Orthorhombic Anisotropic Rock Physics Modeling for Fractured Marine Shale Reservoir in Sichuan Basin

Fractured shale often represents complex anisotropy characteristics. A proper rock physics model can be helpful for accurate prediction of fracture properties. In this paper it proposes an orthorhombic anisotropic rock physics model, considering the distribution characteristics of fracture dip and accurate characterization of VTI (Vertically Transverse Isotropy) characteristics, for the fractured shale reservoir of Wufeng-Longmaxi Formation in Weiyuan-Rongxian area, Sichuan Basin. The modeling results of the well analysis show that the average fracture dip, the standard deviation of fracture dip, and the variation range of fracture dip in shale have significant influence on azimuthal anisotropy. The anisotropy parameters related to S-wave velocity $ {\gamma }^{\left(2\right)} $ and $ {\gamma }^{\left(3\right)} $ are only sensitive to fracture density and insensitive to filled fluids, and can be used for prediction of fracture density in seismic exploration. The anisotropy parameters related to P-wave velocity $ {\varepsilon }^{\left(2\right)} $, $ {\delta }^{\left(2\right)} $, $ {\delta }^{\left(3\right)} $, $ {\varepsilon }^{\left(3\right)} $ are simultaneously affected by fracture density and filled fluid, and can be used to detect filled fluids of fracture by combining the anisotropy parameters related to S-wave velocity. In summary, the rock-physics modelling proposed in this paper is suitable for southern marine fractured shale reservoirs represented by Wufeng-Longmaxi Formation shale, and provides a theory for seismic prediction of fracture properties for shale reservoirs.