Shear-wave anisotropy in the Austin Chalk, Texas, from multi-offset VSP data: case studies

The Austin Chalk is an important reservoir rock in which produc-tivity and direction of fluid flow are predominantly controlled by the presence of fractures. Recent work has shown these fractures to be anisotropic to seismic waves. In this paper, we analyze VSP data from three sites along the Austin Chalk trend to determine the anisotropic structure of the region and investigate the link between anisotropy and reservoir productivity. The anisotropy parameters of polarization direction and time delay are extracted from the data sets using currently available anisotropy estimation techniques. Most of the observed time delay builds up in the near-surface layers, and the polarization directions of the leading split shear waves follow regional fracture and stress directions. Multioffset VSP data from the BP test site near Devine, Texas, are modelled using a combina-tion of crack and thin-layer anisotropy to give a structure with orthorhombic symmetry which yields a good match between observed and synthetic results. Shear-wave data, also from Devine, generated by a P-wave source and by mode conversions are also analyzed and yield similar results to the direct shear-wave data. Zero-offset VSP data from Burleson and Dimmit counties, supplied by Amoco, are also examined. The Burleson VSP was acquired in a producing well. The chalk layer at this site is too thin for observable time delays to build up in transmitted shear waves, but the analysis of reflected arrivals shows a correlation between anisotropy and pro-ductivity in the Austin Chalk