Offset-Dependent Reflectivity Theory and Practice of AVO Analysis

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Investigations in Geophysics Series, Volume 8 Michael R. Cooper, Series Editor

Downloaded 03/27/24 to 118.99.125.10. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/page/policies/terms DOI:10.1190/1.9781560802624.fm

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Offset-Dependent Reflectivity

Theory and Practice of AVO Analysis

Edited by

John P. Castagna and Milo M. Backus

Society of Exploration Geophysicists P.O. Box 702740, Tulsa, OK 74170-2740

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Library of Congress Cataloging-in-Publication Data

Offset-dependent reflectivity: theory and practice of AVO analysis / edited by John P. Castagna and Milo M. Backus.

p. cm.--(Investigations in geophysics series: v. 8) Includes bibliographical references and index.

ISBN 0-931830-46-X: $119.00

1. Petroleum--Prospecting. 2. Natural gas--Prospecting.

3. Amplitude variation with offset analysis. I. Castagna, John Patrick. II. Backus, Milo M., 1932 III. Series

TN271.P4034 1993

622' .1828--dc20 93-14279

CIP

ISBN 0931830-46-X Series ISBN 1-56080-059-3 Volume

Published 1993

Second Printing 1993 Third Printing 1995 Fourth Printing 1997 Fifth Printing 1999 Sixth Printing 2003 Seventh Printing 2007

Printed in the United States of America Downloaded 03/27/24 to 118.99.125.10. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/page/policies/terms DOI:10.1190/1.9781560802624.fm

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Contents

Preface vi Introduction vii

Chapter I: Principles

AVO analysis•Tutorial and review 3 J.P. Castagna

Pseudo-shear•The analysis of AVO 37 R. S. Spratt, N. R. Goins, and T. J. Fitch

Prestack analysis•An integrated approach for seismic interpretation in clastic basins 57 S. K. Dey-Sarkar and S. V. Svatek

Properties of direct AVO hydrocarbon indicators 78

H. W. Swan

Prestack synthetic seismogram of finely layered elastic earth 93 T. K. Kan and C. Y. Young

Weak anisotropic reflections •03

L. Thomsen

Chapter II' Petrophysics

Shear-wave velocity control •5

J.P. Castagna, M. L. Batzle, K. M. Tubman, J. E. Gaiser, and M.D. Burnett

Rock Physics•The link between rock properties and AVO response •35 J.P. Castagna, M. L. Batzle, and T. K. Kan

Chapter III: Processing

Seismic data processing for AVO and AVA analysis •75

J. R. Resnick

Wavelet attenuation and AVO •90 P. C. Luh

Automated extraction of AVA information in the presence of structure 199 Z. Shang, J. A. McDonald, and G. H. F. Gardner

Chapter IV: Interpretation

AVO applications in Saudi Arabia e•

E. F. Chiburis

A model based analysis of AVO in the Sacramento Valley •30 M. R. Hong, J.P. Castagna, and C. J. Sicking

AVO analysis in low- and high-porosity gas sand reservoirs •38 J. Regueiro

Offset-dependent amplitude analysis of data from the Veslefrikk Field, Offshore Norway •50

K. H. Wrolstad

Interpretive aspects of AVO•Application to offshore Gulf Coast bright-spot analysis •67

R. Estill and K. Wrolstad

Chapter V' Inversion

Inversion of P-wave AVO e87

E. Demirbag, C. (7oruh, and J. K. Costain

Elastic inversion of Gulf of Mexico data 303 J. J. Carazzone and L. J. Srnka

Chapter VI: Multicomponent AVO

Linearized AVO inversion of multicomponent seismic data 317 G. J. M. LOrtzer and A. J. Berkhout

Correlation and interpretation of P-P and P-SV data, Zamora gas field, Yolo County, California J. F. Zaengle and C. W. Frasier

Index 346

333

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Preface

This volume is intended to provide a comprehensive review of amplitude-variation-with-offset (AVO) anal- ysis as applied to hydrocarbon exploration. From conception to fruition, it has taken over six years to finish the job.

