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`Investigations in Geophysics Series, Volume 8
`Michael R. Cooper, Series Editor
`
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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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`Ex. PGS 2014
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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
`
`Series
`ISBN 0931830-46-X
`ISBN 1-56080-059-3 Volume
`
`¸ 1993 by Society of Exploration Geophysicists
`All rights reserved. This book or parts hereof
`may not be reproduced in any form without per-
`mission in writing from the publisher.
`
`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
`
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`Preface
`
`vi
`
`Introduction
`
`vii
`
`Contents
`
`Chapter I: Principles
`AVO analysis(cid:127)Tutorial and review
`J.P. Castagna
`37
`Pseudo-shear(cid:127)The analysis of AVO
`R. S. Spratt, N. R. Goins, and T. J. Fitch
`Prestack analysis(cid:127)An
`integrated approach for seismic interpretation in clastic basins
`S. K. Dey-Sarkar and S. V. Svatek
`Properties of direct AVO hydrocarbon indicators
`H. W. Swan
`Prestack synthetic seismogram of finely layered elastic earth
`T. K. Kan and C. Y. Young
`Weak anisotropic reflections
`L. Thomsen
`
`3
`
`(cid:127)03
`
`78
`
`93
`
`57
`
`Chapter II' Petrophysics
`Shear-wave velocity control
`(cid:127)5
`J.P. Castagna, M. L. Batzle, K. M. Tubman,
`J. E. Gaiser, and M.D. Burnett
`Rock Physics(cid:127)The
`link between rock properties and AVO response
`J.P. Castagna, M. L. Batzle, and T. K. Kan
`
`(cid:127)35
`
`Chapter III: Processing
`Seismic data processing for AVO and AVA analysis
`J. R. Resnick
`Wavelet attenuation
`P. C. Luh
`Automated extraction of AVA information in the presence of structure
`Z. Shang, J. A. McDonald, and G. H. F. Gardner
`
`and AVO
`
`(cid:127)90
`
`(cid:127)75
`
`199
`
`Interpretation
`Chapter IV:
`AVO applications in Saudi Arabia
`E. F. Chiburis
`A model based analysis of AVO in the Sacramento Valley
`M. R. Hong, J.P. Castagna, and C. J. Sicking
`AVO analysis in low- and high-porosity gas sand reservoirs
`J. Regueiro
`Offset-dependent amplitude analysis of data from the Veslefrikk Field, Offshore Norway
`K. H. Wrolstad
`Interpretive aspects of AVO(cid:127)Application
`R. Estill and K. Wrolstad
`
`e(cid:127)
`
`(cid:127)30
`
`(cid:127)38
`
`to offshore Gulf Coast bright-spot analysis
`
`(cid:127)67
`
`(cid:127)50
`
`Inversion
`Chapter V'
`e87
`Inversion
`of P-wave AVO
`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
`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
`
`317
`
`333
`
`Index
`
`346
`
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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
`
`vi
`
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`Introduction
`
`reflection coefficient at an inter-
`"The P-wave
`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(cid:127)), 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
`
`VII
`
`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
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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.
`
`eee
`
`VIII
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