Seismic Petrophysics in Quantitative Interpretation

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Investigations in Geophysics Series No. 18

Lev Vernik

Rebecca Latimer, managing editor Tad Smith, volume editor

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00-Vernik_FM.indd 1 12-08-2016 20:04:40 ISBN 978-0-931830-46-4 (Series) ISBN 978-1-56080-324-9 (Volume)

Library of Congress Control Number: 2016945722

All rights reserved. No part of this book may be reproduced, stored in a retrieval system, or transcribed in any form or by any means, electronic or mechanical, including photocopying and recording, without prior written permission of the publisher.

Published 2016 Printed in the United States of America

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About the Author ...... vii

Preface ...... ix

Acknowledgments ...... xi

Chapter 1: Petrophysics of Siliciclastic Rocks ...... 1 Petrophysical classification of siliciclastics ...... 1 Petrographic data and petrophysical classification ...... 1 Application to the core/log database ...... 4 Petrophysical model building ...... 8

Lithologic parameters Vcl and Vsh ...... 8 Total porosity ...... 11 Permeability prediction in siliciclastics ...... 13 Seismic petrophysics ...... 17 Log editing ...... 20 Sonic log ...... 20 Density log ...... 20 Anisotropic correction of sonic logs ...... 23 Chapter 2: Pore Pressure and Stress State ...... 29 Shale-compaction model ...... 29 Porosity reduction ...... 29 Density model for stress computation ...... 31 Vertical effective stress ...... 32 Pore-pressure prediction from shale velocity ...... 34 Sonic velocity versus effective stress in shales ...... 34 Pore-pressure prediction ...... 36

The effective-stress tensor and the Shmin gradient ...... 40 Chapter 3: Seismic Rock Properties and Rock Physics ...... 43 Theoretical models in rock physics ...... 43 Fluid-saturation effect ...... 43 Drained rock-frame moduli ...... 45 Cracks in dry and fluid-saturated rocks: The effect of aspect ratios ...... 46 Drained rock-frame moduli: A mixture of cracks and pores ...... 47 Effects of pore/crack interactions ...... 49 Contact models ...... 51

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Drained frame moduli and effective stress ...... 52 Rock-physics modeling in sand/shale sequences ...... 56 Pore shapes and porosity thresholds in sands ...... 56 Consolidated sandstones ...... 58 Unconsolidated and poorly consolidated sands ...... 61 Sandstone diagenesis models ...... 64 Conventional shales ...... 66 Rock-physics templates in siliciclastic sequences ...... 69

VP-VS and AI-SI relationships in siliciclastics ...... 72 The VP-VS relationship ...... 72 The AI-SI relationship ...... 76 Fluid substitution and the AI-SI template ...... 78 4D modeling of sands and sandstones ...... 80 Chapter 4: AVO Analysis: Rock-physics Basis ...... 87 Linearized AVO equations and their features ...... 87 AVO classification in AI-SI space ...... 89 From the AI-SI template to synthetic-gather models ...... 91 AVO Class III and the zero-gradient case ...... 91 AVO Class IV ...... 93 AVO Classes I and II ...... 95 VTI anisotropy effect ...... 97 The fluid factor in prospect risk mitigation ...... 100 Tuning effects in AVO synthetic modeling ...... 101 Chapter 5: Simultaneous AI-SI Inversion and N/G Computation ...... 105 Log editing and the AI-SI template ...... 105 Anisotropy correction of sonic velocities ...... 106 Additional log editing ...... 107 AI-SI crossplot ...... 107 Modeling N/G ...... 109 Seismic N/G computation from simultaneous inversion ...... 111 AI-SI inversion ...... 111 Sand volume computation and net/gross mapping ...... 114 Effects of lithology and fluid on prestack attributes ...... 118 Chapter 6: Seismic Petrophysics of Unconventional Reservoirs ...... 121 Petrologic data in unconventional shales ...... 121 Rock composition ...... 121 Organic richness and thermal maturity ...... 122 Rock texture ...... 126 Log model for unconventional shales ...... 129 Microstructural observations ...... 129 Kerogen-fraction log ...... 131 Total porosity and kerogen porosity ...... 131 Water saturation ...... 133 Model applications ...... 134 Rock physics of unconventional shales ...... 137 Core measurements of velocity and anisotropy ...... 137 Phase velocity versus group velocity ...... 139 Intrinsic velocity and anisotropy ...... 141 Effect of kerogen on velocities and anisotropy ...... 144

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Stress dependence of velocity and anisotropy ...... 153 Modeling stress dependence ...... 155 Rock-physics model ...... 158

