Interpretation of AVO anomalies

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 [Formula: see text] and S-wave velocity [Formula: see text]. 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 [Formula: see text] contrast. As a result, 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.

Distribution of melt along the East Pacific Rise from 9°30′ to 10°N from an amplitude variation with angle of incidence (AVA) technique

“…1b), we seek to determine the distribution of melt beneath the ridge axis. As a tool for data analysis we use a standard petroleum exploration amplitude variation with angle of incidence (AVA) technique based on the crossplotting of seismic attributes (Castagna et al 1998;Ross 2000;Pelletier 2008;Foster et al 2010). In the literature, amplitude variation with offset (AVO) crossplotting is interchangeably used with AVA to describe the same technique (e.g.…”

“…In the literature, amplitude variation with offset (AVO) crossplotting is interchangeably used with AVA to describe the same technique (e.g. Foster et al 2010;and references therein). However, the two terms, AVA and AVO, can be considered equivalent only for a shallow, horizontal and planar reflector, for which angle of incidence of a given trace can be approximated by its source-receiver offset (Shang et al 1993).…”

“…Another approach uses angle stacks (e.g. Foster et al 2010), with A extracted from a near-angle stack (incidence angle up to ∼20…”

“…2) for oilindustry applications are a background trend characterizing 'nonpay' background and anomaly characterizing possible hydrocarbonbearing regions or anomalous lithology (e.g. Castagna et al 1998;Ross 2000;Foster et al 2010). In practice, the background trend is estimated from either seismic or well data across the interface between reservoir sedimentary rock and seal rock in a region devoid of hydrocarbons.…”

Estimating gas saturation in a thin layer by using frequency‐dependent amplitude versus offset modelling

“…The importance of the problem for CO2 monitoring has also become increasingly apparent in recent years (Xue and Ohsumi 2004;Carcione et al 2006;Daley et al 2008;Ivanova et al 2012). Amplitude variation with offset (AVO) analysis from pre-stack seismic reflection data has been an important tool for fluid and lithology detection (Ostrander 1984;Rutherford and Williams 1989;Castagna and Backus 1993;Russell et al 2003;Foster et al 2010;Simm and Bacon 2014). During AVO analysis, Gassmann's equations (Gassmann 1951;Biot 1956) are often used to simulate changes in bulk and shear moduli from changing pore fluid.…”