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Book Chapter

AVO: Interpretation of AVO anomalies

By
Douglas J. Foster
Douglas J. Foster
1ConocoPhillips, Houston,Texas, U.S.A. E-mail: douglas.j.foster@conocophillips.com; robert.g.keys@conocophillips.com; f.d.lane@conocophillips.com.
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Robert G. Keys
Robert G. Keys
1ConocoPhillips, Houston,Texas, U.S.A. E-mail: douglas.j.foster@conocophillips.com; robert.g.keys@conocophillips.com; f.d.lane@conocophillips.com.
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F. David Lane
F. David Lane
1ConocoPhillips, Houston,Texas, U.S.A. E-mail: douglas.j.foster@conocophillips.com; robert.g.keys@conocophillips.com; f.d.lane@conocophillips.com.
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Published:
January 01, 2010

Abstract

We investigate the effects of changes in rock and fluid properties on amplitude-variation-with-offset (AVO) responses. In the slope-intercept domain, reflections from wet sands and shales fall on or near a trend that we call the fluid line. Reflections from the top of sands containing gas or light hydrocarbons fall on a trend approximately parallel to the fluid line; reflections from the base of gas sands fall on a parallel trend on the opposing side of the fluid line. The polarity standard of the seismic data dictates whether these reflections from the top of hydrocarbon-bearing sands are below or above the fluid line. Typically, rock properties of sands and shales differ, and therefore reflections from sand/shale interfaces are also displaced from the fluid line. The distance of these trends from the fluid line depends upon the contrast of the ratio of P-wave velocity VP and S-wave velocity VS. This ratio is a function of pore-fluid compressibility and implies that distance from the fluid line increases with increasing compressibility. Reflections from wet sands are closer to the fluid line than hydrocarbon-related reflections. Porosity changes affect acoustic impedance but do not significantly impact the VP / VS contrast. As aresult, porosity changes move the AVO response along trends approximately parallel to the fluid line. These observations are useful for interpreting AVO anomalies in terms of fluids, lithology, and porosity.

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Contents

Geophysical References Series

Geophysics Today: A Survey of the Field as the Journal Celebrates its 75th Anniversary

Sergey Fomel
Sergey Fomel
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Society of Exploration Geophysicists
Volume
16
ISBN electronic:
9781560802273
Publication date:
January 01, 2010

GeoRef

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