Abstract

Velocity anisotropy should be taken into account when analyzing the amplitude variation with offset (AVO) response of gas sands encased in shales. The anisotropic effects on the AVO of gas sands in transversely isotropic (TI) media are reviewed. Reflection coefficients in TI media are computed using a plane-wave formula based on ray theory. We present results of modeling special cases of exploration interest having positive reflectivity, near-zero reflectivity, and negative reflectivity.The AVO reflectivity in anisotropic media can be decomposed into two parts; one for isotropy and the other for anisotropy. Zero-offset reflectivity and Poisson's ratio contrast are the most significant parameters for the isotropic component while the delta difference (Delta delta ) between shale and gas sand is the most important factor for the anisotropic component. For typical values of TI anisotropy in shale (positive delta and epsilon ), both delta difference (Delta delta ) and epsilon difference (Delta epsilon ) amplify AVO effects. For small angles of incidence, Delta delta plays an important role in AVO while Delta epsilon dominates for large angles of incidence.For typical values of delta and epsilon , the effects of anisotropy in shale are: (1) a more rapid increase in AVO for Class 3 and Class 2 gas sands, (2) a more rapid decrease in AVO for Class 1 gas sands, and (3) a shift in the offset of polarity reversal for some Class 1 and Class 2 gas sands.

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