Seismic rock physics characterization of anisotropic shale-a Longmaxi Shale case study

s An anisotropic rock physics model based on effective medium theories is constructed for shales and applied to the Longmaxi Shale formation. In the rock physics model, intrinsic VTI (transverse isotropy with a vertical asymmetry axis) anisotropy due to preferred orientations of clay particles is quantified by introducing the clay lamination (CL) index in the Backus averaging method, and additional anisotropy enhanced by bed-parallel fractures is described by the Chapman model for multi-scale pore spaces. Rock physics templates are constructed based on the model in order to better understand the implicit relations between reservoir properties, elastic and mechanical properties and seismic parameters. A model-based method is proposed for inversion of the CL and aspect ratio (AR) of horizontal fractures, as well as VTI anisotropy parameters for the shale formation. Results indicate that the obtained CL index shows a negative correlation with clay content, which is further used in the prediction of S wave velocity (Vs) based on the constructed rock physics model. A more accurate prediction of Vs justifies the application of the constructed rock physics model and the proposed inversion method. Seismic modeling and inversion for the Longmaxi Shale are conducted. Seismic-well tie based on the propagator matrix method indicates that seismic signatures from the target gas shale can be identified by the waveform between a trough and a peak of a strong reflection. Examinations using Rüger's theory of P-wave reflection coefficients indicate that the variations in vertical P-wave velocity VP01 and the anisotropy parameter of the target shale provide interpretations for real seismic amplitudes versus offset (AVO) data. Seismic AVO inversion is then implemented based on Rüger's theory. The inverted anisotropy parameter is further interpreted as the CL index, which reflects the microstructure of the solid matrix of the shale. Seismic inverted layer thickness and the P-wave anisotropy parameter of the target shale reservoir show good agreement with the values obtained at the location of involved well, which justifies the feasibility and applicability of the seismic inversion method.