Abstract

The effect of fluid pressure gradient on AVO response is not easy to discern in the literature and is awkward to communicate. Most explorationists cannot predict how log responses and AVO signatures will change if the fluid pressure gradient is different at a prospective area than at known control (even if their depths are the same): An increase in fluid pressure gradient means a decrease in the hardness values of the acoustic properties (VP, VS, and Rho) for both the shales and the sands, but since AVO varies in a nonlinear way with the contrast in these six variables, it is difficult for us to intuit what the overall effect on AVO will be. The depth may also differ, making the problem even more complex. In Tertiary basins, the most important driver of acoustic properties is compaction (which is primarily a function of depth, but is also affected by fluid pressure gradient). Most modeling packages provide somewhat automated ways to calculate the effect of changes in fluid content, thickness, and porosity—but not fluid pressure gradient. Equations and methods exist (Gassmann fluid substitution, Greenberg-Castagna equations, Gardner equation, Widess diagrams, etc.) to investigate the effects of changes in density, compressional velocity, shear velocity, thickness, fluid content, bandwidth, and many other variables, but fluid pressure gradient is conspicuously absent.

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