VP-VS and AI-SI relationships ...... 161 Chapter 7: Geomechanics of Organic Shales ...... 167 Elastic-property relationships in TI shales ...... 167 Can brittleness be estimated from elastic properties? ...... 168 Static and dynamic elastic parameters in anisotropic mudrocks ...... 169 Elasticity-based brittleness of organic mudrocks: A controversial notion ...... 171

Shmin stress-gradient estimation ...... 173 Uniaxial-strain-based approach ...... 173 Anisotropy prediction in organic and conventional shales ...... 174 Stress profiling from log data ...... 175 Geomechanics of maturation-induced microcracking ...... 176 Chapter 8: Seismic Analysis in Unconventional Shales ...... 183 Reservoir-scale anisotropy estimation ...... 183 Seismic ties ...... 184 TOC estimation from an AI-SI template ...... 186 AI-SI Inversion of Prestack Seismic Data ...... 191 TOC mapping ...... 191

Shmin stress-gradient mapping ...... 194 References ...... 199

Index ...... 209

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Downloaded from http://pubs.geoscienceworld.org/books/book/chapter-pdf/3700021/frontmatter.pdf by guest on 27 September 2021 About the Author

Lev Vernik is a geophysics consultant and owner of Seismic Petrophysics, LLC, in Houston, Texas, USA, and a research profes- sor of geophysics at the University of Houston. He has previously held various geoscience positions with Arco, Vastar Resources, BP, Noble Energy, and Marathon Oil, focusing on seismic petrophysics, AVO modeling/analysis, prestack seismic inversion, and geomechanics. Lev’s long career in subsurface characterization started in the former Soviet Union, where he was involved in drilling and in- vestigating the world’s deepest well on Kola Peninsula. His research continued at the Stanford Rock Physics Project and then in the tech- nology groups of the aforementioned five oil and gas companies.

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Downloaded from http://pubs.geoscienceworld.org/books/book/chapter-pdf/3700021/frontmatter.pdf by guest on 27 September 2021 Preface

Recent advances in seismic acquisition and process- discussed in Chapter 3, “Seismic Rock Properties and ing technologies have led to enormous progress in oil and Rock Physics.” The chapter also addresses certain contro- gas exploration and reservoir characterization. Quanti versial issues, such as parameterization and appropriate tative seismic interpretation also could have improved model selection. The outcomes of this discussion are sev- significantly if we had had the internally consistent meth- eral rock-physics templates that feature the key petrophys- odologies and workflows necessary for seismic modeling. ical groups of siliciclastics in velocity-versus-porosity Seismic petrophysics plays the key role in this respect. space and acoustic-impedance-versus-shear-impedance Because the methodologies of petrophysical analysis (AI-SI) space. Those templates naturally incorporate the lead to uncertainties in estimating key parameters, such as effect of rock compaction and diagenesis and therefore are rock composition, texture, porosity, permeability, and essential in modern seismic inversion interpretation. saturation, it is important to (1) understand the rock for- Chapter 4, “AVO Analysis: Rock-physics Basis,” is mations for their proper characterization and (2) incorpo- the core of the book. This chapter integrates seismic pet- rate proper rock-physics and geomechanics concepts in rophysics with seismic amplitude analysis — notably, predicting the seismic signatures of those formations. with amplitude variation with offset (AVO) or angle of Over the last several years, our understanding of incidence (AVA). The AVO classification scheme and in- seismic rock properties and our ability to model those terpretation methodology offered here differ from the

properties — VP, VS, and density — has improved substan- classic scheme of intercept/gradient analysis and invoke tially. Nonetheless, a certain amount of confusion remains acoustic- and shear-impedance templates instead of inter- with respect to the petroelastic classification of rocks, the cept/gradient crossplots. That allows a user to streamline terminology used, the selection of models, the parameter- AVO behavior predictions during seismic petrophysics ization, and the identification of the primary controls on analysis of the log data early in the workflow. It also pro- the distribution of rock properties in the subsurface. These vides a robust quality assurance during synthetic forward issues are discussed in Chapters 1 and 3 of this book. modeling. In turn, that modeling can be used in seismic Pore-pressure prediction from well-log and seismic processing and gather conditioning for prestack attribute data is critical in drilling operations and also is important computation and simultaneous impedance inversion of in estimations of geomechanical parameters and stress- seismic data. states from seismic data. It is well established that stresses The AI-SI templates and their benefits in prestack in- in the subsurface have a direct impact on rock physics version of sand/shale sequences are exemplified in and, hence, on seismic amplitudes. Indeed, knowledge of Chapter 5. Marked mismatches in frequency content be- effective-stress variations laterally and with depth mark- tween seismic data and log data result in large differences edly reduces interpretation uncertainty and helps separate in vertical resolution and lead to the argument that limita- intrinsic effects from extrinsic influences of pore pressure tions exist in our ability to directly invert for certain res- and stress state on seismic rock properties. An approach ervoir properties (e.g., porosity and permeability) in high- to pore-pressure and vertical-effective-stress analyses is ly heterogeneous sand and sandstone reservoirs. However, discussed in Chapter 2. another valuable reservoir parameter — the net-to-gross The essence of seismic-related rock physics consists thickness ratio — lends itself to fairly accurate estima- of finding relationships between seismic rock properties tions from seismic inversion in many cases. and their petrographic and petrophysical parameters, in- Over the last several years, both the exploration and the cluding their mineralogy, texture, diagenesis, reservoir development of unconventional shales have grown properties, and effective stress. These problems are dramatically and, in fact, many operators have switched to

