Controlling mechanism of shale palaeoenvironment on its resistivity and polarizability: A case study of the Longmaxi Formation in Southern Sichuan

Abstract

The mineral composition, content, and organic matter enrichment in shale are significantly influenced by the sedimentary environment. However, there is a limited understanding of how the sedimentary environment impacts the electrical resistivity and polarization properties of shale. This study conducts experimental tests on shale from the Longmaxi Formation in the extensively explored Sichuan Basin of China, employing techniques such as complex resistivity, X-ray diffraction, organic geochemistry, porosity analysis, elemental geochemistry, and argon-ion polishing scanning electron microscopy. The experimental findings reveal the following key points: 1) The core resistivity of the Longmaxi Formation shale ranges from 14.56 Ω·m to 115.96 Ω·m, with an average value of 64.08 Ω·m. The polarization rate ranges from 0.07 to 0.32, with an average of 0.15. 2) Redox conditions and paleoproductivity primarily influence 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. 3) The interconnected network of organic matter pores with other types of pores and the content of pyrite are the main controlling factors affecting resistivity. The pyrite content also influences the polarization effect of shale. 4) Redox conditions and paleoproductivity positively influence conductivity and polarization, whereas terrigenous input and paleoclimate have inhibitory effects on both. This study provides essential theoretical insights for logging interpretation, electromagnetic exploration of shale gas and subsequent monitoring of hydraulic fracturing.