Shear wave estimation for rock physics reservoir evaluation of jay field in the Niger Delta basin

Abstract

Shear wave logs are one of the key data required for most Amplitude Versus Offset (AVO) studies and quantitative seismic interpretation. Despite its overwhelming importance, it is hardly available in most wells due to its very high cost of acquisition. In the absence of shear wave logs in the field, the Castagna’s empirical relationship that relates P-wave and S-wave velocities and Biot-Gassman’s Fluid substitution equations were adopted for its estimation. The results obtained revealed that Castagna’s empirical relationship underestimated the Shear wave log in the field, even after Biot-Gassman’s Fluid substitution model was applied to define P-wave of the brine filled scenario. A cross plot of Mu-Rho versus Lambda-Rho presented low values for Mu-Rho due to the underestimated shear wave used. To correct for the underestimation, a modified form of the Castagna’s Equation constants were establish. This was achieved by generating several linear regression equations that defines the relationship between P and S-wave for brine sands in the field. The cross plot of Mu-Rho versus Lambda-Rho done with the modified shear wave gave very good results as expected for both fluid and lithology discrimination.

Keywords: Shear Wave, Castagna’s Equation, Biot-Gassman’s Equation, Rock Physics, Cross plots