Controlling Mechanisms of Paleoenvironment on the Resistivity and Polarizability of Shale – A Case Study of the Longmaxi Formation Shale in Southern Sichuan

Abstract

The mineral composition, content, and organic matter enrichment in shale are significantly influenced by the sedimentary environment. However, there is limited understanding of how the sedimentary environment impacts the resistivity and polarizability of shale. This study conducts experimental tests on shale from the Longmaxi Formation (LMXF), employing techniques such as complex resistivity, X-ray diffraction, organic geochemistry, porosity, elemental geochemistry, and argon-ion polishing scanning electron microscopy. The results show that the lower part of the LMXF was deposited in an anoxic environment with high paleoproductivity and low detrital influx, where siliceous shale is developed. This part is characterized by high TOC content, enrichment of biogenic quartz and pyrite, well-developed OM pores, low resistivity, and high polarizability. In contrast, the middle-upper part of the LMXF was deposited in an oxic-dysoxic environment with low paleoproductivity and high detrital influx, which is featured by low TOC content, high clay mineral content, high resistivity, and low polarizability. Redox conditions and paleoproductivity primarily influence the TOC content and the formation of biogenic quartz. The content of pyrite is influenced by redox environments, while clay minerals and terrigenous quartz content are affected by paleoclimate and terrigenous input. The interconnected network of organic matter pores, along with other types of pores, and the content of pyrite are the primary reasons for the high TOC and low resistivity observed in LMXF shale. The pyrite content also influences the polarization effect of shale. Redox conditions and paleoproductivity positively influence conductivity and polarization, whereas terrigenous input and paleoclimate have inhibitory effects on both.