Challenges in shale-reservoir characterization by means of AVA and AVAZ

In most basins, shale reservoirs are bounded by limestone and carbonates at the top and the base. Consequently the isotropic, boundary reflection coefficients are high. Also, shale is anisotropic with or without existing fractures and the anisotropy is high. As a result, the approximate reflection coefficient equations that are used in the standard amplitude vs. angle of incidence (AVA) and azimuth (AVAZ) provide qualitatively and quantitatively erroneous results. These are illustrated in the paper through comparing the results of approximate and exact boundary reflection coefficients as a function of angle of incidence and azimuth for isotropic, vertical transverse isotropy (VTI), horizontal transverse isotropy (HTI), and orthorhombic media. Based on these observations, we recommend using the anisotropic full-reflectivity method for AVA and AVAZ analysis and as the forward modeling engine of the AVA and AVAZ inversion methods.