources from continental blocks, recycled orogens, and magmatic arcs in a standard quartz-feldsparlithics triangular plot (QmFL; Figure 1j). The framework composition to discriminate rift basins associates with four parameters: (i) ln-Qm: 3.078, (ii) ln-Qt: 2.565, (iii) ln-Ls: 1.818, and (iv) ln-F: 1.244 (Figure 2; Table 4).

• 3.2.9 STRIKE-SLIP BASINS

This work considers strike-slip basins as depocenters existing in various tectonic settings (Mann et al., 1983), mostly in transform settings and basins with transcurrent faults (Ingersoll and Busby, 1995).

Some structural styles divide strike-slip basins into different classes: (1) Transtensional basins –basins formed by extension along strike-slip fault systems, (2) Transpressional basins –basins formed by compression along strike-slip fault systems, and (3) Transrotational basins –basins formed by rotation of crustal blocks with vertical axes in strike-slip fault systems.

The composition of sandstone relies on the geometry of the structures like releasing or restraining bends and pull-apart basins (Nilsen and Sylvester, 1995; Noda, 2013). Equally, fractures and faults depend on the bending or (over-) stepping geometry in the

fault zone and plate boundaries. These sectors have irregular collision fronts, where constriction expels pieces of crust or splinters denominated “scholles” (McKenzie, 1972; Burke and Sengor, 1986). The fragmentation of crust supplies a suite of detrital assemblages to sandstones.

Framework petrography of strike-slip basins points out polycyclic sedimentary episodes, under multiple phases of subsidence, uplift of sourcelands, and basin migration (Figure 1k). Sandstone compositions tend to be extraordinarily variable and complex, depending on whether the basins are submarine, sublacustrine, subaerial, or some random combination of these (Nilsen and McLaughlin, 1985).

Strike-slip basins discriminate in composition upon five framework parameters: (i) ln-Qm: 2.244, (ii) ln-P: 2.217, (iii) ln-Lv: 2.156, (iv) ln-Lm: 1.788, (v) ln-Qt: 1.974 (Figure 2; Table 4).

3.3 DISCRIMINANT FUNCTIONS

The analysis identifies three general fields for compositional discrimination; each of them solved by a discriminant function. Overlapping the three general functions are nine fields representing the geometric means for each basin (Figure 3a-k).

The top part of the ternary plot is the discriminant function 1-one (DF1) that is populated by forearc basins, foreland basins, rift systems, accretionary basins, or suture complexes (Figure 3f, g, and j). DF1 has an increase in positive values of Qt, Ls, and Qp. The petrographic indices in discriminant function 1 support the processes of sediment transport, the exhumation of the sedimentary carapace, and a moderate supply of metamorphic rocks (ln-Qt, ln-Ls, and ln-Qp; Tables 2 and 6).

At the low left angle of the plot lies discriminant function 2-two (DF2). It stands for the formation of new source-rocks in collisional zones, continentalmargin arcs, and strike-slip basins (Figure 3e, f, i, and k). DF2 estimates increasing values in Qp, Lm, and Qm. The detrital components near DF2 relate to moderate–high sediment transport, and the exhumation of plutonic or metamorphic source-lands (ln-Qp, ln-Qm, ln-F, and ln-Lm; Tables 2 and 6).

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Accretionary-type basins Confidence

Confidence

Geometric

-99.000%

+99.000%

Mean

1.53000

1.31146

1.74855

85

0.85546

0.60614

ln -Qm

85

2.64433

ln -Ls

85

6.18316

ln -Lm

85

ln -Lv ln -F In-P

Valid N

Mean

ln-Qt

85

ln -Qp

SeReIn X=a SeReIn Y=a

Standard

Median

Variance

1.15026

1.96472

0.58441

0.764468

-0.94610

1.10478

0.36899

0.45548

0.76061

0.872132

-1.32907

2.38374

2.90493

2.35641

2.65006

0.83098

0.911583

1.65828

5.04391

7.32241

5.03846

4.82542

15.88131

3.985136

3.43002

2.43169

1.90745

2.95593

1.28596

2.98839

3.36290

1.833822

-1.00782

85

0.41387

0.02837

0.79938

0.13356

0.09492

1.81851

1.348520

39.32859

85

0.91470

0.75553

1.07388

0.66716

0.95175

0.31002

0.556792

0.28879

85

2.12268

1.75090

2.49446

1.73204

1.95011

1.69132

1.300509

15.27716

85

9.08475

8.69354

9.47596

8.87765

9.35994

1.87270

1.368465

18.20286

85

14.33574

13.75368

14.91780

14.20997

14.12568

4.14555

2.036063

5.23164

Confidence

Confidence

Geometric

-99.000%

+99.000%

Mean

Median

Variance

Deviation

Kurtosis

• VOL. LXII No. 1 • ENERO-JUNIO 2020 •

Back-arc basins Standard

Valid N

Mean

ln-Qt

164

2.60001

2.30146

2.89855

1.79455

2.83344

2.1518

1.46691

-1.18031

ln-Qp

164

1.07687

0.88170

1.27203

0.54429

0.83198

0.9196

0.95895

-0.09579

ln-Qm

164

1.52644

1.30787

1.74501

1.02393

1.23567

1.1534

1.07396

-0.73830

ln-Ls

164

9.67173

7.78380

11.55966

3.89157

6.82876

86.0511

9.27637

0.30670

ln-Lm

164

0.17490

0.16772

0.18207

0.17132

0.17327

0.0012

0.03525

-1.03841

ln-Lv

164

11.52398

9.59361

13.45434

4.84166

9.71538

89.9628

9.48487

-1.37302

ln-F

164

0.82428

0.61721

1.03134

0.43721

0.32794

1.0351

1.01741

3.94849

ln-P

164

5.01983

3.14921

6.89044

0.70855

0.27584

84.4804

9.19132

0.56048

164

11.16871

10.57549

11.76192

10.48242

12.37813

8.4958

2.91476

2.78514

164

29.52323

26.68072

32.36574

25.80397

27.50941

195.0701

13.96675

-1.17381

Confidence

Confidence

Geometric

-99.000%

+99.000%

Mean

Median

Variance

SeReIn X=a SeReIn Y=a

Deviation

Kurtosis

Collisional basins Standard

Valid N

Mean

ln-Qt

52

1.76388

1.66340

1.86437

1.74610

1.69436

0.073333

0.270801

3.680199

ln-Qp

52

0.38149

0.25777

0.50521

0.28500

0.23472

0.111170

0.333422

3.387706

ln-Qm

52

2.02233

1.93504

2.10961

2.00975

2.02049

0.055334

0.235232

2.124866

ln-Ls

52

1.61508

0.98037

2.24979

0.74086

0.92799

2.925957

1.710543

0.722741

ln-Lm

52

7.83858

6.96430

8.71287

6.83540

8.51167

5.551678

2.356200

2.780206

ln-Lv

52

0.55350

0.33432

0.77269

0.28638

0.31569

0.348938

0.590710

0.739361

ln-F

52

0.97127

0.70799

1.23455

0.67378

0.88217

0.503450

0.709542

-0.563966

ln-P

52

1.34867

1.06127

1.63607

1.02338

1.39648

0.599920

0.774545

-0.700760

52

8.19929

8.14934

8.24924

8.19820

8.21248

0.018120

0.134611

-0.491134

52

14.22727

13.25721

15.19733

13.89488

14.89735

6.834701

2.614326

3.978060

SeReIn X=a SeReIn Y=a

18

Deviation

Kurtosis

Forearc basins Confidence

Confidence

Geometric

-99.000%

+99.000%

Mean

1.77826

1.69591

1.86062

443

0.71209

0.66841

ln-Qm

443

1.23448

ln-Ls

443

1.47989

ln-Lm

443

ln-Lv ln-F ln-P

Valid N

Mean

ln-Qt

443

ln-Qp

SeReIn X=a SeReIn Y=a

Standard

Median

Variance

1.53796

1.99751

0.44893

0.670021

0.19197

0.75577

0.56726

0.72541

0.12627

0.355344

-0.12144

1.18552

1.28344

1.14640

1.23452

0.15868

0.398345

0.27315

1.34800

1.61177

0.96593

1.27677

1.15126

1.072969

1.94549

2.75639

2.48527

3.02750

1.65439

2.42289

4.86538

2.205762

-0.25309

443

2.41945

2.10541

2.73350

1.41966

1.67303

6.52838

2.555070

5.31715

443

0.65968

0.58193

0.73743

0.41019

0.49583

0.40014

0.632567

5.56783

443

1.90163

1.59162

2.21164

1.07462

0.97084

6.36146

2.522194

8.52663

443

6.68459

6.60419

6.76499

6.64028

6.95409

0.42788

0.654129

15.24983

443

10.86558

10.42563

11.30552

10.32229

10.00846

12.81189

3.579370

0.61007

Confidence

Confidence

Geometric

-99.000%

+99.000%

Mean

Median

Variance

Deviation

Kurtosis

Foreland basins Standard

Valid N

Mean

ln-Qt

74.000

2.568

2.378

2.758

2.483

2.482

0.383

0.619099

-0.69645

ln-Qp

74.000

0.569

0.367

0.771

0.145

0.125

0.431

0.656589

-1.53047

ln-Qm

74.000

2.804

2.709

2.898

2.786

2.841

0.095

0.307416

1.66909

ln-Ls

74.000

5.212

4.448

5.976

3.903

5.862

6.182

2.486291

-0.48581

ln-Lm

74.000

3.396

2.523

4.269

1.427

3.974

8.068

2.840421

-1.08249

ln-Lv

74.000

1.787

0.966

2.609

0.457

0.541

7.137

2.671588

5.71969

ln-F

74.000

0.563

0.418

0.708

0.359

0.506

0.222

0.471203

9.48428

ln-P

74.000

1.589

1.142

2.035

1.095

0.757

2.110

1.452503

5.01166

74.000

10.434

10.318

10.551

10.427

10.488

74.000

15.217

14.723

15.711

15.139

15.180

Confidence

Confidence

Geometric

-99.000%

+99.000%

Mean

SeReIn X=a SeReIn Y=a

Deviation

Kurtosis

Intra-arc basins Valid N

Mean

ln-Qt

49

3.06331

1.75594

4.37068

ln-Qp

49

2.48617

1.43332

3.53902

ln-Qm

49

2.40301

1.22587

ln-Ls

49

0.71244

ln-Lm

49

2.43610

ln-Lv

49

ln-F ln-P SeReIn X=a SeReIn Y=a

Standard

Median

Variance

1.22275

1.60269

11.64156

3.411973

0.668065

0.95353

1.43219

7.54998

2.747723

0.668065

3.58015

0.85777

0.64569

9.43774

3.072090

0.668065

0.14886

1.27603

0.24678

0.11699

2.16335

1.470833

0.668065

0.58791

4.28430

0.27579

0.10541

23.26525

4.823407

0.668065

16.94799

13.20948

20.68650

7.20390

20.30065

95.19409

9.756746

0.668065

49

2.34080

0.81818

3.86342

0.37522

0.10992

15.79047

3.973722

0.668065

49

10.88974

8.10763

13.67185

5.43568

10.73585

52.71823

7.260732

0.668065

49

14.67324

11.00998

18.33650

11.40568

12.52310

91.40043

9.560357

0.668065

49

36.58722

33.40676

39.76768

35.47031

40.66000

68.89574

8.300346

0.668065

Deviation

Kurtosis

BOLETÍN DE LA ASOCIACIÓN MEXICANA DE GEÓLOGOS PETROLEROS

19

Continental-margin arc basins Confidence

Confidence

Geometric

-99.000%

+99.000%

Mean

2.79578

2.48343

3.10813

0.80219

0.52672

1.07767

189

3.67080

3.26985

ln-Ls

189

0.52768

ln-Lm

189

4.30051

ln-Lv

189

ln-F ln-P

Valid N

Mean

ln-Qt

189

ln-Qp

189

ln-Qm

SeReIn X=a SeReIn Y=a

Standard

Median

Variance

2.00362

2.82697

2.72295

1.650138

-0.61459

0.17837

0.05534

2.11804

1.455349

3.62556

4.07176

2.79836

3.44082

4.48703

2.118262

0.10409

0.32344

0.73192

0.17252

0.10868

1.16425

1.079005

15.67077

3.47505

5.12596

1.23158

2.40112

19.01765

4.360923

-1.48083

6.30710

5.36890

7.24531

2.28523

7.32873

24.56790

4.956602

-1.62622

189

1.23475

1.02428

1.44522

0.64305

1.01936

1.23635

1.111914

1.89923

189

4.90834

4.27192

5.54477

3.20266

4.68158

11.30478

3.362258

-0.27635

189

12.70724

11.97225

13.44224

11.82360

12.97593

15.07770

3.883002

0.40037

189

20.45149

19.91151

20.99147

20.24397

20.90572

8.13810

2.852735

-0.62309

Confidence

Confidence

Geometric

-99.000%

+99.000%

Mean

Median

Variance

Deviation

Kurtosis

• VOL. LXII No. 1 • ENERO-JUNIO 2020 •

Rift basins Standard

Valid N

Mean

ln-Qt

104

3.00653

2.65897

3.35409

2.56567

2.93524

1.82403

1.350566

-0.82718

ln-Qp

104

0.42924

0.29223

0.56625

0.12673

0.09818

0.28345

0.532397

1.06835

ln-Qm

104

3.17774

2.97890

3.37659

3.07604

3.05483

0.59703

0.772679

0.86085

ln-Ls

104

6.50478

4.72934

8.28022

1.81821

3.73095

47.59723

6.899075

-0.67533

ln-Lm

104

4.00631

2.80619

5.20644

1.45858

2.43284

21.74836

4.663514

2.43931

ln-Lv

104

1.90148

0.82004

2.98291

0.34069

0.13150

17.65918

4.202282

4.76897

ln-F

104

2.51631

1.82991

3.20271

1.24411

1.63433

7.11419

2.667244

5.80250

ln-P

104

2.16937

1.40499

2.93374

0.88963

1.61276

8.82232

2.970239

24.97969

104

11.78801

11.55292

12.02310

11.74981

12.25460

0.83451

0.913514

1.82323

104

19.73127

17.18163

22.28091

15.87214

20.81888

98.15881

9.907513

-0.55475

Confidence

Confidence

Geometric

-99.000%

+99.000%

Mean

Median

Variance

SeReIn X=a SeReIn Y=a

Deviation

Kurtosis

Strike-Slip basins Standard

Valid N

Mean

ln-Qt

248

2.25139

2.06384

2.43894

1.97497

2.02671

1.294543

1.137780

5.14998

ln-Qp

248

0.46954

0.38932

0.54977

0.22561

0.34200

0.236888

0.486711

5.70571

ln-Qm

248

2.45959

2.29811

2.62107

2.24461

2.25201

0.959710

0.979648

4.02192

ln-Ls

248

0.32960

0.22083

0.43837

0.11845

0.06812

0.435412

0.659857

33.71980

ln-Lm

248

2.57310

2.26144

2.88475

1.78824

2.20044

3.574667

1.890679

1.56958

ln-Lv

248

3.62629

3.22942

4.02316

2.15667

3.58891

5.796779

2.407650

-0.70913

ln-F

248

1.20569

1.11724

1.29415

0.98720

1.24762

0.287967

0.536626

0.25276

ln-P

248

2.70574

2.48472

2.92677

2.21735

2.89696

1.797902

1.340859

2.87606

248

8.77579

8.49825

9.05334

8.57305

8.72662

2.834969

1.683736

3.69280

248

12.63470

12.27309

12.99631

12.33179

13.04824

4.812447

2.193729

7.24978

SeReIn X=a SeReIn Y=a

Deviation

Kurtosis

Table 4. Basic statistics for nine basins with each detrital mode parameter. Data in sandstone petrography is recalculated using the values from the publications.

