Seismic Inversion of Anisotropic and Isotropic Fluid Indicators and Fracture Density in the TTI Medium

Description of fluid properties, natural fractures, and stress conditions is significant for seismic characterization of unconventional fractured reservoirs. For the first two aspects, traditional methods of solid-liquid decoupling by low-frequency anisotropic Gassmann fluid substitution equation fall short in two aspects: 1) neglecting the combined effects of fractures and pore/fluid and 2) disregarding the influence of fracture inclination on the seismic anisotropy. To address the above two issues simultaneously, this article derives a novel PP-wave reflection coefficient incorporating fluid bulk modulus, vertical effective stress correlation parameter, fracture density, and coupled anisotropic fluid indicator (CFI) in the tilted transverse isotropy (TTI) medium. The derived TTI-saturated stiffnesses with CFI are revealed to provide improved accuracy regarding physical property parameters (porosity, fluid fillings, and matrix mineral content) and fracture parameters (including fracture density and inclination). To invert the above key parameters from offset vector tile (OVT) domain seismic data, the stepwise seismic inversion strategy based on $L_{p}$ quasi-norm sparsity constraints is employed. Synthesized azimuthal seismic data with varying signal-to-noise ratios (SNRs) validate the feasibility and robustness of the proposed inversion method. Ultimately, the innovative method exhibits compelling effectiveness when applied to fractured shale gas-bearing reservoirs in Sichuan Basin, China.