Interpreting AVO responses

Abstract

We investigated the effects of changes in rock properties on AVO responses. In the slope-intercept domain, reflections from wet sands and shales fall on a trend, the Fluid Line. Reflections from the tops of sands containing gas or light hydrocarbons fall on a trend below the Fluid Line; reflections from the base of gas sands fall on a trend above the Fluid Line. The distance of these trends from the Fluid Line depends upon pore fluid compressibility; i.e., distance increases with increasing compressibility. But, if all other factors are equal, base of sand reflections are displaced further from the Fluid Line than top of sand reflections. Consequently, base of sand reflections, which identify down-dip limits and fluid contacts, will be more prominent than top of sand reflections. Porosity changes affect acoustic impedance, but do not significantly impact the Vp/Vs contrast. As a result, porosity changes move the AVO response along trends parallel to the Fluid Line.