20

Meanwhile, discriminant function 3-three (DF3) is the right-side angle grouping back-arc basins forearc basins, intra-arc basins, continental-margin arcs, and strike-slip basins (Figure 3f, h, i, and k). DF3 plots increments of Lv, F, Qp. Function DF3 marks the development of new source-areas and recycling of ancient clastic sequences (ln-F, ln-Lv, ln-Ls, and ln-Qp; Tables 2 and 6).

(A)

(B)

D.F1 Increasing Qt, Ls, Qp

0.25

0.50

0.75

, Lm

Qm

0.75

rea 0.25 sin gL v, F ,Q p

Accretionary basins

(F)

D.F2

D.F3

Forearc basins D.F1

0.75

D.F2

(I)

0.50

0.75

D.F2

0.50

D.F3

D.F2

D.F3

Rift basins

0.25

0.50

0.50

0.25

D.F3

0.75

D.F3

0.50

0.25

0.75

D.F3

D.F2

(K)

D.F1

0.75

0.75

0.50

0.25

(J)

(H) Intra-arc basins 0.25

0.75

D.F1

D.F3

D.F2

D.F1

0.25

Continental-margin arc basin

0.25

D.F3

0.25

0.75

0.75

0.50

0.50

0.50

0.50

0.50

D.F1

0.25

0.75

0.25

Foreland basins

0.75

0.25

D.F1

0.25

(G)

D.F3

(E) Collisional basins

0.75

D.F2

0.75

0.50

0.50

0.25

0.75

0.50

0.75

0.25

0.50

0.50

0.25

D.F2 D.F1

0.75

0.25

0.50

0.25

Back-arc basins

D.F1

0.75

0.75

D.F3

(D)

0.25

0.50

Inc

D.F2

(C)

Back-arc Continentalmargin arc Rift Strike-slip

0.50

n

si rea

Inc

p, gQ

D.F1

Explanation

Forearc Accretionary Collisional Foreland Intra-arc

Strike-slip basins D.F1

0.75

0.25

0.50

0.50

0.25

D.F3

0.75

D.F2

0.75

0.50

0.25

D.F3

Figure 3: Distribution of the discriminant analysis for the composition of sandstones from nine types of basins according to their petrographic indices.

BOLETÍN DE LA ASOCIACIÓN MEXICANA DE GEÓLOGOS PETROLEROS

21

4. CASE STUDY: A DISCRIMINANT FUNCTION FOR CLASTIC SEQUENCES IN NORTHEASTERN MEXICO A case study proves the functionality of the method and its statistical approach, using compositional and modal values from samples collected in northeastern Mexico (Figure 4, Figure 5, and Figure 6). Samples are middle to coarse-grained sandstones from Upper Triassic to Lower Cretaceous formations: El Alamar, La Boca, La Joya, La Casita, and Taraises. The description of these units and their locations, as well as the number of samples included in this work, are available in the next references: Ocampo-Díaz (2007; n=56), Rubio-Cisneros (2008; n=20), Ocampo-Díaz (2011; n=108), Rubio-Cisneros et al. (2011; n=7), Ocampo-Díaz et al. (2012; n=56), Rubio-Cisneros (2012; n=129), and Ocampo-Díaz and Rubio-Cisneros (2013; n=300), Ocampo-Díaz et al. (2014; n=16). Based on previous works, the samples and their corresponding sequences deposited in the following basins: rift, back-arc, forearc, continental-margin arc, and strike-slip.

Explanation

Triassic-Lower Jurassic sequences Upper Jurassic Sequences

25° N

N

Lower Cretaceous Sequence

50 km

0

Upper Cretaceous Sequences

• VOL. LXII No. 1 • ENERO-JUNIO 2020 •

Village

Monterrey

Thrust fault

Saltillo

Santiago Allende Montemorelos

100°30'0''W

25°30'0''N

N

25° N

W

Rayones

San Rafael

MONTERREY

'0''

10

1°

10

W

'0''

50

0°

10

100°10'0''W

S

Triassic and Juriassic Sequences

Villa de Santiago

SALTILLO C O A H U I L A

San Roberto

Allende

i

Galeana

e

Montemorelos

Linares

Iturbide

100°W Locations of the sampled formations El Alamar Formation

r

101°W

r

a

M

NU E V O L E Ó N

a

Linares

d

Galeana

r e

La Boca Formation

Iturbide

San Roberto

Potrero Prieto

O

r

La Joya Formation

a l n t i e

La Casita Formation

La Asención

USA

La Escondida

El Barretal

Aramberri

MEXICO

Tinajas

Miquihuana

Study area

C I U DA D V I C T O R I A Detail in Figure 5

T A M A U L I PA S

99°50'0''W

99°30'0''W

0

99°10'0''W

30

60 km

Figure 4: Geologic sketch and map of northeastern Mexico. The map with the coordinate system has two highlighted zones showing the area of study. The map in the upper right-hand locates each formation for this work with a star or polygon in a circle; it also details the units studied herein (modified form Ocampo-Díaz et al., 2012). The second highlighted zone is near the locality of Ciudad Victoria, described in Figure 5. See text for citations and Figure 6 to extend the description of the units analyzed.

22

2611000

473000

474000

475000

476000

1300

2610000

1200

Jn

Jlbi

2609000

1200

Ji 0

Ji

Ji

El Huizachal

‘

Ji

1000

00 14

1100

1100

10 00 90

0

La Joya Verde Cementerio

2608000

1000

100 0

Jlbs

1200

Jo

1

Aguas de Minas

Jn

Jlj 1400

Graphic scale

Geologic contact Locality Inferred geological School contact Coordinate system: UTM Topografic base map from INEGI: Ciudad Vioctoria F-14-A-20

Jc

1300

Ji

1200

Ku

1400

Symbols

Ji

La Anacahuita

2607000

Undifferentiated Cretaceous strata (Ku) La Casita Formation (Jc) Olvido Formation (Jo) Novillo Formation (Jn) La Joya Formation (Jlj) 00 16 Rhyollite (Ji) La Boca Formation upper member (Jlbs) La Boca Formation lower member (Jlbi)

El Madrono

1100

0

478000

Explanation

N

130

477000

13 00

2 km

12

00

1100 120 0

1300

Figure 5: Generalized modified geologic map of Valle de Huizachal (Rubio-Cisneros and Lawton, 2011). Stars and polygons indicate the units studied in this work, and correlate to those formations explained in Figure 4. Map symbols: Jlbi—lower member of La Boca Formation; Jlbs—upper member of La Boca Formation; Ji—rhyolite domes; Jlj—La Joya Formation; Jn—Novillo Formation; Jo—Olvido Formation; Jc—La Casita Formation; Ku—undifferentiated Cretaceous strata.

4.1 REGIONAL GEOLOGIC FRAMEWORK

The central region of the state of Nuevo Leon is a world-class outcrop for visiting the clastic sequences for the stratigraphic column of the Sierra Madre Oriental, specifically within Monterrey Trough. The rock sequences appear mainly in the vicinity of the towns of San Pablo Tranquitas near Galeana, and Rayones. The clastic sequence extends south to Ciudad Victoria in the state of Tamaulipas (Figure 4).

The clastic sequences in the Monterrey Trough include fluvial-alluvial deposits of Upper Triassic age, alluvial-lacustrine-fluvial deposits of the Early-Middle Jurassic, deposits from a deltaic coastline of the Upper Jurassic, delta deposits influenced by tides, and also marine marginal sedimentary environments of the Lower Cretaceous (Michalzik, 1991, Michalzik and Schumann, 1994, Ocampo-Díaz, 2007, Ocampo-Díaz et al., 2008, Barboza-Gudiño et al., 2010; Ocampo-Díaz, 2011, 2012, Rubio-Cisneros, 2012).

4.2 GENERAL COMPOSITION OF SANDSTONES

A modal analysis demonstrates significant changes in the composition of sandstones in northeastern Mexico, indicated by the contents of lithics and feldspars (Figure 7). The lithics vary in composition correspondingly to the concentration of metamorphic rock detritus of low- mid-grade and volcanic fragments. All samples from the clastic sequences plot in the BOLETÍN DE LA ASOCIACIÓN MEXICANA DE GEÓLOGOS PETROLEROS

23

4000 m Lithostratigraphic units

Parras Form. + Difunta Gruoup

Foreland related to Laramide orogenic event

Age

Maestricht.

Méndez Formation

Campanian

San Felipe Form. Santoniano Coniacian Agua Nueva Form.

Turonian

3000 m

Epoch

Cuesta del Cura Form.

Cenomanian

Tamaulipas superior Formation

Albian

Upper

SE

Cretaceous

NW

La Peña Formation

Cupido Formation

Barremian

La Casita Formation Zuloaga Form.

Valanginian

Berriasian Tithonian Kimmeridgian Upper

• VOL. LXII No. 1 • ENERO-JUNIO 2020 •

Taraises Formation

Hauterivian

Jurassic

2000 m

Tamaulipas inferior

Lower

Aptian

Oxfordian

1000 m

Callovian

Middle

Triassic

Upper

Minas Viejas Formation La Joya Form.

La Boca Formation Lower-Middle Jurassic

Explanation

0m

Alamar Formation External shelf carbonates

Exposed areas, nondeposition

Open shelf carbonates

Platform border carbonates Restricted marine evaporites (Sabkha) gypsum and anhydrite

Coarse grained continental sediments

Moderatly deep marine calcareous shale, silstone (phospatic lime mudstone) Basinal deep-water carbonates and shales High energy carbonate grainstone (shoal) Restricted marine evaporite (sabkha), salt

Outer ramp mudstone and shale External shelf carbonates Lagoonar carbonates, peritidal carbonates Nonmarine to marginal marine coarse clastics Transgressive sequence

Regressive sequence

Figure 6: Lithostratigraphic column of northeastern Mexico based on Michalzik (1991) and others cited herein. Stars and polygons represent the same sampled formations as in Figure 4.

24

QFLt diagram contrasting compositional polygons, allowing interpreting how sedimentary petrology evolved in the basin and the periods of supply from source-rocks (Figure 7).

Upper Triassic sandstones from the Alamar Formation (Michalzik, 1991; Rubio-Cisneros, 2008a; Rubio-Cisneros, 2008b) have a high textural maturity and plot in the upper part of the recycled orogen field (Figure 7). La Boca Formation contains the most immature sandstones with high lithic content (Rubio-Cisneros and Lawton, 2011; Figure 7). The provenance of both the La Boca and La Joya formations evolve from a dissected arc to continental transition (Rubio-Cisneros et al., 2011; Figure 7).

Late Jurassic to Early Cretaceous sandstones from La Casita Formation plot between the fields of transitional continental and recycled orogeny (Figure 7). The feldspar content indicates recycling during the deposition of the La Joya and further enrichment of the overlying formations of Upper Jurassic to Lower Cretaceous (Figure 7). The stratigraphy of Upper Triassic to Lower Cretaceous sandstones northeastern Mexico comprises four main periods of supply from source-areas. Also, the sequence of sandstone composition discriminates three types of basins, starting with the rift, then forearc, and finally strikeslip (Figure 8).

(1) During the Late Triassic, there is a supply of sediment and recycling from crystalline basement rocks and Paleozoic sedimentary carapace. Samples of Upper Triassic discriminate into a rift basin against all other data, increasing positive values of Qp, Qt, and Ls. Sandstone composition associates with high indices from metamorphic lithics, polycrystalline and monocrystalline quartz. Rock fragments derive from low–middle-grade metamorphic, volcanics, and in minor proportion sedimentary rocks.

(2) The Early–Middle Jurassic sandstones have an abrupt change in the composition of source-rocks from newly formed source-areas. The Lower Jurassic sandstones discriminate into a forearc basin, with significant increments of Lv, F, and Qp concerning volcanic fragments, feldspars from volcanic and metamorphic sources (Figure 7 and 8).

(3) During the Middle Jurassic, most unstable framework components progressively disappear with subsequent low-rate recycling of Upper Triassic and Lower Jurassic formations, explained by the rise of Qm and the abundance of monocrystalline quartz with syntaxial overgrowths.

(4) Throughout the Late Jurassic to Early Cretaceous, tectonic activity recycled the preexisting sedimentary sequence in northeastern Mexico, predominating strike-slip basins. The Upper Jurassic sandstones increase in feldspars, volcanic, and metamorphic rock fragments produced by a small lapse of exhumation/unroofing of plutonic and basement rocks. Further on, the sedimentary lithics and monocrystalline quartz content prove moderate recycling of sedimentary rocks (Figure 7). The modal composition of Lower Cretaceous sandstones indicates an increase in feldspar content, volcanic detritus, and metamorphic sources. The composition correlates to the formation of new source-rocks (Figure 7).

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Qm

Q

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F

Tra nsi ti

LJ Al

LC Mixed

Dissected Arc

37

nt u plif t

Recycled Recycled Orogen Orogen

Transitional Arc

Transitional recycled

Transitional Arc

Undissected Arc

L

50

50%

LB

me

25

Dissected arc

Ba se

13

Ba sem

ent U

plif

t

LB

Qu recyartzose cled

AL Al

LC

ona l co ntin ent al

Cra

LJ

Cra ton

El Alamar Formation (Al) La Boca Formation (LB) La Joya Formation (LJ) La Casita Formation (LC)

ton

45

Tra nsi tion al C

ont ine nta l

18

in te rio r

Explanation

Int

erio

r

3

50%

F

Lithic Undissected recycled arc

Lt

Figure 7: Ternary diagrams QFLt and QmFLt with confidence regions for tectonic environments (Dickinson et al., 1983). The scattered compositional data is for Late Triassic–Early Cretaceous clastic sequences in northeastern Mexico (Rubio-Cisneros and Ocampo-Díaz, 2010).

DF 1

0.00

1.00

0.75

0.25

0.50

0.50

0.25

0.75

0.00

1.00

DF 2

Explanation

DF 3

Forearc Accretionary Collisional Intra-arc

Back-arc Continentalmargin arc Rift Strike-slip NE Mexico Formations El Alamar Fm. La Boca Fm. La Joya Fm. La Casita Fm.

Figure 8: A ternary diagram with the discriminant functions in its vertices and confidence regions for the composition of nine basins analyzed. The figure overlaps the petro-tectonic evolution of clastic sequences of Late Triassic to Early Cretaceous in northeastern Mexico.

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5. DISCUSSION The framework composition of sandstones from Late Triassic to Early Cretaceous in northeastern Mexico served for testing a discriminant diagram. The analysis indicates the development of rift, forearc, and strike-slip basins (Figure 8; Table 4). Previous works reported rift sequences for Late Triassic in the region (cf., Michalzik, 1991; Rubio-Cisneros, 2008a; Rubio-Cisneros, 2008b; Barboza-Gudiño et al., 2010, Dickinson and Gehrels, 2010; Ocampo-Díaz, 2011). The discriminant function in this work suggests a partial relationship to Late Triassic samples in a rift system. The framework petrography remarks positive indices of metamorphic lithics and quartz (Qt, Qp, and Qm). Sources are rocks of low–midgrade metamorphism, volcanic, and in minor proportion sedimentary rocks.

Considering that our data from the rift samples represent the west of the southernmost expression for the circum-Atlantic rifting, the composition of sandstones correlates to no magmatic history of lithosphere that thinned at preexisting weak zones. Else, the samples tend to belong to strike-slip basins (Ocampo-Díaz, 2011; Ocampo-Díaz et al., 2014). The Late Triassic Alamar Formation lacks in composition associated with bimodal volcanism, which is evidence for an amagmatic extension over a dense crust of crystalline basement.

We suggest a strict rift laying more to the east in the Gulf of Mexico, were rifting started under a tectonic force by magmatic accommodation and further subsidence (Buck, 2004).

Back in northeastern Mexico, the thermally dense lithosphere thinned followed by uplifting source-rocks and a type of rift sequence formed from isostatic subsidence.

During the Early to Middle Jurassic, there was a magmatic arc with extensional activity developing a forearc basin. New source-rocks supplied the basin, showing the most significant increments of Lv, F, and Qp related to volcanic components. Besides rock fragments from volcanic genesis are feldspars from metamorphic sources. In the course of this event, some angular unconformities developed after thinning the low-density crust.