Failures of flesh and bone, editorial changes, job reassignments and relocations, and the more ordinary obstacles, conspired to delay completion. In 1987 we had a core group of six papers (Thomsen; Castagna et al. a and b; Chiburis; Wrolstad; and Estill and Wrol- stad). The remaining papers were added in 1989 and 1990 with the exception of the tutorial which was added in 1991. Many of the papers have been revised

"organically" over the years. Perhaps due to the quality of the original contributions, or possibly as a

consequence of the important and controversial sub- ject matter, we feel that the delay did not materially

decrease the significance of these papers.

We are deeply grateful to the thirty-five authors whose patience, cooperation, and encouragement en- abled us to complete the task. Thanks also to Sabra Gronemeier whose contributions are too many to enumerate. Special acknowledgments are owed to Lynn Griffin without whose remarkable skill and ded- ication this volume could not have been completed.

J. P. Castagna

ARCO Oil and Gas Co.

M. M. Backus

Univ. of Texas at Austin

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Introduction

"The P-wave reflection coefficient at an inter-

face separating two media is known to vary with angle of incidence. The manner in which it varies is strongly affected by the relative values of Poisson's ratio of the two media. For moderate

angles of incidence, the relative change in reflec- tion coefficient is particularly significant when Poisson's ratio differs greatly between the two

media.

Theory and laboratory measurements indicate that gas sands tend to exhibit abnormally low Poisson's ratios. Embedding a low velocity gas sand into sediments having 'normal' Poisson's ratios should result in an increase in reflected P-wave energy with angle of incidence. This phenomenon has been observed on conventional seismic data recorded over known gas sands."

With these words, W. J. Ostrander ushered in a new era in seismic interpretation. Although many workers were aggressively moving forward with amplitude variation with offset (AVO) analysis before 1982, Ostrander's presentation at the 52nd Annual Interna- tional Meeting of the Society of Exploration Geophys- icists popularized the concept as an exploration tool.

The presentation proved to be a precursor to the avalanche of literature on the subject which has ap- peared during the past decade.

The tremendous promise of the isolation and analy- sis of offset-dependent reflectivity lies in the theoreti- cal relationship between reflection coefficient, incident angle, and the variation in compressional-wave veloc- ity (V•), shear-wave velocity (Vs), and density (p) across an interface. These contrasts are, in turn, dependent on rock properties variations. In particular, an interface between gas-saturated and water-satu- rated unconsolidated or poorly consolidated sediments exhibits distinctive relationships between these pa- rameters. Thus, bright spots caused by gas reservoirs potentially can be distinguished from high amplitude events of the same polarity caused by coal or other low impedance units. In addition to direct hydrocarbon indication for gas and high GOR oils, other potential applications include lithology or porosity indication, extraction or "true" zero-offset reflectivity, and im- proved imaging resulting from proper treatment and exploitation of waveform variations with offset.

All too often, however, the potential of AVO anal- ysis has not been realized. Under proper (sometimes

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fortuitous) circumstances, AVO analysis has proved to be a useful prospecting tool (success in the Sacra- mento Valley is particularly notable). In other situa- tions, the technique has simply failed (although the literature seems to be strongly biased toward success- ful examples). Table 1 shows questions concerning AVO analysis raised at the AVO workshop conducted at the 1983 SEG convention. These questions remain relevant 10 years later.

