Genesis Mechanism of Low Resistivity in the Lower Cambrian Qiongzhusi Formation Shale and Its Response...

In recent years, extensive low-resistivity shale reservoirs have been discovered in the Sichuan Basin, especially in the Lower Cambrian Qzs Fm (Qiongzhusi Formation). However, significant variations in gas production from these low-resistivity reservoirs across different regions have posed challenges in selecting and predicting sweet spots for the exploration and development of shale gas. Therefore, the shale samples were subjected to the analysis of a series of experiments, including core resistivity measurements, thermal simulations, and Raman spectroscopy, in order to explore the genesis mechanisms behind low-resistivity shale reservoirs and analyze their response characteristics to pore structure. The results show that HMG-OM (highly mature graphitized organic matter) and formation water stored in pores and fractures provide more conductive pathways, which are the primary mechanisms driving the low-resistivity characteristics of shale within the study area. As graphitization increases, shale resistivity significantly decreases, while maturity increases. Additionally, there are notable changes in the shale mineral composition and pore structure, including quartz breakage, clay minerals transforming into quartz, feldspar, and other minerals, and the dissolution of calcite and pyrite, which leads to a significant increase in inorganic pores. Meanwhile, the graphitization of OM causes the continuous deformation, shrinkage, and collapse of OM pores, resulting in poor development of OM pores. In this study, we integrate theoretical analysis and the experimental results and identify the Weiyuan-Ziyang area in southwestern Sichuan as a key area for future shale development in the Qzs Fm, with the aim of providing theoretical guidance for the exploration and development of shale gas in the Sichuan Basin.