The framework composition and the values of the Sedimentary Recycling Index (SeReIn) of La Boca and La Joya formations correlate with an unconformable stratigraphy (OcampoDíaz and Rubio-Cisneros, 2013; Table 4). Perhaps this long-term effect of stretching continental lithosphere turned into regional subsidence proven by angular unconformities in most outcrops across northeastern Mexico. Some unstable framework components in La Joya Formation disappear upsection while recycling from underlying units ceased all supply; however, monocrystalline quartz (Qm) occurs with syntaxial overgrowths.

Subsidence accommodated a thick sedimentary sequence of La Casita Formation, forming a strike-slip basin on Late Jurassic–Early Cretaceous (Ocampo-Díaz et al., 2014). Rock fragments in sandstones like feldspars, volcanic, and metamorphic lithics eroded from plutonic rocks, basement units, and sedimentary sources.

The data presents some degree of scattering, and it seems plausible that previous works classified the types of framework components using limited compositional parameters.

Also, unconsolidated sands rather than sandstones may be a factor that introduced some bias into the analysis (e.g., Dickinson et al., 1983). Albeit these challenges, the library of framework petrography (QmFL) solved compositional indexes in a discriminant analysis for correlating sand and sandstones to a basin.

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Basins associated with suture zones were excluded from this work for its lack of statistical representation since only three samples were available in the literature (e.g., Garzanti et al., 1996). Any additional statistics require the use of more sandstones for better confidence in explaining the compositional discrimination with the type of suture.

6. CONCLUSION

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Herein we test how framework composition and statistical parameters discriminate the provenance of sandstones among detrital populations from nine types of basins.

The method in this work uses a ternary diagram in conformity to some conventional representations for sandstone provenance. The diagram is suitable for either ancient or recent clastic successions to plot sands and sandstones in a sedimentary basin. In general, the mean modal compositions of sandstones can distinguish among rock components in basins with newly formed source-rocks and the process of unroofing. Meanwhile, other compositional indices discern basins where deeper structural levels are brought to the surface and eroded.

Each vertex from the ternary plot is a discriminant function, representing an increase in specific compositional parameters 1) Qt, Ls, and Qp support the processes for sediment transport, the exhumation of the sedimentary carapace, and a moderate input of metamorphic rocks at rift systems, suture complex basins, foreland, and forearc basins; 2) Qp, Lm, F, and Qm relate to moderate–high sediment transport, and the exhumation of plutonic or metamorphic source-lands at forearcs, collisional zones, continental-margin arcs, and strike-slip basins; and 3) Lv, F, Qp, and Ls mark new source-areas and recycling of ancient clastic sequences in basins such as back-arc, intra-arc, strike-slip, continentalmargin arcs, and forearc.

The dataset is a compilation of different works, considering the limits on the interpretation of sands or sandstones by the authors. Some restrictions in the data are sampling method or how the author classified samples from its framework petrography (e.g., Dickinson, 1970).

Any sample not corresponding to a compositional field is discarded from the analysis and does not qualify for any specific type of basin. We consider that at each discrete point in the stratigraphy, the composition of the basin is not limited by time, undertaking instead lateral variability in the stratigraphy driven by autogenic processes. Therefore, samples not corresponding to any compositional field at every time in the evolution of the crust fit neither basin nor the “hybrid” category.

The tool of this method supports some interpretations of the composition of sandstone, source-rocks, tectonic setting, and processes within basins. The analysis correlates framework compositions to some genetic factors in basins; endogenic or exogenic events may match for specific stratigraphic records of sandstones. Besides, its application reduces ambiguities in sandstone provenance analysis with the parent basin and improves the history of well-established tectonic settings.

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Nowadays it exists controversy about the tectonic architecture and evolution of suture basins, either by the amalgamation of multiple terranes or accretion regarding a single forearc (Ingersoll and Schweickert 1986; Mossakovsky et al., 1993; Sengör et al., 1993; Busby and Ingersoll, 1995). This work leaves an open possibility to examine the connection between the composition of the orogenic source and its basin. For a better understanding of Suture zone basins, consult Dewey (1977), Miall (1999), and Todd (2000).

Any interpretation of sandstone provenance may be misled if not considering the type of basin or tectonic settings that produced them, even if extensive knowledge is available about source-rock lithologies, sediment texture, and modal composition. The analysis in this work shares the same fundaments of multivariate statistics and discriminant functions in the SeReIn, but for basin discrimination. Successive geologic events at any tectonic setting can significantly affect detrital modes in sands and sandstones differently among basins.

Compositional variation in basins reflects an increase in the heterogeneity of principal components in sandstones. The sedimentary indexes provide sufficient data to guide a better interpretation of source areas, basins, unconformities, and loading histories.

ACKNOWLEDGMENTS This research was partly funded by stewardships of CONACyT, DAAD-GOAL, IAS, and SEPM. The authors appreciate the help from Martín Guerrero-Suastegui, Margarita Martínez-Paco, Huna, Ixba, Abhijit Basu, Nadia Rubio-Cisneros, John Holbrook, Timothy Lawton, Eduardo Garzanti, and Juan Alonso Ramírez Fernández. We are also thankful for the mathematical support given by Aarón Ulises Ocampo-Díaz, Roberto Soto-Villalobos, and Ernesto Sotuyo.

Finally, we acknowledge Pilar López Rodríguez and Juan Rogelio Román Ramos for reviewing and improving the writing.

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Analysis of mineralogy and porosity on a carbonaceous mudstone of the Pimienta Formation, western margin of the Tampico Misantla Basin, Mexico Análisis de mineralogía y porosidad en mudstone carbonáceo de la formación Pimienta, en el margen centro-oeste de la Cuenca Tampico Misantla, México

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Carlos Vega-Ortiz(a)1 , Bryony Richards(b), John D. McLennan(a,b), Raymond Levey(b), Néstor Martínez-Romero(c)

(a)

Department of Chemical Engineering, The University of Utah, Salt Lake City, UT 84112

(b)

Energy & Geoscience Institute, The University of Utah, Salt Lake City, UT 84108

(c) 1

Universidad Nacional Autónoma de México, Mexico

Correspondencia autor principal: carlos.vega@utah.edu, Carlos Vega Ortiz

ABSTRACT

The Pimienta Formation of the Tampico Misantla Basin offers a high potential for unconventional reservoir development. Among other attributes, this is due to its organic properties and sedimentary depositional environment. It is well known that the porosity and permeability of shales differ significantly from those of conventional reservoirs; therefore, a proper accurate characterization at micro- and nanoscales is valuable for understanding the implications of the petrophysical microstructure, and porosity distribution on the storage of hydrocarbons, fluid flow, and permeability. A key observation is the identification of the mineral composition depending on the location and depth of sampling with respect to the geologic structure and sedimentary environment. A total of 12 samples of the carbonaceous mudstone of Pimienta Formation, obtained from Mexico’s Geological Core Repository (Litoteca Nacional CNH), provide representative coverage of the analyzed block at the foothill of the Sierra Madre Oriental. A series of Scanning Electron Microscopy (SEM) images provide detailed analyses of the chemical composition, morphology, organic matter, and porosity distribution.

RESUMEN

La formación Pimienta de la Cuenca Tampico Misantla ofrece un alto potencial para el desarrollo de yacimientos no convencionales. Entre otras propiedades, se debe al alto contenido orgánico y el ambiente de depositación sedimentario. Es ampliamente conocido que las características de permeabilidad y porosidad son diferentes a las de yacimientos convencionales, por lo que es necesario elaborar una precisa descripción a escalas micro y nano dimensionales para el entendimiento de las implicaciones en las propiedades petrofísicas, así como también de la distribución de porosidad, para efectos de almacenamiento de hidrocarburos, transporte de fluidos y permeabilidad. Es de particular interés identificar la composición mineralógica de acuerdo con la localización y profundidad de la formación dentro de la estructura geológica de la cuenca y el ambiente de depositación. Se analizan un total de 12 muestras del mudstone carbonáceo de la formación Pimienta, obtenidas de la Litoteca Nacional de la Comisión Nacional de Hidrocarburos, las cuales proveen una cobertura representativa de la zona prospectivas analizada al pie de la Sierra Madre Oriental. Una serie de pruebas de 34

caracterización como son imágenes de microscopia de barrido con capacidad de análisis mineralógico, así como pruebas de difracción de rayos x, proveen un análisis detallado de la composición química, morfológica, materia orgánica y distribución de porosidad.

1. INTRODUCTION / INTRODUCCIÓN The development of unconventional reservoirs in Mexico is a topic of great interest provided by the successful unconventional shale oil and gas developments in North America (Bowker, 2007; Pollastro, 2007; Stevens and Moodhe, 2015). There is anticipation of high economic potential for the Jurassic Pimienta Formation in the Tampico Misantla Basin (TMB), as it has good quality source rock with potential for production (GranadosHernández et al., 2018; Jarvie and Maende, 2016; Morelos-García, 1996; Vega-Ortiz et al., 2020a) comparable to successful unconventional plays elsewhere in the world (US-EIA, 2013). As of 2019, the estimated reserves in the TMB are 1P 804 MMBOE, 2P of 3098 MMBOE, and 3P of 5570 MMBOE (CNIH Reservas, 2019). In 2015, Mexico’s Secretary of Energy (SENER), collaborating with Mexico’s National Hydrocarbon Commission (CNH), issued a 5-year development plan establishing areas of prospective fields including conventional and unconventional reservoirs in order to expand the country’s hydrocarbon production (SENER, 2019).

El desarrollo de yacimientos no convencionales en México es un tema de gran interés, dado el éxito logrado en yacimientos de shale oil y shale gas en EUA (Bowker, 2007; Pollastro, 2007; Stevens and Moodhe, 2015).

Se anticipa que la formación Pimienta del Jurásico Titoniano Superior tiene un alto potencial económico dadas las buenas condiciones de calidad de roca generadora (Granados-Hernández et al., 2018; Jarvie and Maende, 2016; Morelos-García, 1996; Vega-Ortiz et al., 2020a), y su potencial de producción que es comparable con plays de yacimientos no convencionales en diferentes zonas del mundo (US-EIA, 2013). A la fecha de la redacción de este documento, 2019, las reservas estimadas en la Cuenca Tampico Misantla (CTM) son 1P = 804 MMBOE, 2P = 3098 MMBOE, y 3P = 5570 MMBOE (CNIH Reservas, 2019). En 2015, la Secretaria de Energía (SENER), en colaboración con la Comisión Nacional de Hidrocarburos (CNH) de México, emitieron el Programa Quinquenal de Licitaciones para la Exploración y Extracción de Hidrocarburos 2015-2019, en el cual se establecen las zonas prospectivas de yacimientos no convencionales, con lo que se busca expandir las capacidades de producción del país. (SENER, 2019).

The area of study (AOS) designated for this study is a block that covers an area of 69.6 km2, located in the central-western sector of the TMB. The AOS is at 150 km to the NW of the prolific Chicontepec Basin and Golden Lane field (CNH-SENER, 2010; Viniegra and Castillo-Tejero, 1970), including the territory of San Luis Potosi and Veracruz states (Fig 1). It is geographically delimited in the north by the Tamaulipas Arch, to the east by the Gulf of Mexico, to the south by the Teziutlan Masif and the Veracruz Basin and to the west the by the Sierra Madre Oriental (Fitz-Díaz et al., 2018). The AOS was explored for commercial development of conventional zones with 42 wells drilled from 1912 to 2000, although no successful targets were encountered (CNH México, 2020; López-Ramos, 1952). Geological samples of the Pimienta formation are available in only seven wells, with location shown in Figure 1. Provided the crescent interest in the international oil and gas market towards unconventional reservoirs development, it is of critical importance to perform thorough technical analysis in these legacy areas to elaborate on the previous assessments of reserves quantification and production feasibility in Mexico.

Geográficamente la CTM es delimitada al norte por el Arco de Tamaulipas, al oriente por el Golfo de México,, al sur por el Macizo de Teziutlán y la Cuenca de Veracruz, y al occidente por la Sierra Madre Oriental (SMO) (FitzDíaz et al., 2018). El área de estudio (AE) designada para este análisis es un bloque que cubre un área de 69.6 BOLETÍN DE LA ASOCIACIÓN MEXICANA DE GEÓLOGOS PETROLEROS

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km2, ubicado en el sector centro-occidental de la CTM, al piedemonte de la SMO. El AE se encuentra a 150 km al NO de la prolífica Cuenca de Chicontepec y de la Faja de Oro (CNH-SENER, 2010; Viniegra and CastilloTejero, 1970), dentro de los límites territoriales de los estados de San Luis Potosí y Veracruz (Fig 1). El AE fue explorada para desarrollo comercial de yacimientos con 42 pozos perforados desde los años 1912 hasta el 2000, sin haber encontrado algún yacimiento con condiciones aptas para producción convencional (CNH México, 2020; López-Ramos, 1952). A la fecha de elaboración de este estudio, en la Litoteca Nacional únicamente hay existencia de muestras geológicas de la formación Pimienta de siete pozos, cuya localización se muestra en la figura 1. Dado el creciente interés de los mercados petroleros internacionales hacia el desarrollo de yacimientos no convencionales, es de suma importancia realizar un análisis desde la perspectiva actual para evaluar zonas potencialmente productoras omitidas al momento de perforación, y contribuir con información técnica para la evaluación del yacimiento, cuantificación de reservas y factibilidad de producción en México.

Figure 1. Location of the area of study, located in the western-central edge of the Tampico Misantla Basin. Location of the seven wells with Pimienta fm samples available and granted for this study. (Base Map from CNIH Mapas (2020) and (SGM, 2020)).

Figura 1. Localización del area de estudio, ubicada en el margen centro-occidental de la cuenca Tampico Misantla. Ubicación de los siete pozos con muestras disponibles de la formacion Pimienta, facilitadas para la realización de este estudio (Mapas de referencia CNIH Mapas (2020) y (SGM, 2020)).

In present-day terminology, unconventional reservoirs refer to a scenario where the source rock also acts as reservoir rock (Mccarthy et al., 2011). The organic content of a rock is one indicator of its potential as the source rock for hydrocarbon extraction (Jarvie, 2015; Jones, 1984; King et al., 2015). The key of the success of unconventional oil and gas resources relied on the systematic implementation of hydraulic fracturing techniques, aiming to increase the area of flow from the reservoir into the well (Algadi et al., 2014; Fernandez-Badessich et al., 2016; Jabbari, 2013). The proper planning and execution of the drilling, fracking, and completions requires the thorough understanding of the geological formation characteristics (Rybacki et al., 2016; Zhang et al., 2016), as no all shale reservoirs can be approached in the same manner (Rickman et al., 2008). Additional to recognizing the inherent low permeability and porosity is necessary to characterize porosity size, distribution, and interconnectivity, as well as the chemical composition and distribution of the minerals that comprise the Pimienta formation in the AOS, which is the objective of this study.

En terminología actual, los yacimientos no convencionales, comúnmente conocido como plays tipo shale-, se refieren al escenario en donde la roca generadora actúa también como roca almacén (Mccarthy et al., 2011). El contenido orgánico de la roca es uno de los principales indicadores del potencial productor de hidrocarburos (Jarvie, 36

2015; Jones, 1984; King et al., 2015). La clave del éxito en yacimientos no convencionales radica en la implementación sistemática de técnicas de fracturamiento hidráulico, enfocado a incrementar el área de flujo de los fluidos del yacimiento hacia el pozo productor (Algadi et al., 2014; Fernandez-Badessich et al., 2016; Jabbari, 2013). La planeación y ejecución adecuados de los programas de perforación, fracturamiento hidráulico y completaciones requiere de un entendimiento detallado de las propiedades geológicas y petrofísicas de los yacimientos (Rybacki et al., 2016; Zhang et al., 2016), ya que no todos los plays tipo shale pueden ser abordados bajo el mismo criterio (Rickman et al., 2008). Además de identificar la problemática inherente de las reducidas permeabilidad y porosidad, es necesario caracterizar el tamaño de los poros, su interconectividad y distribución, así como también la composición química y distribución de los minerales que conforman la estructura de un play como los es la formación Pimienta, lo cual es el objetivo de investigación en este estudio.