The enormous potential value of the method and the corresponding need for education and further research on the subject are the motivation for this volume. We make no attempt to present a robust methodology for AVO analysis. Such a methodology requires special data acquisition and processing considerations as well as an integrated approach to interpretation, and is certain to remain proprietary within the confines of those organizations which are successfully exploiting AVO analysis in the quest for hydrocarbons. We also make no claim to exhaustively treat the subject. It is our intention simply to (1) provide a general introduc- tion to the subject and review the current state-of-the- art, (2) provide useful reference materials and data, and (3) present recent original contributions at the leading edge of AVO technology. It is hoped that this volume will be useful to explorationists and students, as well as to researchers in the field. Thus, we have solicited a wide variety of original contributions. Some papers are at a basic tutorial level with broad appeal, while others will attract only a select audience. Some are applied or empirical while others are highly theo- retical. A diversity of approaches and orientations will become apparent to the reader who ultimately will have to synthesize this information and utilize it in a practical way.

This volume is divided into six chapters. Chapter 1 covers the basic principles of offset-dependent reflec- tivity as applied to hydrocarbon exploration and de- velopment. Two papers, (1) Spratt, Goins, and Fitch and (2) Dey-Sarkar and Svatek present integrated approaches to practical AVO analysis as applied by two major oil companies. The properties of AVO indicators, like those presented throughout this vol- ume, are then investigated by Swan. Kan and Young discuss lessons learned from full-waveform elastic modeling while Thomsen describes the complicating effects of realistic earth media which exhibit weak anisotropy.

Chapter II covers the fundamental petrophysical Downloaded 03/27/24 to 118.99.125.10. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/page/policies/terms DOI:10.1190/1.9781560802624.fm

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Table 1. Questions raised at the 1983 AVO workshop, 53rd Annual International Meeting, Society of Exploration Geophysicists

--What is the current state of our knowledge of operative in-situ rock properties?

--Under what plausible real earth conditions do we expect a significant change in reflection coefficients with angle?

wWhat are the important rock property variables and what is the sensitivity of angle dependence to these variables?

--What are the available methods of seismogram synthesis and how do they compare?

--What have we learned about the generalized reflectivity of layers, layer sequences, and transition zones?

--What are some of the important characteristics of solid earth synthetic seismograms obtained for realistic earth models?

--What are the effects of anisotropy?

--In what circumstances have we been able to exploit angle dependent reflectivity effects in exploration?

--Are special data acquisition, processing, and display techniques helpful in isolating the derived target characteristics?

--What are the most promising likely applications in the short term?

--How significant are the changes in waveform and amplitude observed in practice?

--Should we recast our basic model for certain environments?

--What is the relative potential merit of shear impedance estimates made from (a) P-P AVO, (b) P-S-P conversions, (c) P-S-S conversions, (d) S-S AVO?

--What is the likelihood of providing multiparameter subsurface images for exploration use?

--What are the most significant gaps in our knowledge, understanding, and capabilities with regard to the change in reflectivity with offset?

principles which form the basis for AVO analysis. The first paper (Castagna, Batzle, Tubman, Gaiser, and Burnett) describes various techniques for measuring or estimating shear-wave velocities, while the second (Castagna, Batzle, and Kan) describes the relation- ships between rock properties and wave propagation parameters such as velocity and attenuation.

Chapter III contains three papers relating to seismic data processing for AVO analysis. Resnick illustrates the special processing necessary for AVO analysis, Luh shows the importance of Q compensation, and Shang, McDonald, and Gardner present an automated method for amplitude-versus-angle extraction in struc- turally complex areas.

Five AVO case studies with real data and real

exploration objectives constitute Chapter IV. The authors (1) Chiburis, (2) Hong, Castagna, and Sicking, (3) Regueiro, (4) Wrolstad, and (5) Estill and Wrolstad provide examples of AVO interpretation from four

continents.

Chapters V and VI, covering elastic inversion and multicomponent AVO, have a distinct eye on the future. Demirbag, Coruh, and Costain present an inversion approach which is computationally practi- cal, while Carrazone and Srnka provide a real data example of AVO inversion. L6rtzer and Berkhout study multicomponent inversion, while Zaengle and Frasier provide a case study of multicomponent AVO analysis.

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