Insights into the Multiscale Conductivity Mechanism of Marine Shales from Wufeng–Longmaxi Formation in the Southern Sichuan Basin of China

Abstract

Gas-bearing capacity is an important feature in the evaluation of the different properties of shale. The calculation of adsorbed gas and free gas content is the focus of the shale gas-bearing capacity evaluation, for which gas saturation is a key parameter. In the present study, the target area was the marine shales of the Wufeng–Longmaxi Formation in the Dingshan, Jiaoshiba, and Changning areas of the southern Sichuan Basin in China, while the purpose of the study was the more effective characterization of Langmuir’s volume and Langmuir’s pressure using well-logging data. The application of new well-logging technologies in the evaluation of shale gas-bearing capacity is seldom studied, and the conventional sand-mudstone saturation models calculate the shale gas-bearing capacity with low accuracy. Therefore, this study systematically analyzed the shale conductivity mechanism, which laid the foundation for a new calculation model for shale gas saturation. The analysis results of the influencing factors of shale conductivity in the study area showed that the resistivity of shale in the interlayer is mainly affected by the thin low-resistivity layers, and the resistivity of shale in laminates is affected by clay minerals, pyrite, overmature conductive organic matter, and pore fluids. Moreover, this study further clarified the main controlling factors of the conductivity mechanism by implementing a multiscale analysis. Herein, on the meter-scale, the influence of thin low-resistivity layers on the shale resistivity was characterized based on a horizontal resistivity model; on the centimeter-scale, the influence of pore fluids on shale resistivity was investigated based on the rock electrical experiments; and on the nanometer-scale, the influence of clay minerals, pyrite, and organic materials on shale resistivity was examined based on digital rock technology and numerical simulation of the electrical properties. The results showed that the factors affecting the conductivity of the shale, from the strongest to the weakest, are conductive organic matter, thin low-resistivity layer, clay mineral, pore water, and pyrite, respectively.