2. GEOLOGICAL BACKGROUND / MARCO GEOLÓGICO The stratigraphic sequence and the structural frame in the AOS discussed in this section is shown in figure 3, which is a North-South fence diagram with the strata identified the wells DD, HH, GG and AA, based on the formation tops from drilling reports and available GR logs.

The depth in this diagram is referred to sea level. The basement of the TMB is a sequence of continental sediments identified as red rocks and metamorphic complex, deposited in the western margin of the Pangea, shoreline with the paleo-pacific ocean.

In the early Jurassic, the breakup of Pangea and it is associated continental rifting produced a sequence of marine transgressional and receding shoreline events (Cantú-Chapa, 2001). The Huayacocotla formation is related to the post-rift shallow marine and basinal environment, identified with black shales and siltstones (Rueda-Gaxiola et al., 1993). Later in the mid-Jurassic, the Yucatan platform southbound drift (Marton and Buffler, 2016) and the opening of the Gulf of Mexico produced a marine regression and a continental basin deposition (Pindell and Kennan, 2009), where the Cahuasas formation was deposited.

La secuencia lito estratigráfica y el marco estructural del AE descrito en esta sección se ilustra en la figura 3, que es un diagrama seccional de los estratos identificados en los pozos DD, HH, HH y AA, basado en las cimas de formación reportadas durante la perforación y los registros geofísicos de rayos gamma (GR) disponibles. La profundidad indicada el diagrama es referenciado al nivel medio del mar. El basamento de la CTM es una secuencia de depósitos sedimentarios continentales que se identifican como lechos rojos y complejo metamórfico, depositado en el extremo occidental de Pangea, en el litoral del paleo-océano pacífico.

En el Jurásico Inferior, la división continental de Pangea y el rift continental asociado originaron una secuencia de eventos transgresivos y retrocesos de la lineal litoral (CantúChapa, 2001). La formación Huayacocotla está relacionado con ambientes post-rift de aguas someras y de cuenca, identificado por las intercalaciones de lutitas y areniscas (Rueda-Gaxiola et al., 1993). Posteriormente en el Jurásico Medio el desplazamiento de la Plataforma de Yucatán hacia el sur y la apertura del Golfo de Mexico, ocasionaron un ambiente de deposición de cuenca continental por el retroceso marino (Pindell and Kennan, 2009), en donde fue depositada la formación Cahuasas.

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The late Jurassic period is dominated by a marine transgression, which created a low-energy, shallow water sea with a reducing environment (Peterson, 1983). According to Goldhammer and Johnson (2001), sediments of fine grain and organic-rich marine lithofacies deposited over preexisting carbonate ramps, that would become the source rock for the petroleum systems in the TMB. The Oxfordian Santiago and Zuloaga formations, identified elsewhere in the TMB are sparsely present in the AOS. The Kimmeridgian produced three different formations; The Taman formation is associated with a shallow marine, oxic to anoxic environment, with a organic content up to 1.6 wt. %, as reported by Vega-Ortiz et al, (2020a) analyzing samples from the well GG. The Chipoco Formation is formed from deposits near the shoreline, deducted from its mixed composition of oolitic limestone, developed in dynamic intertidal waters, and the high content of clastic minerals from fluvial deposition, as observed in the well DD in this study. The calcarenite San Andres formation is interlayered with the Taman formation, and often considered as lateral continuation of the Tithonian Pimienta formation, depending on the area of deposition (Cantú-Chapa, 1992).

El periodo Jurásico Superior fue dominado por una transgresión marina, que genero condiciones marinas someras de baja energía de ambiente reductor (Peterson, 1983). De acuerdo con Goldhammer y Johnson (2001), sedimentos de grano fino y facies marinas de alto contenido orgánico se depositaron sobre rampas de carbonatos preexistentes, que eventualmente producirían las rocas generadoras de hidrocarburos en los sistemas petroleros de la CTM. Las formaciones de Santiago y Zuloaga del Oxfordiano son identificadas en otras zonas de la CTM, sin embargo, con escasa presencia en el AE. El Kimeridgiano produjo tres diferentes facies: La formación Taman depositada en aguas someras de ambiente anóxico, con contenido orgánico de 1.6 wt.% como se reporta en Vega-Ortiz et al, (2020a) analizando muestras del pozo GG. La formación Chipoco es asociada en la zona de rampa del litoral en la Plataforma San Luis-Valles, deducido de la composición mezclada de calizas oolíticas producidas en aguas dinámicas intermareales, y el alto contenido de minerales clásticos de deposición fluvial, observada en el pozo DD de este estudio. La formación calcarenita de San Andrés se encuentra intercalada con la formación Taman, y en ocasiones considerada como continuación lateral de la formación Pimienta del Titoniano, dependiendo del área de depositación (Cantú-Chapa, 1992).

The Pimienta formation was deposited during the Upper Jurassic Tithonian period. A marine transgression from the Gulf of Mexico flooded the area that today corresponds to the TMB (Fig. 2), extending to the Yucatan Peninsula. The flooding event created a shallow marine, low-energy depositional environment (Peterson, 1983), where the organic-rich, carbonaceous mudstone of the Pimienta formation created a homogeneous and conformable stratum on top of the Kimmeridgian (Salvador, 1987). The organic sediments of the Pimienta formation became the source rock for the giant offshore Sonda de Campeche fields in the southern Gulf of Mexico (Magoon et al., 2001; Santamaria Orozco, 2000). In the AOS, the exposed land of the Valles-San Luis Platform (Carrillo-Bravo, 1971) created a shoreline with a marine slope and basin dipping towards the SE into the TMB, allowing the accumulation of kerogen mixed Type II/Type III (Vega-Ortiz et al., 2020a). The typical petrology description of the Pimienta formation in the AOS from drilling reports (CNH México, 2020) is black to dark brown carbonaceous mudstone, with recrystallized fragments, with thin intercalations of black carbonaceous mudstone, light gray carbonates, and gray cryptocrystalline limestone.

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La formación Pimienta fue depositada durante el periodo Jurásico Superior, en la era Titoniano. La transgresión marítima del Golfo de México cubrió el área que hoy en día corresponde a la CTM (Fig. 2), extendiéndose desde el centro de México hasta la Península de Yucatán. La transgresión dio lugar a un ambiente marino somero de baja energía (Peterson, 1983), donde el mudstone carbonáceo rico en materia orgánica de la formación Pimienta, genero un estrato homogéneo y conformable sobre los sedimentos del Kimerigdiano (Salvador, 1987). Dichos sedimentos orgánicos en la formación Pimienta dieron lugar a la roca generadora en los principales yacimientos de México , incluidos los de la Faja de Oro, Cuenca de Chicontepec y la Sonda de Campeche (Magoon et al., 2001; Santamaria Orozco, 2000). En el AE, el bloque expuesto de la Plataforma Valles-San Luis Potosí (Carrillo-Bravo, 1971) originó un litoral y pendiente con buzamiento en dirección SE, hacia el centro de la CTM, permitiendo la formación de kerógeno mezclado tipo II/tipo III (Vega-Ortiz et al., 2020a). La descripción petrográfica de la formación Pimienta en el AE en general se describe como mudstone carbonáceo gris oscuro a negro, con fragmentos recristalizados y laminaciones delgadas de lutitas negras, fracturas selladas con calcita y caliza criptocristalina gris.

Figure 2. Jurassic paleographic map displaying the shallow marine depositional environment of the Pimienta Formation in the Tampico Misantla basin, (Adapted from (Salvador, 1987)) Figura 2. Mapa paleografico del Jurasico, indicando el ambiente de depositacion marino somero de la formación Pimienta. (Adaptada de Salvador, 1987)

During the Cretaceous, the compressional tectonic events resulting from the Farallon plate subduction underneath the North American plate (Martini, 2018) originated the Laramie orogeny that uplifted the Sierra Madre Oriental (SMO). According to Mitrovica et al. (1989), the subsequent tilting of the North American plate formed a foreland basin in the current area of the TMB. In addition to this, the higher eustatic sea levels of the mid to late Cretaceous resulted in a major oceanic transgression originating the North American Interior Seaway (Slattery et al., 2015), and a deep-water environment in the TMB area. In the area of the AOS, a paleographic, morphostructural and stratigraphic study by (Eguiluz de Antuñano et al., 2000), concluded that the Huayacocotla sector of the SMO dominated the structural and sedimentary environment, developing evaporitic carbonate platforms. The Tamaulipas group of the Lower Cretaceous, and the Mendez, San Felipe and Agua Nueva limestones were formed during this period (Aranda-Gomez et al., 2000).

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Durante el Cretácico los esfuerzos compresionales resultantes de la subducción de la placa Farallón por debajo de la placa Norteamericana (Martini, 2018) causaron la orogenia Laramide que produjo el levantamiento de la SMO. De acuerdo con estudios realizados por Mitrivica et al. (1989), la subsecuente inclinación de la placa de Norteamérica originaron una cuenca tipo ante-país en el área de la CTM. Adicionalmente, el elevado nivel eustático del mar en la época Cretácico superior provocó una transgresión marina dando lugar al llamado Mar Interior de Norteamérica (Slattery et al., 2015), y un ambiente de aguas profundas en la región centro-oriental de la CTM. Eguiluz de Antuñano et al. (2000), a través de un estudio paleográfico, morfoestructural y estratigráfico, concluyó que el sector Huayacocotla de la SMO dominó el ambiente estructural y de depositación en el AE, desarrollando plataformas de carbonatos evaporíticos. El grupo estratigráfico de la formación Tamaulipas y los depósitos de carbonatos de las formaciones Méndez, San Felipe, y Agua Nueva, fueron originados en el Cretácico Inferior (Aranda-Gomez et al., 2000).

Finally, in the Cenozoic when the Gulf Of Mexico receded (Cossey et al., 2016), the foreland basin was filled with clastic material that was eroded from the SMO, creating today’s coastal plain and allowing the deposition of conglomerate strata like the Chicontepec formation, where vast quantities of hydrocarbons migrated to produce oil and gas reserves, allowing the development of production fields in the prolific Golden Lane field and Chicontepec Basin (Estrada et al., 2010).

Por último, en la Cenozoico después del retroceso del Golfo de México (Cossey et al., 2016), los depósitos clásticos y fluviales que fueron erosionados de la SMO rellenaron la cuenca antepaís, originando la actual planicie costera y permitiendo la acumulación de conglomerados como la formación Chicontepec, donde vastas cantidades de hidrocarburos emigraron desde las rocas generadoras del Jurásico para formar yacimientos productores de aceite y gas natural en la cuenca de Chicontepec y la zona de la Faja de Oro (Estrada et al., 2010).

3. SAMPLE DESCRIPTION / DESCRIPCIÓN DE LAS MUESTRAS The rock samples of the Pimienta Formation for the mineral analysis were obtained from Mexico’s Geological Core Repository under the authorization of the Ministry of Energy (SENER) and CNH, from the wells in the AOS (Fig. 3). The geological layout of the Pimienta Formation in the AOS includes elevated areas to the NW, dipping slightly towards the SE. X-ray diffraction (XRD) Mineral composition was done in samples from wells AA, BB, DD, EE, GG, HH, and II. Scanning electron microscopy (SEM) microphotographs were taken from wells DD and GG. No preservation technique used on the samples during storage.

Las muestras geológicas para este estudio fueron obtenidas de la Litoteca Nacional de la Comisión Nacional de Hidrocarburos, con autorización de la Secretaria de Energía (SENER) como convenio académico para tesis doctoral. La localización de la muestras y la clave de los nombres de los pozos se indica en la Figura 3. La estructura geológica en el AE muestra una elevación en el sector NO, con buzamiento hacia el SE. Análisis de difracción de rayox X se realizó en las muestras de los pozos AA, BB, DD, EE, GG, HH e II. Toma de imágenes con microscopio de barrido electrónico SEM, fueron tomados en los pozos DD y GG. 40

Figure 3. Geological structure of the Pimienta Formation in the Area of Study (AOS). The northeastern has higher elevation due to its proximity to the Valles-San Luis Platform shoreline. The thickness of the Pimienta formation varies from 40 to 120 m Figura 3. Estructura del área de estudio delineado con líneas isocronicas. El área noroccidental tiene una mayor elevación debido a la cercanía con el litoral de la Plataforma Valles-San Luis. El espesor de la formacion Pimienta varia entre 40 y 120 m.

4. METHODS / MÉTODOS 4.1 X-RAY DIFFRACTION (XRD) / DIFRACCIÓN DE RAYOS X (XRD)

X-ray diffraction (XRD) analysis was completed at the Energy and Geoscience Institute at the University of Utah using a Bruker D8 Advance and subsequently interpreted using Topaz software. The bulk sample preparation for XRD consisted of grinding and milling to a particle size of < 44 μm by using a 325-mesh screen. Clay sized particles were further separated to 5 μm size fraction by mixing the powder with deionized water, stirring, and allowing to settle for 37 min. Subsequently, a 100 ml aliquot is centrifuged at 1500 rpm for 15min. The resulting slurry is mixed with ethylene glycol and dried in an oven at 65°C for 12 hrs. The operating parameters for Bruker DS8 diffractometer are 0.02o2θ step size, 40 kV and 40 mA for CU-K-α radiation, and 0.6 and 0.4 seconds per step, for bulk and clay samples, respectively.

El análisis de difracción de rayos X (XRD) fue realizado en el Instituto de Energía y Geociencias (EGI) de la Universidad de Utah, empleando un instrumento Bruker D8 Advance y utilizando el software Topaz para la interpretación de resultados. La preparación de muestras crudas consistió en molienda y trituración a un tamaño de partículas < 44 μm usando una pantalla de malla 325, seguida de una segunda separación para partículas arcillosas de 5 μm, mezclando el polvo con agua desionizada, agitando y permitiendo decantación de sólidos por 37 min. Posteriormente una alícuota de 100 ml es centrifugada a 1500 rpm por 15 min.

La mezcla resultante es combinada con etilenglicol y secada en un horno a 65 °C por 12 hrs. Los parámetros de operación del instrumento de difracción Bruker 8 son 0.02o2θ de incremento de fase, 40 kV y 40 mA para la radiacion CU-K-α, así como 0.6 y 0.4 segundos por paso, para las muestras crudas y de arcillas, respectivamente.

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Energy-Dispersive X-Ray Spectroscopy (EDS) measures the x-ray wavelength of the photons emitted after an incident electron interacts with the electrons on the surface of a mineral. Each element will produce a single x-ray, which is used as a signature for the identification of that particular chemical element present in the sample. The EDS analysis was completed with an X-ACT PentaFet Precision detector at the University of Utah’s Energy & Geoscience Institute (EGI). The samples were polished in a Struers Tegrapol-21, using a 220 grit polishing disc with water as a lubricant.

La técnica de Espectroscopía de Energía Dispersiva, (EDS por sus siglas en inglés EnergyDispersive Spectroscopy) mide la longitud de onda de los rayos-X que son emitidos después de la interacción de un electrón incidente sobre la superficie de un mineral, enfocado en detectar presencia de elementos químicos y elaborando un mapa de la distribución del elemento en la zona analizada.. El análisis EDS se realizó en los laboratorios el EGI con un instrumento X-ACT PentaFet Precision Detector. Las muestras fueron preparadas con desbaste en la máquina Struers Tegrapol-21, con un disco pulidor de 220 grit, usando agua como lubricante.

