An effective azimuthal reflectivity modeling tool for generating seismic data in anisotropic shale reservoirs

ABSTRACT An organic-rich shale generally undergoes complex geologic, physical, and chemical evolution and exhibits obvious anisotropic characteristics. Azimuthal seismic data processing and interpretation provide support for quantitatively characterizing shale reservoirs. To improve the simulation of complex seismic wavefield responses for interbedded shales, we develop a MATLAB toolbox named the azimuthal reflectivity modeling (AzRM) tool to model seismic reflection amplitude variations with incidence angle and azimuth (AVAZ). In the AzRM tool, we develop how to compute the exact reflection and transmission coefficients of the stratified anisotropic model based on analytical solutions and iterative algorithms. To ensure calculation accuracy and efficiency, we use the 1D fast Fourier transform (FFT) to generate synthetic seismic gathers. The advantage of the seismic modeling method in the AzRM tool is that it considers the impact of multiple waves and transmission losses within interbedded shales on reflection amplitudes, unlike convolution models that only consider primary reflection wave simulations. Based on the effective rock-physics model developed for shale rocks, we analyze how geologic sweet spot parameters (maturity, total organic carbon [TOC] content, porosity, and clay content), layer thickness, and fracture parameters affect the AVAZ responses in organic-rich shale reservoirs. The results show that changes in maturity, TOC content, and fracture density have a greater impact on the AVAZ responses, whereas clay content has a relatively small impact on them. We illustrate the main algorithms, explain the structure of the source codes, and conclude that an AVAZ response analysis for stratified anisotropic media may provide theoretical guidance for seismic inversion and reservoir characterization in organic-rich shales.