Anisotropy of complex resistivity of the shale in eastern Guizhou province and its correlations to reservoir parameters of shale gas

The electrical method is widely used to recognize the best shale reservoirs due to its low cost and high efficiency compared with the seismic method. However, there are a few studies about the electrical properties of shale, especially the anisotropy of complex resistivity and its correlations to parameters of shale gas reservoirs, which limits the application of the electrical method in shale gas exploration. In this paper, we discuss the anisotropic properties of complex resistivity in 3 orthogonal directions and the relationships between resistivity and reservoir parameters of shale gas such as Total Organic Carbon (TOC) content, brittle minerals content and maximum vitrinite reflectance. All discussions are based on the experimental data obtained through analyzing six typical shale samples collected from eastern Guizhou Province. Research suggests that : (1) The anisotropy of amplitudes of complex resistivity varies with the frequency. In low and medium frequency bands, the vertical amplitude of complex resistivity is significantly greater than the strike and dip ones. However, with the increasing of frequency, the differences of amplitude among 3 orthogonal directions become smaller. Even at a certain frequency, the anisotropy of amplitude disappears. (2) The double Cole-Cole model fits well with the measured complex resistivity data of shale. (3) The TOC content has significant linear positive correlations with the strike resistivity and dip resistivity. Thus, it is possible to predict TOC content through the strike or dip resistivity. (4) No evident relationships are found between the resistivity, brittle mineral content and maximum vitrinite reflectance. The reason is the complexity of the influence of brittle minerals content and vitrinite reflectance on resistivity. The presented research results are beneficial to gain a comprehensive understanding of the electrical characteristics of shale and provide a new way to evaluate shale gas reservoirs.