Genesis, Identification Method, and Exploration Potential Evaluation of Marine Low-Resistivity Shale Gas Reservoirs

Low-resistivity shale gas reservoirs with high and low gas yields are widely distributed in the southern Sichuan Basin. Research on the genesis, identification methods, and exploration potential of these reservoirs is lacking, leading to increased risks in shale gas exploration and development. This paper focuses on the Longmaxi Formation shale in western Chongqing as the research object. Based on the experimental results from XRD, the carbon sulfur analyzer, asphalt reflectivity, XPS, particle helium porosity, measured gas content, and logging curve data, a scatter plot of the gas content and logging resistivity is generated to ascertain the resistivity limit of low-resistivity shale gas reservoirs. By conducting correlation analysis on logging resistivity, mineral composition, TOC, R o, graphitization degree, water saturation, porosity, and fracture development degree, the genetic types of low-resistivity shale gas reservoirs are identified in this paper. By integration of the resistivity difference quotient, a genetic type and exploration potential identification chart for low-resistivity shale gas reservoirs are developed. The results indicate that the upper limit of logging resistivity for low-resistivity shale gas reservoirs is 15 Ω m. In the study area, there are primarily three types of low-resistivity shale gas reservoirs: high water-cut fracture type (D1 in the middle of the Yunjin syncline), high water-cut fracture-organic matter graphitization type (D2 in the south of Yunjin syncline, D4 in Linjiang syncline), and organic matter graphitization type (D3 in the south of Yunjin syncline, D5 in Huguosi syncline). Shale gas reservoirs with normal resistivity, characterized by logging resistivity exceeding 15 Ω m, as well as organic graphitized low-resistivity shale gas reservoirs, with logging resistivity below 15 Ω m and a resistivity difference quotient lower than 0.3, exhibit exploration potential.