Bayesian Seismic Inversion of Geological and Engineering Sweet-Spot Parameters in Monoclinic Media Embedded in a VTI Background

Given that fractures in the shale gas-bearing reservoirs are tilted and horizontal, it is appropriate to model the fractured formation as the monoclinic medium with tilted fractures embedded in the vertically transverse isotropic (VTI) background. Within a monoclinic medium, our contribution focuses on the direct azimuthal seismic inversion of the comprehensive geological and engineering sweet-spot (GESS) parameters for fluid identification, rock brittleness assessment, stress determination, and the detection of two distinct natural fracture sets. With the anisotropic Gassmann fluid substitution, we first derive the highly precise stiffness coefficients of saturated monoclinic media. Subsequently, a new linearized PP-wave reflection coefficient integrating fluid bulk modulus, brittleness index (BI), vertical effective stress correlation parameter, as well as horizontal and tilted fracture densities, is derived. A stepwise Bayesian strategy is then employed to invert above GESS parameters from the offset-vector-tile (OVT) domain seismic data. The robustness of the method is validated by the synthetic azimuthal data with varying signal-to-noise (S/N) ratios. Finally, the effectiveness is evidenced through its application to fractured gas-bearing shale reservoirs in the Sichuan Basin, China, systematically substantiated by well data, geological structures, and microseismic events. It provides predictions of the fluid bulk modulus, BI, vertical effective stress correlation parameter, horizontal and tilted fracture densities.