Electrical Anisotropy Analysis by a Modified Thomas-Stieber Model

Analysis of electrically anisotropic reservoirs has been challenging with traditional petrophysical analysis. Several techniques were proposed as a framework for using graphical cross-plots to evaluate shaly-sand reservoirs. However, there has never been a clear workflow to define shale laminations and shale anisotropy. In this study, we incorporate a depth-dependent Thomas-Stieber model to describe the shale laminations and use core observation to describe the existence of shale anisotropy. From the vertical and horizontal resistivity, an electrical anisotropy template was built in conjunction with the modified Thomas-Stieber model and core observations. The template generated assuming isotropic shale underestimated the hydrocarbon volume. However, the template generated considering the shale as anisotropic improved the estimations of hydrocarbon presence, permitting a global assessment of the hydrocarbon potential of the shaly-sand reservoirs. Using the depth-dependent Thomas-Stieber model we showed the presence of compacted shale laminations. We also showed that electrical anisotropy is a function of shale laminations, shale compaction and sand cementation. Using core observations, we confirmed the shale layers are anisotropic. Our electrical anisotropy template enhanced the accuracy of hydrocarbon identification in the anisotropic reservoir and permitted identification of more pay zones from vertical and horizontal resistivity data.