Joint inversion of PP and PS AVAZ data to estimate the fluid indicator in HTI medium: a case study in Western Sichuan Basin, China

Abstract

The existence of fractures in an otherwise isotropic medium induces anisotropy in the medium. Because of existing in situ stress, most fractures are often aligned close to vertical, rendering a reservoir azimuthally anisotropic in character. Joint interpretation of PP and PS seismic data generally provides greater details in resolution of the estimated subsurface rock properties and geological structures than conventional PP seismic data. Here we report on the applicability of PP and PS azimuthal amplitude variation with offset (AVAZ) data in fracture characterization. The theory is based on a linear slip model and the Born formula such that PP- and PS-reflection coefficients are sensitive to fracture weaknesses. First we demonstrate numerical experiments with synthetic PP-AVAZ, PS-AVAZ and joint inversion to estimate fluid indicator. Results show that when the fractures have low saturation of gas, the fluid indicator estimated from PP-AVAZ data is fairly accurate. However, when gas saturation reaches up to 70%, joint inversion can help to improve the resulting poor quality in PP-AVAZ data inversion. For high values of gas-saturation, both PP inversion and joint inversion are sensitive to errors in background Poisson's ratio. Based on the result of our numerical experiment with synthetic data, we analyze a field dataset from the Western Sichuan Basin in China. The inversion result is consistent with well log based interpretation. All known reservoirs are accurately depicted by the estimated fluid indicator while the false gas zones interpreted by other methods are eliminated. When displayed as an inline section, the distribution of reservoirs appears consistent with the interpretation of the stratigraphy and geological structures.