Application of shear wave estimation based on Poisson’s ratio model in Block TII of the eastern margin of the Pre-Caspian Basin

The inversion of pre-stack wave impedance is closely related to the parameters such as incident angle, P-wave velocity, S-wave velocity, and density. Compared to post-stack inversion, the inclusion of both P- to S-wave velocities allows for the derivation of more comprehensive elastic parameters. This enhances the ability to differentiate lithology and hydrocarbon-bearing formations, providing a more accurate foundation for drilling decisions. Shear-wave velocity data are particularly critical for pre-stack impedance inversion. However, in the TII block, only one set of S-wave logging data is available, and its reliability is uncertain due to factors such as mud invasion. As a result, accurately estimating shear-wave velocity especially its hydrocarbon-sensitive variations are a crucial step in elastic parameter inversion for pre-stack analysis. Given the characteristics of the main target formations, KT-I and KT-II, in the TII block, porous carbonate reservoirs with strong heterogeneity, this study proposes a shear-wave velocity estimation method based on an empirical Poisson’s ratio model. This approach effectively enhances the sensitivity of pre-stack seismic attributes to hydrocarbons. Through test processing, parameters of the Poisson’s ratio-based estimation model were selected to the geological conditions of the TII Block. A reservoir-sensitive threshold for the v_P/v_S ratio was identified as 1.8. Based on the v_P/v_S parameters of KT-II_G formation, the thickness of effective reservoir is predicted. The predicted thickness is more than 85% consistent with well verification, which provides a strong support for exploration and evaluation of well location deployment.