Relationship Between Electrical Properties and Sedimentary Environment of the Longmaxi Formation Shale, Southern Sichuan

Abstract

The Lower Silurian Longmaxi Formation in the south of Sichuan is a key player in the exploration and development of shale gas in China. Due to a highly complex topographic area, electromagnetic methods (EM) become important exploration means in this area. Many studies have been conducted on the shale mineral composition and electrical properties of shale, however, the correlation between sedimentary environments and the electrical properties of shale remain poorly understood. The electrical properties and sedimentary environment of the organic-rich shale of the Longmaxi Formation have been studied by means of X-ray diffraction, organic geochemistry, scanning electron microscopy and complex resistivity measurements. The discovered high quartz content of the Longmaxi Formation shale results in low resistivity. Deep-water shelf biogenic quartz contributes lower resistivity more than that of shallow-water terrigenous quartz. The deep-water anoxic and organic sedimentary environment led to major enrichment of pyrite, leading to a high polarization effect in shale. We present the correlation between the lithofacies types and electrical properties of Longmaxi Formation. The mixed siliceous shale lithofacies is the most favorable among the three lithofacies, which is characterized by high total organic carbon (TOC) content, high brittleness mineral content, high polarization and low resistivity ("three high and one low"). This feature is an effective identification of shale gas reservoirs by electromagnetic prospecting. Our study can provide constraints on electrical parameters of rocks for electromagnetic "sweet spot" exploration of shale gas, and so this has important geological significance to shale gas exploration and development.

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

“…CuSO 4 electrodes were employed to minimize the polarization effect of the electrodes, and a symmetrical quadrupole connection method was utilized for electrode connection (Adao et al, 2015). AB was supplied with a speci c alternating current, and the impedance of the plug sample was then measured and read through MN (Yang et al, 2020). Throughout the measurement, both ends of the sample remained immersed in a saturated liquid to prevent evaporation and maintain the core's saturated condition, ensuring the experimental results' accuracy.…”

“…Through studying the dispersion characteristics of the LMXF shale, it has been discovered that mineral composition is the main factor affecting resistivity (Xiang et al, 2014), especially the content of pyrite and quartz (He et al, 2017). With further research, the impact of porosity on electrical properties has also been found to be substantial, particularly organic matter pores (Wang et al, 2016;Yang et al, 2020;Zhu et al, 2021Zhu et al, , 2022Zhang et al, 2023). Some researchers have employed geochemical methods to reconstruct the paleoenvironment of shale deposition, revealing a connection between mineral and organic matter content and the paleoenvironment (Loucks and Ruppel, 2007;Dowey and Taylor, 2017;Doner et al, 2019).…”

Differential Thermal Regimes of the Tarim and Sichuan Basins in China: Implications for Hydrocarbon Generation and Conservation

“…The Sichuan Basin, developed on the Upper Yangtze platform basement, experienced a Sinian-Middle Triassic cratonic depression stage and a Late Triassic-Quaternary foreland basin stage (He et al, 2020;Yang et al, 2020). Correspondingly, the strata of the Sichuan Basin, with a total thickness of 5-10 km, consists of two sedimentary systems, including the lower marine carbonate sedimentary system and the upper continental clastic sedimentary system (Xu et al, 2018).…”

Prediction of Shale Gas Pressure based on Multi‐channel Seismic Inversion in Fuling

“…However, with the complication of geological conditions, the requirements for reservoir interpretation accuracy are gradually increasing. The wave impedance based on single-channel inversion always differs greatly from the actual wave impedance through acoustic logging Yang et al, 2020).…”