4.3 SCANNING ELECTRONIC MICROSCOPY (SEM) / MICROSCOPIA DE BARRIDO ELECTRÓNICO

The SEM images were obtained using a FEI Quanta 600F scanning electronic microscope, located at the Utah Nanofab, University of Utah. This Quanta SEM uses a high-resolution field emission source (FEG) to analyze samples, allowing higher-magnifications and resolution than a standard tungsten SEM. The samples analyzed were uncoated and unpolished, in order to observe the topography of the surfaces in their natural state after splitting the rock sample into small pieces suitable as SEM specimens. The settings for the microscope were adjusted to mitigate charging effects as much as possible: electron beam 2 KeV, Spot 5, high vacuum pressure, working distance WD 10.

Las imágenes de microscopía electrónica fueron obtenidas en un instrumento SEM FEI Quanta 600F, en el laboratorio NanoFab de la Universidad de Utah, de alto vacío y fuente emisora de alta resolución. Las muestras no tuvieron preparación, sin pulido ni recubrimiento, para observar la topografía y textura de las rocas en su estado natural después de ser fracturadas en pequeños fragmentos de ~ 2 mm. Los parámetros de operación del SEM fueron ajustados para mitigar los efectos de saturación electroestática: haz de electrones de 2 KeV, spot 5, alto vacío, distancia operativa WD 10.

5. RESULTS / RESULTADOS 5.1 XRD MINERALOGY / MINERALOGÍA XRD

The XRD mineralogy analysis of the samples indicates that the predominant mineral content in the Pimienta formation is carbonates; the full results included in Table 1 and XRD diffractograms shown in Appendix A. Calcite (CaCO3) is the most abundant mineral overall, with an average of 70.4 wt. %, dolomite (CaMg(CO3)2) is also present in a lower quantity with 1.3 wt. % 42

average content and Mg-rich calcite ((Ca,Mg)CO3) found in the well EE at 1.0 wt. %. The sandstone is composed of quartz (SiO2) with an average of 16.9 wt. %. The metamorphic tectosilicates of plagioclase (NaAlSi3O8 – CaAl2Si2O8) and K-Felspar (KAlSi3O8) are considered as part of the sandstone mineral composition, with an average of 0.74 and 2.28 wt. %, respectively.

A relevant observation is that the K-feldspar occurred only in the shallower wells DD, BB and EE, and the plagioclase is observed in the deeper wells HH, GG, II, and AA. Clay minerals are identified as illite (K 0.65Al 2.0[Al 0.65Si 3.35O 10](OH) 2), the most abundant constituent with an average of 6.4 wt. %, and kaolinite (Al2(Si2O5)(OH)4) only present at the deepest sample obtained from well AA, with 2 wt. % mineral abundance. Traces of interlayered illite-smectite ((Na,Ca)0.33(Al,Mg)2(Si4O10)(OH)2 · nH2O) was measured from wells HH, GG and AA, being the illite to smectite proportion greater than 90 %. Other lower quantities of minerals were identified as traces of pyrite (FeS2), present in most of the samples, barite (BaSO4) with a content of 1 wt. % in the well BB, and anhydrite (CaSO4), with a relatively high average concentration of 3 wt. % in the shallower well DD, and smaller quantities in wells GG and AA. El análisis de mineralogía XRD indica que el mineral predominante en las muestras de la formación Pimienta son carbonatos. La tabla 1 muestra la lista del contenido mineral, los respetivos difractogramas están en el Apéndice 1. La calcita (CaCO 3) es el mineral más abundante, con un promedio de 70.4 wt.%. Dolomita (CaMg(CO3)2) también está presente en menor proporción con 1.3 wt.%, destacando la muestra del pozo EE en donde se encontró calcita rica en Magnesio ((Ca,Mg)CO3) con 1 wt.%. Las areniscas se identifican con el mineral de cuarzo (SiO2) con un contenido promedio de 16.9 wt.%. Los tectosilicatos metamórficos de plagioclase (NaAlSi3O8 – CaAl2Si2O8) y K-Felspato (KAlSi3O8) se consideran como parte de la composición de areniscas, con una mínima cantidad de 0.74 y 2.28 wt.%, respectivamente.

Una observación relevante es que el K-Feldespato ocurre en los pozos DD, BB y EE, que son los que se encuentran en el sector elevado NO del AE,

mientras que el Plagioclase está contenido en los pozos HH, GG, II y AA. Los minerales arcillosos identificados son Ilita (K0.65Al2.0[Al0.65Si3.35O10](OH)2), siendo el más abundante con 6.4 wt. % promedio, y Caolinita (Al2(Si2O5)(OH)4) con 2 wt.%, identificado en el pozo AA. Trazas de Ilita-Esmectita interlaminada ((Na,Ca)0.33(Al,Mg)2(Si4O10)(OH)2 · nH2O) fue detectada de las muestras de los pozos HH, GG y AA, siendo la proporción Ilita a Esmectita de más de 90%. Otros minerales identificados en menor cantidad son Pirita (FeS2), el cual está presente en todas las muestras, Barita (BaSO4) con contenido de 1 wt.% en el pozo BB, y Anhidrita (CaSO4), con una concentración relativa alta de 3 wt.% en el pozo somero DD, y en menor cantidad en los pozos GG y AA. Different authors have reported the implications of different minerals in the reservoir and source rocks.

Diagenetic filamentous illite reduces permeability by coating the pore walls (Pallatt et al., 1984). Swelling clays like smectite may increase the pore pressure by altering the geological stress distribution in the formation. Anhydrite reacts with organic matter through the reaction CaSO4 + 2CH2O = CaCO3 + H2O + CO2 + H2S, producing calcite that can be leached into the porosity of the rock, and sulfidic acid that becomes soluble and generating sour oil and gas (Selley, 1997). Kaolinite is also a cementing clay that will occupy the pore spaces after precipitating from the solution. In terms of wellbore stability and design of drilling fluid systems, appropriate fluids like oilbased mud and silica-based mud (Soric et al., 2004) might be considered in those regions where mineral clay and anhydrite content makes the borehole susceptible to swelling. A detailed analysis of the effect of mineral composition on the geomechanical properties of the Pimienta formation trough multistage triaxial tests is presented by Vega-Ortiz et al.

(2020b).

Diferentes autores han reportado las implicaciones de los minerales contenidos en las rocas generadoras y almacenadoras. La ilita diagenética filamentosa reduce la permeabilidad cuando recubre las paredes de los poros (Pallatt et al., 1984). Algunas arcillas como la esmectita se dilatan en contacto con agua, lo cual altera la distribución de esfuerzos geológicos

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en la roca y por consecuencia modificando la presión de poro. La anhidrita reacciona con materia orgánica a través de la reacción CaSO 4 + 2CH 2O = CaCO 3 + H2O + CO2 + H2S, produciendo calcita que se filtra en la porosidad de la roca sellando fracturas y poros interconectados, y ácido sulfhídrico que se mezcla con los fluidos orgánicos de la formación produciendo crudo de alto contenido de azufre (Selley, 1997). La caolinita también es un agente cementante cuando se precipita de la solución acuosa, que reduce la permeabilidad de la roca. En términos de estabilidad geomecánica del pozo y diseño de fluidos de perforación en las zonas donde existen arcillas susceptibles a dilatación. Un análisis detallado de los efectos de la composición mineral en las propiedades geomecánicas de la formación Pimienta a través de análisis triaxial multi-etapa es presentado por VegaOrtiz et al. (2020b).

X-ray Diffraction (XRD) results are also presented in a mineral composition ternary diagram (Fig. 4), grouping the different constituents according to the Mineral classification from (Dana et al., 1997): Carbonates (14: ANHYDROUS NORMAL CARBONATES), Sandstones (75: TECTOSILICATES Si Tetrahedral Frameworks and 76: TECTOSILICATES Al-Si Framework) and Clays (71: PHYLLOSILICATES Sheets of Six-Membered Rings). The ternary diagram is superimposed over the “sCore: A Classification for Organic Mudstones”

by Gamero-Diaz et al. (2013). The distribution of the mineral composition confirms the classification of the Pimienta formation as ‘silica-rich carbonate mudstone, rather than the more generic term ‘shaly limestone”, as reported in drilling reports (CNH México, 2020). Carbonates dominate the composition for all samples. The clay -illite- content remains relatively constant for all the samples averaging 6.6 wt. %, with a minimum of 3 wt. % and a maximum of 23 wt. %.

In contrast, the sandstone content ranges from 2 to 41 wt. %, averaging 19.9 wt. %. A noticeable trend in the quartz content is based on the burial depth of the Pimienta formation; shallower samples at wells BB and DD contain higher quartz content, compared to the rest of the wells in deeper areas.

Los resultados del análisis XRD también son presentados en el diagrama ternario de composición 44

mineralógica (Fig 4), consolidando los minerales observados en tres grupos minerales de acuerdo a la clasificación de Dana: Carbonatos (14: Carbonatos normales anhídridos), Areniscas (75: Tectosilicatos – Estructuras de Silicio) y Arcillas (71: filosilicatos – laminas o aros hexagonales). El diagrama ternario es superpuesto sobre la clasificación “sCore: clasificación de mudstones orgánicos” de Gamero Diaz, 2013, que identifica a la formación Pimienta como mudstone carbonáceo, lo cual es una descripción más adecuada comparada con la comúnmente conocida de caliza arcillosa (shaly limestone) como se observa en los reportes de perforación. Los carbonatos dominan la composición mineral. Las arcillas se cuantifican en un rango de 3 a 23 wt.%. En contraste, las areniscas se encuentran en un rango de 2 a 41 wt.%. Una tendencia relevante es el contenido de cuarzo basado en la profundidad de la formación Pimienta; los pozos más someros BB y DD tienen mayor contenido de cuarzo, comparado con el resto de los pozos en áreas profundas.

5.2 ENERGY-DISPERSIVE X-RAY SPECTROSCOPY (EDS) / ESPECTROSCOPÍA DE ENERGÍA DISPERSIVA The sample selected for EDS analysis is a segment of the core ID 261344, recovered from the well DD. According to the mineral content the bulk of the sample is “mixed mudstone”. XRD mineral composition for this sample (Table 1) is Calcite 50 wt. % , Quartz 36 wt. %, K-Feldspar 6 wt. %, Illite 5 wt. %, Anhydrite 2 wt. %, and traces of Pyrite. Fig A6 shows the XRD Difractogram of this sample. Form the core slab fragment (Figure 5) the texture indicates an extremely fine-grained compact black mudstone, showing a cemented fracture that is inclined to bedding, indicating that the infilling material was transported and precipitated after deposition. A ~1 mm thick vein of calcite and a thinner occurrence of pyrite were deposited in the fracture. Considering that the density of pyrite (5.01 g/c3) is higher than calcite (2.71 g/c3), it may be assumed that the pyrite occupies the lower side of the fracture. Figure 5 also display the wireline logs at the interval where the core was retrieved, cutting through a ~4m thick layer of limestone, identified from the shift in the GR curve

to 100 GAPI with respect to the shale line (200 GAPI), a near-zero porosity, overlap of density and porosity curves provided with the appropriate compatible scale in the log header. Also displayed the results from pyrolysis analysis with an average TOC = 1.2 wt.%. Note that top of the Pimienta formation is taken from drilling reports, although the GR curve shows a transition from sand line at a shallower depth.

Las muestras seleccionadas para el análisis EDS es de un fragmento del núcleo ID 261344, recuperado del pozo DD. Se identifica como mudstone mixto, de acuerdo con la composición mineralógica que consiste en Calcita 50 wt. % , Cuarzo 36 wt. %, K-Feldspato 6 wt. %, Ilita 5 wt. %, Anhidrita 2 wt. %, y trazas de Pirita, El difractograma del EDS se muestra en la Fig. A6. En el fragmento del núcleo usada en el análisis EDS (Fig 5) se aprecia la textura de los granos es muy fina y compacta. Se distingue una fractura la cual se desarrolla en un alta pendiente, cuasi-paralela al eje longitudinal del núcleo (y del pozo, el cual es vertical). Los minerales en solución de agua que infiltraron la fractura fueron precipitados, cementando con calcita y pirita como se observa en la vena de ~ 1mm de espesor. Considerando que la densidad de la pirita (5.01 g/c3) es mucho mayor que la calcita (2.71 g/c3), se puede asumir que la pirita ocupa el espacio inferior de la fractura. En la Figura 5 se presentan los registros geofísicos del intervalo donde fue obtenido el núcleo, penetrando una capa de ~4m de espesor de caliza (limestone), identificada por una reducción el de GR a ~100 GAPI (comparado con la línea de lutitas a ~200 GAPI), nula porosidad NHPI, el empalme de curvas densidad y porosidad en el segundo canal con las escalas adecuadas compatibles, así como un incremento en la curva DT de sónico de porosidad.

Se observa una ligera diferencia en las curvas de resistividad siendo la curva profunda RT ~20 ohm-m más resistiva que la curva superficial RS. Así como también se muestra el contenido orgánico total, obtenido por pruebas de pirolisis hídrica en esta muestra con promedio de TOC = 1.2 wt.%.

K-Feldspar

Illite

36

6

5

261342 (DD)

34

1

23

15

261331 (BB)

51

4

41

1

260551 (EE)

84

6

261338 (BB)

95

3

2

261049 (HH)

65

tr

21

1

13

260973 (GG)

89

1

5

Tr

4

tr

261318 (GG)

70

19

2

6

tr

1

261341 (GG)

51

2

34

5

7

1

261011 (II)

94

2

3

261006 (II)

96

2

2

261347 (AA)

66

1

10

1

1

17

Barite

Quartz

tr

Pyrite

Dolomite

50

Kaolinite

Calcite

261344 (DD)

Mg Calcite

(Well)

Sample ID

Anhydrite

Sulfite/ates

% Illite in I/S

Clays

Illite/Smectite

Sandstones

Interlayered

Carbonates

GROUP

Plagioclase

MINERAL

tr

2

23

1

4

3

tr

8

tr tr

2

>90

tr

>90

tr

tr

>90

2

1

2

Table 1 XRD Mineral Abundance Pimienta Formation. Samples are sorted in-depth, increasing order.

Tabla 1 Composición mineral XRD de las muestras analizadas de la formación Pimienta. Las muestras se muestran en orden creciente de profundidad.

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Figure 4. Mineral Composition of the Pimienta Formation displayed in a clay/sandstone/limestone ternary diagram, superimposed over the “sCore: A Classification for Organic Mudstones” by (Gamero-Diaz et al., 2013). The classification of the Pimienta formation is carbonate mudstone. Major shale plays of the world are mapped, identifying the Eagle Ford and Bakken from North America, and Hanifa from Saudi Arabia, as analogous to the Pimienta Formation.

Figura 4. Composición mineralogica de las muestras analizadas de la formación Pimienta, mostradas en diagrama ternario arcillasarenas-carbonatos, de acuerdo a la clasificación “sCore: A Classification for Organic Mudstones” por Gamero-Diza et al. (2013). La formacion Pimienta puede ser clasificada como mudstone carbonáceo. Se muestran también plays productores tipo no convencional, identificanfo a las formaciones Eagle Ford y Bakken de Norteamérica, y Hanifa de Arabia Saudita, como análogos a la formacion Pimienta.

Figure 5. Top view of a core fragment of Pimienta formation, sample ID 261344, from well DD. Extremely fine-grained black mudstone conforms to the bulk of the sample. A vein of calcite-pyrite crosses the core at a high angle with respect to the vertical. The areas in the blue squares are the selected spots for EDS analysis. Zone A is selected over the calcite-pyrite vein. Zone B covers an area over the bulk carbonaceous mudstone. Right side is are a set of wireline logs over the interval of Pimienta formation.

Figura 5. Vista lateral de un fragmento de slab de la formación pimienta, muestra ID 261344, obtenida del pozo DD. La matriz de la muestra esta compuesta de mudstone negro de grano muy fino. Una fractura sellada con calcita y pirita corta el núcleo, paralela al eje longitudinal. Las zonas A, B y C son seleccionadas para análisis EDS. La zona A cubre la vena de calcita y pirita. La zona B analiza una zona de la matriz de mudstone. En la derecha se muestran los registros geofísicos en el intervalo correspondiente a la formación Pimienta.