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unconventional plays. It is often thought that these plays Palciauskas, 1994), Seismic Data Analysis (Yilmaz, also require unconventional approaches to reservoir charac- 2001), Quantitative Seismic Interpretation (Avseth et al., terization. In Chapter 6, “Seismic Petrophysics of 2005), and The Rock Physics Handbook (Mavko et al., Unconventional Reservoirs,” it is demonstrated that the fun- 2009). Many theoretical models presented here originated damental workflows typical for conventional reservoirs still from the work of Mark Kachanov (Tufts University) with are applicable in organic shales. However, the complexity various coauthors. of rock-physics modeling increases in proportion to the ad- Methodology and workflows described in this book ditional parameters of interest, such as total organic carbon have been successfully applied in many exploration and (TOC) content and thermal maturity, and their effects on reservoir characterization projects worldwide and result- organic porosity, overpressure, and the stress sensitivity of ed in significant oil and gas discoveries and accurate re- seismic velocities. Important geomechanical issues related serve calculations, particularly in the Gulf of Mexico, to predicting horizontal-well-completion quality — issues West Africa, North Sea, Norwegian Sea, and Eastern such as brittleness, least-horizontal-stress gradient, and Mediterranean provinces. hydrocarbon-generation-induced microcrack development, Seismic Petrophysics in Quantitative Interpretation is in organic mudstones — are discussed in Chapter 7. written for oil and gas industry professionals and academi- Finally, Chapter 8, “Seismic Analysis in Uncon cians who are concerned with the use of seismic data in ventional Shales,” presents examples of seismic forward petroleum exploration and production. In addition, it modeling and prestack inversion for some of the most le- should prove useful toward thoughtful applications of those veraging parameters of unconventional shale reservoirs data by geotechnical engineers. Seismic interpretation can — both the intrinsic parameters (e.g., TOC content) and be made simple and robust by integrating rock-physics the extrinsic ones (e.g., horizontal stress gradient). It ap- principles with the seismic and petrophysical attributes that pears that AI-SI templates modified for organic shales can bear on the properties of conventional reservoirs (thick- be adapted for those evaluations, and the organic richness ness, net/gross, lithology, porosity, permeability, and satu- and fracability of organic mudrocks can be mapped on the ration) and of unconventional reservoirs (thickness, lithol- basis of simultaneous seismic inversion results. ogy, organic richness, and thermal maturity). Practical so- This book is, of course, complementary to other pub- lutions can be used to address existing interpre- lications on related topics, such as Acoustics of Porous tation problems in rock-physics-based AVO analysis and Media (Bourbie et al., 1987), AVO (Chopra and Castagna, prestack seismic inversion in order to streamline the 2014), Introduction to the Physics of Rocks (Gueguén and workflows in subsurface characterization.

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00-Vernik_FM.indd 10 12-08-2016 20:04:40 Acknowledgments

I wish to thank Marathon Oil Corporation for permis- have helped to focus on what is important. Special thanks sion to publish this book. Special thanks are extended to go to Amos Nur (Stanford University) for the jumpstart he David Brimberry, Dicman Alfred, Jim Allen, Robert provided me in my early years in the United States. I am Blanchard, Vladimir Grechka, Glory Kamath, Yulia also enormously grateful to Rebecca Latimer, Tad Smith, Khadeeva, Scott Koza, Jadranka Milovac, and Chris Konstantin Azbel, and Marco Perez for their thoughtful Tuttle (all presently or formerly with Marathon Oil), reviews and many helpful suggestions. Mark Boggards and Niven Shumaker (Noble Energy), Special thanks also go to the SEG publication group, and Tim Lane and Leon Thomsen (ex-BP). Cooperation led by Susan Stamm, for their diligence in the book’s with Mark Kachanov (Tufts University) over the last five early review and later composition and printing phases. years has been very useful in bringing me fresh perspec- Anne Thomas’s help with style editing and making the tives on theoretical elasticity, whereas discussions with book consistent with the SEG standards is most Brian Russell, Tad Smith, Kitty Milliken, and Öz Yilmaz appreciated.

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