Azimuthal anisotropy analysis of wide-azimuth P-wave seismic data for fracture orientation and density characterization in a tight gas reservoir

The role of anisotropy in fracture detection has dramatically increased with the advent of wide-azimuth (WAZ) and high-density seismic acquisition. Fracture prediction using horizontal transverse isotropy (HTI) anisotropic theory is a useful tool for identifying reservoir characteristics. We have developed an approach for fracture density and orientation estimation based on the combination of a velocity variation with azimuth (VVAZ) and an amplitude variation with azimuth (AVAZ) analysis workflow. First, we sort the prestack WAZ data into offset vector tile (OVT) sectors and perform VVAZ inversion by elliptical velocity fitting of measured azimuth-differential traveltimes. In this step, we can predict the fast P-wave velocity, slow P-wave velocity, and fracture orientation. Second, we apply AVAZ inversion to extract more accurate predictions of the anisotropic gradient and fracture orientation. We implement the method with 3D prestack WAZ seismic data acquired in the Sichuan Basin, from the southwest part of China. The field data example indicates that the inversion results agree with geologic information and well-log imaging data, which confirm the effectiveness of this technology.