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• MINERAL SPECTRUM ANALYSIS

Energy-Dispersive X-Ray Spectroscopy (EDS) analysis consists of mineral mapping over the zones areas highlighted in Figure 5. Higher accuracy on the mineral characterization is achieved by selecting individual spots for spectrum analysis. A total analysis of 12 spectrum spots were completed in the sample, distributed in 3 spots (S1-S3) for Zone A, four spots (S4-S7) in Zone B and five spots (S8-S12) Zone C. The EDS results have units of weight percent (wt. %), which are used to predict the mineral composition, estimated from the proportion of each of the elements to that of the chemical composition. Figures 6A, 6B, and 6C show SEM images of the different zones and the selected spots for spectrum analysis. The results of elemental quantification from EDS analysis are displayed in table 2.

El análisis EDS se realizó en la zonas señaladas en la figura 5, así como también se seleccionaron 12 puntos para análisis espectral (Sx) el cual provee análisis detallado en áreas reducidas, distribuidas en tres puntos (S1-S3) en la zona A, cuatro puntos (S4-S7) en la zona B, y cinco puntos (S8-S12) en la zona C. El análisis mapeo espectral se realizó para los elementos Calcio, Oxigeno, Carbono, Silicio, Hierro y Azufre. Los resultados de análisis espectral tienen unidades de peso porcentual wt.% por elemento químico analizado, desplegados en la Tabla 2.

Spectrum

C

O

Map Zone A

22.7

40.5

Spectrum 1

18.4

Label

Spectrum 2

Na

Al

Si

S

Ca

Fe

Zn

Total

0.2

0.1

3.2

8.5

17.6

7.1

0.2

100

+ Traces Clay amd Dolomite

55.2

1.4

25.1

100

Calcite (CaCO3)

31.3

3.2

100

Quartz (SiO2)

100

Kerogen (C,H,O,N)

67.3

22.7

1.2

Map Zone B

20.7

52.7

0.1

0.4

0.2

3.0

1.0

4.7

2.3

0.9

22.1

0.7

100

48.8

3.1

36.8

100

11.3

Spectrum 5

66.9

24.4

Spectrum 6

11.4

43.9

Spectrum 7

12.3

53.3

4.8

Map Zone C

20.0

40.2

10.6

11.2

9.3

Spectrum 8

4.6

58.6

35.3

0.8

0.8

Spectrum 9

14.8

8.7

0.8

45.3

Spectrum 10

16.9

51.7

Spectrum 11

22.2

49.9

Spectrum 12

Predicted Mineral

65.5

Spectrum 3 Spectrum 4

Mg

29.8

1.0

3.4

0.8

4.3

100

44.7

100

29.6

100 8.7

Quartz (SiO2) Calcite (CaCO3) Calcite (CaCO3) + Quartz (SiO2)

100 100

30.5

Pyrite (FeS2) Kerogen (C,H,O,N) +

100

0.9

0.5

29.3

100

0.6

1.0

25.0

1.2

100

29.3

6.8

34.1

100

Quartz (SiO2) Pyrite (FeS2) Calcite (CaCO3) + Quartz (SiO2)

Kerogen (C,H,O,N) Pyrite (FeS2) /

Anhydrite (CaSO4)

Table 2 EDS Spectrum analysis results for each of the locations as selected from Figure 5, sample ID 261344.

Tabla 2. Resultados de Espectroscopía de Energía Dispersiva EDS en las zonas indicadas en la Figura 5. Muestra de formación Pimienta del pozo DD.

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The Zone A in Figure 6A has a scale of 500 μm. The upper region is the homogeneous mudstone. The brighter (therefore higher density) diagonal vein is the pyrite. Underneath, the calcite vein is parallel with the distinguishable microfractures in the perpendicular direction. Spot S1 has a high content of Ca, C, and O, indicating positively that the white infilling material of the natural fracture is calcite.

Spot S2 is predominantly Si and O, which are the constituting elements of quartz. No other elements typical of clays or feldspars, such as Al, K, Na, are identified in this particular spot analysis. The sharp edges of this object could indicate that it is part of a network of microfractures in the calcite vein. The microfractures are ~20 μm wide and run perpendicular to the calcite and pyrite vein. Spot S3 is a representative area of the darkest, low density, an irregular spot in the mudstone area. The predominant mineral occurrence of C and O confirms that this particular location is organic material or kerogen.

Note for Zone A mineralogy: Besides the mineral quantification in the spots S1, S2 and S3, there are traces of Mg, Al, and Zn, measured by mineral mapping in the entire zone. These are components of clays that are intrinsically present in the sample.

La zona A en la Figura 6A tiene una referencia de escala de 500 μm. La región superior-izquierda de la imagen es mudstone homogéneo. La franja brillante (densidad alta) es la depositación de pirita, que es paralela a la vena de calcita que se distingue por las múltiples microfracturas. La esquina inferior derecha es nuevamente la matriz de mudstone. El punto S1 tiene alta concentración de Ca, C y O, confirmando que el material cementante de la fractura es calcita.

El punto S2 es predominantemente Si y O, que son los elementos que constituyen el cuarzo. Los ángulos bien marcados en ese cuerpo sugieren que es un grano dentro de la red de microfracturas dentro de la vena de calcita. Las microfracturas tienen un espesor de ~20 μm, con una orientación perpendicular a la vena principal, es decir a la fractura cementada. El punto S3 es un área obscura (baja densidad) en una zona irregular de la matriz mudstone. Los elementos predominantes son C y O, confirmando que es material orgánico o kerógeno. Adicionalmente en el 48

mapeo general de la zona A se encontraron trazas de Mg, Al y Zn, que son elementos de las estructuras de arcillas, instrinsicamente contenidas en la muestra.

T h e m i n e r a l s o f Zo n e B i n F i g u r e 6 B a re homogeneously, and well-sorted grains conform to the bulk sample of the carbonate mudstone. Grain size is in the order of 10 μm. Features in this zone are pyrite framboids and distinctive oval and concave patterns, presumably resulting from organic matter or microfossils. Spot S4 performed on a distractive pyrite framboid, confirmed with high Fe and S content. Spot S5 measured one of the dark spots arranged in a distinctive concave pattern, presumably a section of a microfossil. The high content of C and O in mineral analysis confirms this spot as organic matter. Spot S6 is selected from a gray spot or grain, which is delimited by a perimeter of darker, less dense material. The identified minerals are Ca, C, and O, which are the constituents of Calcite. Spot S7 is chosen from a brighter, higher density spot, although due to imminent charging affects the brightness and contrast may not truly represent the morphology.

The EDS spectrum analysis indicates that the composition in this spot is dominantly calcite, with a minor proportion of Si. Additional elements identified in Zone B are traces of Na, Mg, and Al, which are components of clay.

Los minerales de la zona B (Fig. 6B) están constituidos en un agregado homogéneo y coherente que forman la matriz mudstone de la roca. El tamaño de los granos es del orden de 10 μm. En la zona se identifican framboides de pirita y característicos núcleos ovalados y cóncavos resultados de materia orgánica y microfósiles. El análisis espectral en el punto S4 se realiza en un framboide de pirita, confirmado con la detección de elementos S y Fe. El punto S5 caracteriza la región obscura con forma cóncava, posiblemente de un fósil, confirmado como materia orgánica por el alto contenido de C y O. El punto S6 se realiza en un área con matiz gris claro, delimitado por un contorno obscuro de baja densidad, obteniendo concentraciones de Ca, C y O, lo cual son constituyentes de la calcita. El punto S7 es elegido en un punto brillante de alta densidad, sin embargo por el efecto de saturación electrostática

no se puede observar la textura de la zona en detalle.

El análisis espectral arroja composición de calcita, con una menor proporción de Si. Otros elementos identificados en la zona B son trazas de Na, Mg, y Al, que son componentes de arcillas.

The EDS analysis in the Zone C in Figure 6C is completed over a similar region as Zone A, centered around the pyrite vein, identifying the spots: S8 Quartz, S9 Pyrite, S10 Kerogen, S11 Kerogen, and S12 Framboid.

El análisis EDS en la zona C (Fig. 6C) se realiza sobre zonas similares a la Zona A, centrado alrededor de la vena de pirita. Los elementos identificados del análisis espectral concluyen la distribución de minerales: S8 Cuarzo, S9 Pirita, S10 Kerógeno, S11 Kerógeno, y S12 Framboide de Pirita.

• MINERAL MAPPING / MAPEO DE ELEMENTOS QUÍMICOS

A 2D distribution map for each element is constructed from the spectral analysis over a surface. Figure 7 shows the compositional distribution maps of the Zone A in Figure 5 for the elements Ca, C, O, Fe, S, and Si. El mapeo superficial de la distribución de elementos químicos de se muestra en la Figura 7, realizado en la zona A para los elementos Ca, O, C, S Fe y Si. Mapping of calcium (Fig. 7A) and oxygen (Fig. 7B) indicate a strong presence of the elements throughout the bulk sample of carbonate mudstone, determining the lithology of the rock matrix deposited during the formation of the Pimienta formation. Calcite crystals are also present as infilling material in the naturally occurring fractures. Carbon (Fig. 7C) is also present in most of the analyzed surface, but there is a higher concentration in spot S3, that was identified as organic matter, suggesting the presence of kerogen.

El mapeo de los elementos Ca (Fig. 7A), O (Fig. 7B) y C (Fig. 7C) indican una distribución dominante en toda la extensión de la superficie de la zona A, determinando la matriz litológica como mudstone carbonáceo de la formación Pimienta. Cristales de calcita también están presentes en el material de cementación de

la fractura, aunque depositados en diferente evento geológico. El elemento carbono presenta una alta concentración en el punto S3, que fue determinado como kerógeno en el análisis espectral.

Sulfur (Fig. 7D) is mostly present in the pyrite vein, follows the same distribution as the iron mapping (Fig. 7E). The isolated spots where S concentrates (other than pyrite), but not correlating to Fe concentration over the same area, could be an indication of other isolated minerals containing S, such as the anhydrite (CaSO 4), as reported in XRD results. A significant consequence of sulfur in the kerogen is the potential for H2S generation as a maturation byproduct.

La distribución de S y Fe, en las Figuras 7D y 7E tiene una marcada presencia en la vena de pirita, así como en las estructuras framboides. También se identifican puntos de alta concentración de S, que no se replican en la distribución de Fe con lo que se descarta que sean núcleos de Pirita, sin embargo podrían ser identificados como anhidrita, basado en el análisis de XRD. Una consecuencia importante de contenido de azufre es la generación de H2S como producto de la maduración del yacimiento.

Finally, the mapping for Silicon (Fig. 7F) concentrates mostly in the lower region. The EDS mapping demonstrates that Si occurs in minor veins, 20 μm thick. The focused presence of Si indicates that the silica-dominated infiltration into the carbonate mudstone matrix in aqueous solution occurred. This infiltration followed channels and microfractures in the calcite vein but was unable to permeate beyond the pyrite vein.

Finalmente, el mapeo de Si (Fig 7F) indica que la distribución se concentra en el sector inferiorderecha, así como en las microfracturas de la vena de calcita, que tienen un espesor de 20 μm. La distribución de los minerales sugiere que la secuencia de depositación del cuarzo ocurrió en un evento no detrítico, post-fracturamiento y cementación de calcita, como filtración en una solución acuosa, permeando en las microfracturas de la calcita y el mudstone, y en menor proporción a través de la vena de pirita.

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Figure 6A. SEM image of the area of analysis Zone A. Scale 500 μm. Spot S1 is selected in order to analyze the calcite vein. S2 is selected from a low-density area that is at the boundary of the white calcitic vein. S3 is an analysis of a cluster of organic material.

Figura 6A. Imagen de microscopia electronica de barrido, SEM de la zona A. Referencia 500 μm. El punto S1analiza la vena de calcita.

El punto S2 es un punto de baja densidad (coloración obscura) en el margen de la vena de calcita. El punto S3 es un nódulo de material orgánico.

Figure 6C. Scanning electron microscopy (SEM) image of the area of analysis Zone C. Scale 250 μm. Spot S8 performed on a region of slightly darker gray hue, next to the pyrite vein. S9 is over the pyrite vein. S10 and S11 analysis completed on organic dark spots. S12 is completed on an isolated high density, bright spot.

Figura 6C. Imagen de microscopia electronica de barrido, SEM de la zona C. Referencia 250 μm. El punto S8 se realiza en una región identificada por la tonalidad gris obscuro -en la escala de color de SEM indica poca conductividad- cercano a la vena de pirita, donde se ubica el punto 9. Los puntos 10 y 11 se realizan sobre regiones de material orgánico. El punto 12 caracteriza un punto de alto brillo.

5.3 POROSITY ANALYSIS / ANÁLISIS DE POROSIDAD

Figure 6B. Scanning electron microscopy (SEM) image of the area of analysis Zone B, Scale 100 μm, located in the Mudstone area. The quality of the image is affected by the charging effect due to the high siliceous (Si) content. Spot S4 is taken over a Pyrite framboid. S5 is selected from non-conductive dark spots characteristic of organic matter. S6 targets a gray grain/cluster delimited by a distinctive darker perimeter. S7 is an analysis of a highly conductive area.

Figura 6B. Imagen de microscopia electronica de barrido, SEM de la zona B. Referencia 100 μm.LA calidad de la imagen es afectada por el efecto de carga electrostática por el alto contenido de Silicio (Si). El punto S4 es tomado en un framboide de pirita. El punto S5 es seleccionado en una región no conductiva característico de materia orgánica. El punto S6 se enfoca en una región no conductiva delimitado por un contorno obscuro. El punto S7 analiza un área altamente conductiva.

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The sample selected for porosity analysis is the sample ID 261318 from well GG. Porosity obtained with N2 porosimeter indicates 0.7 p.u. Lithologic classification is that of a clay-rich carbonate mudstone with the XRD analysis of carbonates as calcite 70 wt. %, silicates from quartz 19 wt. % and plagioclase feldspar 2 wt. %, clay as illite, 6 wt. %, Sulfates in the form of barite 1 wt. % and anhydrite 1 wt. %, and traces of pyrite. Pyrolysis analysis indicates medium-high total organic content (TOC) of 2.51 wt. % (Vega-Ortiz et al., 2020a). The high TOC ensured that there would be organic matter (kerogen) present in the rock matrix. Similar to the sample from well DD, the sample ID 261318 contains microfractures filled with calcite. The features observed in this sample are believed to be representative of the Pimienta formation at this particular depth and location.

La muestra seleccionada para análisis de porosidad es la muestra ID 261318 del pozo GG. La porosidad medida con porosímetro de N2 es de 0.7 p.u. La

clasificación de litología es mudstone carbonáceo rico en arcilla, con contenido mineralógico 70 wt.% calcita, areniscas de 19 y 2 wt.% de cuarzo y feldespato plagioclase, respectivamente, contenido de arcillas como Ilita en 6 wt.%, así como sulfatos en forma de barita 1 wt%, anhidrita 1 wt.%, y trazas de pirita. Análisis de pirolisis indica un valor medio de contenido orgánico total TOC de 2.5 wt.%, como se reporta en Vega-Ortiz et al, (2020), asegurando que en esta muestra asegura que hay presencia de kerógeno en la matriz. Al igual que la muestra analizada del pozo DD, esta muestra tiene fracturas selladas con calcita.

Ramsay (1980) studied the formation of veins trough the crack-seal mechanism. Geological stress deforms and induces microfractures in the rock matrix, creating new surfaces. The deposition of crystallized minerals transported by aqueous solution leads to the growth of the infilling crystals, sealing the cracks, and forming the vein (Fyfe et al., 1978).

Ramsay (1980) estudió el mecanismo fractura-cementación en la formación de venas. Los esfuerzos geológicos deforman e inducen microfracturas en la matriz de la roca, creando nuevas superficies. La depositación de minerales cristalizados a través de soluciones acuosas permite el crecimiento de cristales que sellan y cementan la fractura, formando las venas de minerales (Fyfe et al., 1978).

