Genetic mechanism of low resistance in shale analyzed by triaxial compression test

【Objective】The gas shale of Wufeng-Longmaxi Formation in the southern Sichuan Basin shows the low-resistance anomaly in multiple areas. Statistical analysis reveals that the low-resistivity zone (<10 Ω·m) in the Wufeng-Longmaxi shale is spatially associated with tectonic faults. To reveal the coupling mechanism between shale resistivity and tectonic fault, 【Methods】this paper determined the petrological and geochemical characteristics of the outcrop samples obtained from the Wufenglongmaxi shale of Southern Sichuan Basin through analyses of thin-section identification, X-ray diffraction, laser Raman spectroscopy, whole-rock asphalt reflectance, and conventional physical property. Subsequently, the characteristics of shale resistivity change during the deformation were revealed using the triaxial compression tests. The impact of the fracture system generation and the conductive fluid intrusion on low-resistance shale was also clarified.【Results】The results show that the resistivity of shale samples, primarily composed of uniform clay minerals, pyrite, and organic matter with high thermal maturity, significantly decreased after being saturated with various salinity brines. There is a positive correlation between the reduction of resistivity (95.07%-98.70%) and the brine salinity. After reaching the limit of compressive strength, the resistivity of brine-saturated samples generally experienced a further resistivity decrease of 5.7 Ω·m to 25.7 Ω·m (with an average value of 13.3 Ω·m). This decrease maintained a linearly positive correlation with the resistivity observed after cracking.【Conclusion】The intrusion of conductive fluid and the generation of the fracture system in shale were the primary controlling factors for shale resistivity reduction, and the resistivity after cracking was governed by both the intruded brine salinity and the fracture density. This research elucidates and supplements the influence mechanism of tectonic fault zones on shale resistivity, enriching the theoretical framework for understanding the genesis of low-resistivity shale, and holds significant implications for exploration and development of low-resistivity shale formations.