EFFECTS OF TRANSVERSE ISOTROPY ON P-WAVE AVO FOR GAS SANDS

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 (DELTAdelta) 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 (DELTAdelta) and epsilon difference (DELTAepsilon) amplify AVO effects. For small angles of incidence, DELTAdelta plays an important role in AVO while DELTAepsilon 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.