Figure 7. EDS Compositional map of the Zone A from sample ID 261344. Calcium, oxygen, and carbon are present in the carbonate mudstone and calcite vein, analyzed by spot S1. A high concentration of C occurs at spot S3, where kerogen is identified. Sulfur and iron occur in the pyrite vein. Isolated sulfur spots may correspond to anhydrite. Quartz is seen in the O and Si maps, verified by the spectrum analysis in spot S2.

Figura 7. Imagen de Barrido Electronico de Dispersion., EDS, de la zona A, realizada en la muestra ID 261344, identificando los elementos Calcio (Ca), Oxigeno (O) y Carbono (C) en la región de la matriz de mudstone y en la fractura sellada con calcita (punto S1). Una alta concentración de C ocurre en el punto 3 donde se puede identificar un nódulo de kerogeno. Los elementos Hierro (Fe) y Azufre (S) están presented en la vena de pirita. Los puntos aislados de S son indicación de cristales de Anhidrita -se descarta presencia de pirita por la ausencia de Fe en los mismos puntos-. Depositos de arenas, o cuarzo, es observada por el mapeo del elemento Silicio (Si), verificado por el análisis espectral del punto S2

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Figure 8. Large scale image of Pimienta formation sample. The location of the sample for SEM is indicated in the wireline logs over the Pimienta formation interval.

Figura 8. Imagen de una muestra de la formación pimienta. En la derecha se indica la ubicación de la muestra analizada usando los registros geofísicos como referencia.

The overall layout of the sample is shown in Figure 8, an SEM image with magnification 30x of an unpolished sample. Clay-rich carbonate mudstone is noted as the dominant lithofacies. The vein is formed by two parallel crystal layers of calcite and pyrite, deposited along the fracture surface.

At this magnification of 30x, the texture and grain sorting of the mudstone looks evenly distributed with random and isolated clusters of minerals in the form of irregular shapes or bright, denser spots.

Also recognizable at this relatively large scale is a fracture (dashed red) that runs across the mudstone section parallel to the vein, likely as a consequence from in-situ geological stress. The vein is ~0.5 mm wide. The calcite crystals in the vein are identified by characteristic cleavage planes, which are perfect rhombohedral {10ī4} (Rachlin et al., 1992), produced from the hexagonal, space group R3c calcite structure given by Chessin (1965). A thinner pyrite layer, width ~ 0.1 mm, is observed in the vein.

The wireline logs of the Pimienta formation are displayed in Figure 8, indicating the location where the sample for SEM analysis is taken. The GR in the suprajacent formation of K Tamaulipas Inferior (K-TI) is a low 20 GAPI curve. The J-K boundary sets an increase in the GR curve, which is the typical signature 52

of the Pimienta formation. The porosity curve derived from Sonic log shows a slight increase (no neutron porosity curve is available). The last track displays the organic content with a peak at TOC 1.8 wt.% in the Pimienta formation. The Kimmeridgian Taman formation below shows a higher TOC up to 2.2 wt.%.

La configuración de los minerales en la muestra se explica en la Figura 8, es tomada con SEM a una magnificación de 30x en una muestra sin pulir, siendo la facie litológica dominante el mudstone carbonáceo rico en arcillas. La vena está formada por dos capas paralelas cristalinas de calcita y pirita depositadas a lo largo de la superficie de la fractura.

En la magnificación de 30x la textura y distribución de los granos parece homogénea, con distribución aleatoria y aislada de grupos de cristales de formas irregulares. También se identifica una fractura (línea punteada roja) que se desarrolla a lo largo de la matriz de mudstone y paralelo a la vena. El espesor medido de la vena de calcita es de ~0.5 mm, así como también una capa de pirita delgada de ~ 0.1 mm de espesor.

Los cristales de calcita en la vena se identifican por los planos de clivaje característicos que consisten en romboedros {10ī4} (Rachlin et al., 1992), producidos por la estructura hexagonal de calcita, grupo R3c.

(Chessin et al., 1965).

Los registros geofísicos en la formación Pimienta se muestran en la figura 8. Indicando la profundidad a la cual se obtuvo la muestra para análisis SEM. La curva de GR en la formación K Tamaulipas Inferior (K-TI) indica valor bajo 20 GAPI. El contacto J-K se identifica con un incremento en las curvas de GR, característico de la formación Pimienta. La curva de porosidad derivada de registro sonico muestra in leve incremento a ~3 p.u. (no existe curva de neutrónporosidad). El ultimo registro es el contenido orgánico total, con un máximo de TOC=1.8 wt.% para la formación Pimienta y 2.2 wt.% en la formación Taman.

• INTERGRANULAR POROSITY / POROSIDAD INTERGRANULAR

An example of the intergranular porosity is observed in Figure 9, which has a magnification of 1924X. The texture of the carbonate mudstone bulk section of the sample shown is composed of flake-like grains, approximately 5.0 x 5.0 µm. All of the grains align with the same orientation with respect to the vertical, producing an aggregated accumulation of well-sorted grains. Intergranular pore space is highlighted in the red frame. The dimensions of this pore are 3.3 x 2.6 µm. The intergranular porosity is limited, and the connectivity between pores is limited. This likely considerably restricts the overall permeability of the rock matrix.

Un ejemplo de porosidad intergranular se observa en la Figura 9, que tiene una magnificación de 1924x.

La textura del mudstone carbonáceo tiene una composición de granos tipo hojuela de dimensiones 5.0 x 5.0 µm. Todos los granos tienen la misma alineación, produciendo un compuesto agregado de granos bien organizados. El espacio poroso intergranular, así como la conexión entre los poros, es limitado lo cual restringe el transporte de fluidos y ocasiona una permeabilidad muy baja en la matriz de la roca.

• INTERCRYSTALLINE POROSITY /POROSIDAD INTERCRISTALINA

At 181x magnification, a crystalline cluster was detected in the clay-rich carbonaceous mudstone (Fig. 10A). The crystal cluster is embedded within the mudstone matrix. A 1025x magnification of the inset square (Fig. 10B). That view shows a

clear example of intercrystalline porosity. The crystals in Figure 10B are interpreted as anhydrite (CaSO 4), which is orthorhombic and classified in the space group Amma. (Shindo and Nozoye, 1992).

The characteristic anhydrite cleavage is the result of the crystal nucleation and growth precipitated from aqueous solution (Pina, 2009). The presence of anhydrite confirms the hypothesis of shallow marine conditions since calcium sulfate minerals are known to form in tidal environments (Young and Chan, 2017).

The gaps between the crystals produce voids with dimensions in the range of 2 to 20 µm. By analyzing the morphology of the crystals, it can be concluded that these pores are interconnected and contribute to the effective porosity. This connectivity, therefore, represents favorable conditions for fluid transport within the rock.

A una magnificación de 181x se observa una agrupación de cristales incrustada en la matriz de mudstone (Fig. 10A). A la magnificación de 1025x (Fig. 10B) se observan los cristales y la porosidad intercristalina con mayor detalle. La estructura ortorrómbica de los cristales los define como anhidrita, clasificados por Shindo y Nozoye (1992) dentro de grupo espacial Amma. El clivaje característico de la anhidrita es resultado de la nucleación cristalina y crecimiento de la precipitación de solución acuosa (Pina, 2009). La presencia de anhidrita confirma la hipótesis de condiciones marinas someras dado que los minerales de sulfato de calcio se desarrollan en ambientes sedimentarios de marea (Young and Chan, 2017). Los espacios intercristalinos producen una porosidad de dimensiones en el rango de 2 a 20 µm. Analizando la morfología de los cristales se observa que los poros están interconectados contribuyendo positivamente a la porosidad efectiva y la permeabilidad.

• MICRO-FRACTURE POROSITY / POROSIDAD DE MICROFRACTURAS

At the boundary between the calcite vein and the carbonate mudstone, there is a microfracture that runs parallel to the area of contact between the two rock components, observed in Figure 11 with magnification 131X. The width of this microfracture is 5 µm. A mechanism for the microfracture occurrence is the

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poor bonding between the matrix and the precipitate material in the veins, combined with the geomechanical deformation and fracture caused by in-situ stresses.

This type of porosity may represent major channels for fluid transport. When the reservoir is subject to pressure differential, the fluid stored in the intergranular and intercrystalline porous networks and will flow through microfractures. Unfortunately, these fractures may likely be closed at in-situ stress conditions.

Figure 9 Clay-rich carbonate mudstone composed of wellsorted fillosilicate flake shaped grains deposited uniformly. The intergranular porosity is in the order of 3 μm Figura 9. Imagen de alta resilucion (escala 30 um) de una muestra de mudstone carbonáceo rica en arcilla. La textura es de filosilicatos -hojuelas- apiladas, depositados uniformemente. La porosidad intergranular es del orden de 3 μm.

En la unión entre la vena de calcita y la matriz de mudstone existe un espacio entre los dos grupos minerales, como se observa en la Figura 11 que tiene magnificación 131x. El espesor de esa microfractura es de 5 µm. El mecanismo de formación de la fractura es la débil unión entre la matriz y el material de cementación, en combinación con los esfuerzos geológicos de deformación. Este tipo de porosidad representan canales amplios para el transporte de fluidos del yacimiento hacia el pozo, en caso de permanecer abiertos.

Figure 10. Crystalline structure imbedded in the limestone matrix. The porous network between the anhydrite crystals is interconnected, creating favorable conditions for storage and permeability. Pore dimensions are on the order of 2 to 20 μm Figura 10. Estructura cristalina de anhidrita incrustada en la matriz de mudstone carbonáceo. La red de porosidad entre los crsitales de anhidrita están interconectados, crando condiciones favorables para almacenamiento y permeabilidad. El tamaño de los poros es en el orden de 2 a 20 μm.

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• ORGANIC MATTER – KEROGEN / MATERIA ORGANICA - KERÓGENO

Kerogen is a geopolymer that derives from accumulated organic material that was deposited and buried with the rock. Depending on the depositional environment, the accumulated organic material can be terrestrial, lacustrine or marine, origin, composed of micro or macroscopic forms of life. The composition of the kerogen is a complex arrangement of organic fabric and tissues. The organic material may be preserved in an anaerobic environment where decomposition is minimized. Further burial exposes the organic matter to elevated temperature and pressure, leading to diagenetic processes, which are the chemical reactions that convert the kerogen into lineal chains of over geological time.

The type of hydrocarbon generated from the kerogen will depend on the initial elemental constituents, particularly C, H, O, and N. The products vary from a range combination of light compounds C1-C4 natural gas and volatiles to heavier chains that produce heavy wax and bitumen. The byproducts CO2 and H2S are also commonly found in oil and gas reservoirs.

El kerógeno es un geo polímero que se deriva de la materia orgánica que fue acumulada y enterrada junto con los depósitos sedimentarios. Dependiendo del ambiente de depositación, el material orgánico puede ser de origen terrestre, marino o lacustre, compuesto de tejidos orgánicos complejos o residuos de paleo organismos microscópicos.

Los residuos orgánicos pueden ser conservados en ambientes anaeróbicos, en donde la descomposición es inhibida, sin destruir o transformar los compuestos orgánicos en elementos minerales. Siguiendo el mecanismo tectónico de subducción, la materia orgánica llega a ser sometida a condiciones elevadas de presión y temperatura dando lugar al proceso de diagénesis, que consiste en las reacciones químicas que convierten la materia orgánica en kerógeno, y eventualmente en la transformación en cadenas lineales de hidrocarburos a través de los tiempos geológicos. El tipo de hidrocarburos generados dependen de los elementos constitutivos, particularmente C, H, O y N. Los productos generados varían en un rango de compuestos ligeros como gas natural y crudo ligero (C1-C4) hasta cadenas más pesadas que producen crudo pesado y bitumen. Los productos colaterales como CO2 y H2S también son comúnmente encontrados en yacimientos de aceite y gas.

With field emission (FEG) SEM, the backscattered secondary electron images can infer the presence of kerogen as dark, lower density spots. The kerogen in Figure 12 is identified as isolated clusters of organic matter in a fined grained mudstone matrix. This dispersed occurrence is consistent with deposition having occurred in low energy, shallow marine reducing environment.

En las imágenes obtenidas con SEM, el kerógeno se identifica como áreas de tonalidad obscura debido a material de baja densidad. En la Figura 12 están señalados los nódulos de kerógeno incrustados en la matriz de mudstone, que son los que proporcionan el contenido orgánico total TOC en el análisis de pirólisis.

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Figure 11. Shear-induced micro-fracture in the interface between calcite and shaly limestone, caused by the difference in the geomechanical properties of each of the materials and poor bonding between the surfaces. These fractures may have been the result of distressing during sample recovery from the in-situ environment.

Figura 11. Microfractura inducida por esfuerzos de cizalla en la interface de la vena de calcita y la matriz de mudstone, causada por las diferentes propiedades geomecánicas de los minerales y la reducida cementación entre las superficies. Estas fracturas pudieron haber sido resultado de la reducción de los esfuerzos in-situ, en la recuepracion del núcleo a superficie.

Figure 12 SE- SEM Image carbonaceous mudstone from Pimienta Formation at 10 KeV. Kerogen appears as darker spots. Limestone topographic features are observed on a grayscale. The areas covered with pyrite material exceptionally bright, caused by the excess of scattered electrons in the surface.

Figura 12. Imagene de SEM de la formación Pimienta, mudstone carbonáceo con 10 KeV. La textura del mudstone se observan en escalas de grises. El kerogeno se observa como manchas obscuras.

Las zonas cubiertas con pirita se manifiestan como alto brillo, causado por el exceso de electrones dispersados en la interfase metálica.

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6. MONTE CARLO SIMULATOR CASINO / SIMULADOR NUMÉRICO CASINO The Monte Carlo CASINO (Université de Sherbrooke, Québec, 2000) is used to simulate the electron trajectories upon penetration on a surface of defined material (Fig. 13). Simulations below show the electron trajectories for the three components (organic matter, mudstone, and pyrite) for beam voltages of 2 and 10 KeV. All of the images have the same scale z=1500, x=2000.

Scanning electron microscopy (SEM) imaging relies on the interaction between an electron beam and the surface layers of a sample. The electrons will either penetrate the sample or will be scattered back into the SEM chamber, where the secondary electrons detector can measure, quantify and process the signal produced from the scattered electrons and translate this into a meaningful 2D image. The image contrast will vary according to the sample composition. For this particular sample of the Pimienta formation, there are three components: rich clay carbonate mudstone (SiO2-CaCO3) is the primary matrix constituent, residual kerogen is present as organic material. Pyrite, along with calcite, infilled the naturally occurring fractures in the rock.

El simulador CASINO utiliza método MonteCarlo (Université de Sherbrooke, Québec, 2000) para predecir la trayectoria de los electrones incidentes después de interactuar con capas de distintos minerales, simulando el cañón de electrones operado a 2 y 10 keV. En el caso de la muestra analizada de la formación Pimienta, existen tres componentes: la matriz mudstone, la vena cementada con calcita y pirita, y los nódulos de kerógeno (Fig. 13). Las imágenes de SEM se producen de la interacción entre el haz incidente de electrones y los átomos en la superficie la muestra, pudiendo ser reflejados en caso de materiales de alta densidad, o bien penetran hacia el interior de la muestra si no existen las interacciones necesarias para ser reflejado. El cañón de electrones y el detector se ubican encima de la muestra. El haz de electrones recorre el área expuesta de la muestra, enfocado secuencialmente en pequeñas áreas a manera de pixeles; el detector mide, cuantifica y procesa la energía de los electrones reflejados en cada uno de estos pixeles. La compilación de la respuesta de cada uno de los pixeles genera una imagen representativa de la textura de la muestra analizada. El contraste y matiz de cada píxel depende de la composición de elementos químicos en cada punto en particular; metales densos reflejan una alta cantidad de electrones, produciendo imágenes brillantes, mientras que elementos de baja densidad reflejan una menor cantidad de electrones, produciendo imágenes obscuras.

For the simulations of the images taken with 2 keV, the electron interacts only with the surface layers of the material, leading to shallow penetration. The contrast and brightness of the images are exceptionally homogeneous, and the topography features and textures of the sample are identifiable, although the grey hue scale is not sufficient to distinguish between mineral components of the sample.

La simulación de la interacción con 2 keV, la interacción de los electrones con la muestra es mínima, limitada a la capa superficial. El contraste y brillo de las imágenes producidas son excepcionalmente homogéneas, siendo difícil distinguir entre los distintivos en la textura de los componentes.

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Figure 13. Numerical Monte Carlo simulation of the possible trajectories of the electron inside different materials, using the CASINO® software. At 2 KeV, the interaction volume is small, and the electrons from the SEM beam remain on the surface. At 10 KeV, the electrons will penetrate deeper into the material: Organic matter offers the least resistance, and the image will appear darker. In the Pyrite metallic elements, the electrons will remain closer to the surface and will be scattered back to the detector, creating charged surfaces and brighter images.

Figura 13. Simulacion numerica utilizando el método Montecarlo del programa CASINO, indicando las trayectorias de penetracion del haz de electrones del SEM a energías de 2, 5 y 10 KeV, dispersas hacia dentro de la muestra después de interactuar con la superficie de los materiales. A 2 KeV, el volumen de interacción es reducido y los electrones incidentes del SEM permanecen el la superficie. A 10 KeV, los electones penetran a una mayor profundidad en la estructura de la muestra: El material orgánico ofrece la menor resistencia permitiendo una penetración mayor, -sin reflejar electrones hacia el exterior-, produciendo una imagen obscura. En las superficies metalicas de pirita, los electrones tendrán poca penetración, ya que en su mayoría son reflejados hacia el exterior de la muestra, produciendo una imagen muy brillante y saturada.

The mineral response will be identifiable using higher energy in the electron beam, as displayed in simulations corresponding to 10 KeV. For organic-rich samples, it is particularly useful for kerogen identification. Since organic matter is a complex organic structure of C, H, O, N and S, the electron beam penetrates deep into the subsequent layers from the surface in a narrow distribution of trajectories, and fewer electrons are scattered back to the detector, producing darker areas. On the other hand, pyrite is composed of crystalline grains containing the heavier metallic elements FeS2, which offers a higher resistance path for the incident electrons. The electron trajectories are distributed in a shallower and wider region in the layers closer to the surface, which produces an excess of electrons scattered back to the chamber; hence the high amplitude of the signals in the detector produces a saturated bright image in Figure 12.

La respuesta de los minerales es distinguible usando energía más elevada en el cañón de electrones, como se muestra en las simulaciones a 10 keV. Es particularmente útil para muestras que contienen alto contenido orgánico para identificación de la distribución del kerógeno. Dado que la composición química con elementos C, H, O, y N, el haz de electrones penetra en capas más profundas, con pocos electrones reflejados hacia el detector. En contraparte, la pirita está compuesto de elementos metálicos Fe y S , ofreciendo una mayor resistencia en la trayectoria de los electrones, provocando trayectorias más dispersas y cercanas a la superficie, ocasionando que gran parte de los electrones incidentes sean reflejados hacia el detector y produciendo imágenes brillantes y saturadas.

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7. MICROFOSSILS / MICROFÓSILES Microfossils are remains of life that were buried in sediments and are used as geological markers and diagnostics of depositional environments. Marine organisms often lived in shallow water, where the proper conditions of temperature, solar radiation were favorable to support life. Figure 14A is an example of fossils observed in the Pimienta formation sample ID 261344 from well DD. The oval shape is a sectioned microfossil with a welldefined layer in the perimeter of the body of the organism. In the lower section of the body, there is a characteristic spiral structure rotating along the longitudinal axis, resembling the shell of present-day organisms. The length of these microfossils is about 50-100 µm. This particular microfossil matches the description of a tapered morphogroup (Bernhard, 1986), in Figure 14B. In a study by (Omaña and González Arreola, 2008), benthic foraminifera microfossils are described from a Kimmerigdian formation outcrop in a nearby location, Figure 14C. Los microfósiles son restos de organismos que fueron enterrados y preservados en forma mineral estudiados en el área de micropaleontología, y que son usados como marcadores geológicos y métodos de diagnóstico para ambientes de deposición. En las zonas neríticas existe abundancia de organismos marinos por las condiciones de temperatura y radiación solar, óptimas para el sustento de formas de vida. La Figura 14A es un ejemplo de los fósiles observados en la muestra ID 261344 del pozo DD. El contorno cónico-ovalado es una sección de un microfósil de 50 a 100 µm de longitud. En la sección inferior se distingue una característica geometría espiral de las conchas o exoesqueletos de los moluscos encontrados en la actualidad. El microfósil tiene características similares a los morfogrupos descritos por Berhnhard (1986) en la Figura 14B. En un estudio por Omaña y González Arreola (2008) describe un ejemplo de foraminífero bentónico encontrado en un afloramiento del Kimeridgiano en una locación cercana, Figura 14C.

Figure 14. A) Microfossils found in the Tithonian Pimienta formation core. B) Classification of foraminifera morphogroups by (Bernhard).

C) Kimmeridgian Foraminifera (Kurnubia palastiniensis) as described by (Omana and Gonzalez Arreola, 2008) Figura 14. A) Microfosiles observados en la muestra de formación pimienta del pozo DD. B) Clasificación de morfo-grupos foraminiferos, descritos por Berhnard (1986). C) Ejemplar de foraminífero del kimmerigdiano (Kurnubia palastiniensis), descrito por Omaña y González Arreola, (2008).

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8. PIMIENTA FORMATION PROSPECTIVE REVIEW / COMPARATIVO DEL POTENCIAL DE LA FORMACIÓN PIMIENTA

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A source rock analysis in the Pimienta formation studied in the central-western margin by Vega-Ortiz et al, (2020a) reports a range of TOC of 0.25 to 2.3 wt.%, being the structurally deeper areas with the higher organic content. The maximum Hydrogen Index is 179 mg HC / g TOC, whereas the Oxygen Index is in a range 20-151 mg CO2 / g TOC. Since most of the samples are thermally mature it is not possible to determine the thermal maturity, presenting Tmax values from 425 to 451 º C. Other studies of Pimienta samples in the south of the TMB (Granados-Hernández et al., 2018; Morelos-García, 1996) reports a TOC range of 0.8 to 8 wt.%, and kerogen type II and III deducted from HI less than 600 mg HC / g TOC. Based on the mineral composition, it is determined that the Pimienta formation is analogous to the Eagle Ford and Bakken formations from North America (Anderson, 2014; Bromhead et al., 2017). As of 2018, the reserves estimates in these tight oil plays are 4,734 and 5,862 MMBBL, respectively (EIA, 2020). Although, the geochemical properties of the Eagle Ford report TOC of 2 to 12 wt.%, with kerogen type II-III prone to produce oil and gas, with porosity 8-12%. According to Whitson and Sunjerga (2012), the fluids produced in the Eagle Ford and other plays in North America vary from condensate oil to gas condensates API 28-62º. The production in the Eagleford as of March 2020 is 1,230 BODP and 4,194 Mscf/d. The Bakken formation properties are average TOC of 11 wt.%, kerogen types I and II, with some Type III in the shallower flanks (Sonnenberg, 2011), producing 1,250 BOPD and 2000 Mscf/d in March 2020.

La evaluación de roca generadora en la formación Pimienta realizada en el AE por VegaOrtiz et al, (2020a), reporta un valor de TOC en el rango de 0.25 a 2.3 wt.%, siendo las zonas estructuralmente más profundad las de mayor contenido orgánico, como se describió anteriormente. El Índice de Hidrogeno (HI) máximo es de 179 mg HC / g TOC, mientras que el Índice de Oxigeno (OI) está en un rango de 20-151 mg CO2 / g TOC. Dado que la mayoría de las muestras son térmicamente maduras, no es posible definir un tipo de kerógeno, presentando valores Tmax de 425 a 451 º C. Otros estudios realizados en la formación Pimienta en el sur de la CTM (Granados-Hernández et al., 2018; Morelos-García, 1996) reportan TOC en un rango de 0.8 a 8 wt.%, y kerógeno tipo II y III, deducido de HI menor a 600 mg HC / g TOC. Basado en la composición mineral, se determina que la formación Pimienta es análoga a las formaciones Eagle Ford y Bakken de Norte América (Anderson, 2014; Bromhead et al., 2017). Al ano de 2018, las reservas estimadas en estos plays de baja permeabilidad es de 4,734 y 5,862 MMBBL, respectivamente (EIA, 2020). Sin embargo, las propiedades geoquímicas de la formación Eagle Ford consideran TOC de 2 a 12 wt.%, con kerógeno tipo II-III potencial productor de aceite y gas, y porosidad en el rango de 8 a 12%. De acuerdo con Whitson y Sunjerga (2012), los fluidos producidos en la formación Eagle Ford son condensados de aceite y gas condensado de grado API 28-62º. Las propiedades de la formación Bakken son promedio TOC de 11 wt.%, con kerógeno tipo I y II, con presencia de tipo III en los flancos someros (Sonnenberg, 2011).

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9. CONCLUSIONS / CONCLUSIONES The analysis of SEM images demonstrates that the clay-rich carbonate mudstone of the Pimienta formation is comprised of a complex arrangement of sediments, crystalline structures, and textures. The distribution and geological structure layout of the Pimienta formation influences strongly on the mineral composition of the samples, identifying presence of 46 wt.% silicates and 49 wt.% carbonates in the shallow shoreline ramp in the north of the AOS, and a deeper marine basin in the south where carbonaceous mudstones dominate, with up to 96 wt.% carbonates.

The images provide enough resolution to determine that the grains are sorted uniformly over the area of interest, with isolated nucleation of organic matter (kerogen) that produces TOC of up to 2.3 wt. % in the deeper sector of the area of study. The most relevant observation is the very low porosity in the form of intergranular and intercrystalline porosity. In addition to low permeability (poor porous space interconnectivity), which reduced the intercrystalline pores are present to the order of 2-3 µm. The presence of embedded anhydrite crystalline structures is postulated to improve the permeability of the rock since the gap in between the crystals is larger than 20 µm, improving the porosity and permeability conditions of the rock.

En análisis de las imágenes de microscopia electrónica SEM demuestran que la formación Pimienta en el área analizada se clasifica como mudstone carbonáceo, compuesta de una compleja de sedimentos, estructuras cristalinas y texturas. La distribución y estructura geológica de la formación Pimienta influye en la composición mineral, encontrando 46 wt.% silicatos y 49 wt.% carbonatos en la zona superficial de rampa del litoral, y hasta 96 wt.% de carbonatos en la zona más profunda de cuenca marina.

Las imágenes proveen suficiente resolución para analizar la distribución uniforme y compacta de granos fino, con nucleaciones de material orgánico (kerógeno), que producen TOC de hasta 2.5 wt.% en el sector SE del área estudiada. Una de las observaciones más relevante son las dimensiones de la porosidad intergranular e intercristalina en el orden de 2 a 20 µm, resultando en una porosidad extremadamente reducida. Se estipula que la presencia de estructuras cristalinas en la matriz del mudstone, como la anhidrita que incrementan la permeabilidad y porosidad efectiva en la roca debido a la porosidad intercristalina medida en 20 µm.

The crack-seal veins, although closed -infilled with precipitated calcite, provide the conditions for microfractures to occur in between the boundaries of the different materials due to the weak bonding and effect of tectonic events and geo-stresses.

Las fracturas cementadas con calcita y pirita, a pesar de sellar los conductos originados por la fractura, proveen posibles canales de flujo a través de las microfracturas observadas en la interfase entre los diferentes minerales debido al enlace deficiente y los esfuerzos compresivos que deforman las rocas.

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be possible to find structural spots where the geochemical and petrophysical properties feature potential commercial unconventional reservoirs, upon assessment on producible reserves.

La información recopilada de las formaciones Pimienta concluye que si bien la formación Pimienta en el bloque analizado no tiene propiedades optimas como sus análogos en Norteamérica, es posible encontrar zonas estructurales en donde las propiedades geoquímicas y petrofísicas puedan conformar un yacimiento no convencional con potencial comercial, una vez que se realice la evaluación de posibles reservas.

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10. ACKNOWLEDGMENTS / AGRADECIMIENTOS The geological samples for this study were obtained from México’s National Core Repository (Litoteca Nacional), under the authorization from the National Hydrocarbon Commission (CNH) and Secretary of Energy (SENER). Carlos Vega-Ortiz is a holder of the National Hydrocarbon Scholarship, by CONACYT and SENER. Authors are thankful to the University of Utah and the Energy and Geoscience Institute for the facilities provided for this study.

Las muestras geológicas utilizadas en este estudio fueron aprobadas por la Litoteca Nacional de la Comisión Nacional de Hidrocarburos de Mexico. El autor Carlos Vega-Ortiz realiza tesis doctoral con fondos de la Beca Nacional de Hidrocarburos de la Secretaria de Energía y de CONACYT. Los autores agradecen al Energy and Geoscience Institute de la Universidad de Utah por las facilidades otorgadas para este estudio.

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APPENDIX A.

XRD DIFFRACTOGRAMS

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Fig A1. Pimienta sample from well GG, depth 2453 m. Composition: Carbonates: Calcite 70%. Clastics: Quartz 19%, Plagioclase 2%, Pyrite Trace. Shales: Illite 6%, Barite 1% and Anhydrite 1%.

Fig A2. Pimienta sample from well BB, depth 1838 m. Composition: Carbonates: Calcite 51%, Dolomite 4%. Clastics: Quartz 41%, K-Feldspar 1%, Pyrite Trace. Shales: Illite 3%

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Fig A3. Pimienta sample from well BB, depth 1877 m. Composition: Carbonates: Calcite 95%, Dolomite 3%. Clastics: Quartz 2%.

Shales: None

Fig A4. Pimienta sample from well GG, depth 2453 m. Composition: Carbonates: Calcite 51%, Dolomite 2%. Clastics: Quartz 34%, Plagioclase 5%, Pyrite Trace. Shales: Illite 7%, Barite 1%

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Fig A5. Pimienta sample from well DD, depth 1570 m. Composition: Carbonates: Calcite 34%, Dolomite 1%. Clastics: Quartz 23%, K-Feldspar 15%, Pyrite 1%. Shales: Illite 23%, Anhydrite 4%,

Fig A6. Pimienta sample from well DD, depth 1568 m. Composition: Carbonates: Calcite 50%, Dolomite Traces. Clastics: Quartz 36%, K-Feldspar 6%, Pyrite Traces. Shales: Illite 5%, Anhydrite 2%,

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Fig A7. Pimienta sample from well AA, depth 3038 m. Composition: Carbonates: Calcite 66%, Clastics: Quartz 17%, Plagioclase 1%, Pyrite 2%. Shales: Illite 10%, Kaolinite 2%, Anhydrite 2%.

Fig A8. Pimienta sample from well EE, depth 1859.6 m. Composition: Carbonates: Calcite 84%, Mg Calcite 1%, Dolomite 1%.

Clastics: K-Feldspar 6%, Pyrite Traces. Shales: Illite 8%.

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Fig A9. Pimienta sample from well II, depth 2605.7 m. Composition: Carbonates: Calcite 96%, Dolomite 2%. Clastics: Quartz 2%, Shales: None

Fig A10. Pimienta sample from well II, depth 2603.9 m. Composition: Carbonates: Calcite 95%, Dolomite 2%. Clastics: Quartz 3%, Shales: None

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Fig A11. Pimienta sample from well GG, depth 2396 m. Composition: Carbonates: Calcite 89%, Dolomite 1%, Clastics: Quartz 5%, Plagioclase Traces, Pyrite traces. Shales: Illite 4%.

Fig A12. Pimienta sample from well HH, depth 2155 m. Composition: Carbonates: Calcite 65%, Dolomite Traces, Clastics: Quartz 21%, Plagioclase 1%. Shales: Illite 13%.

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