Pore microstructure and its controlling effects on gas content of deep shale reservoirs in the Wufeng-Longmaxi formations, Da'an area, western Chongqing

The Da'an area, located in western Chongqing within the Sichuan Basin, serves as a promising new target for deep shale gas exploration in the Wufeng-Longmaxi formations. However, the pore microstructure characteristics of shale reservoirs in this area and their controlling effects on gas content are yet to be clarified, posing a challenge for shale gas exploration in depth. Using the analytical and test data from scanning electron microscopy (SEM), lowtemperature gas adsorption, and nuclear magnetic resonance (NMR) experiments, we investigate the features of pore microstructures of various lithofacies in deep shales in the Wufeng-Longmaxi formations, as well as their controlling effects on the differences in gas content. The results indicate the presence of four lithofacies in deep-seated shales in the Da'an area: siliceous shales, mixed siliceous shales, clayey siliceous shales, and mixed clayey-siliceous shales. Pores in the shales are dominated by organic pores, followed by intercrystalline pores, dissolution pores, and interlayer pores in clay minerals. Meanwhile, fractures also observed in the shales consist of organic matter-filled fractures, interlayer fractures in clay minerals, and tectonic stress-induced fractures. Primary pore structures in the deep-seated shales include mesopores (pore size ranging from 2 to 50 nm) and macropores (pore size greater than 50 nm). An increased total organic carbon (TOC) content is conducive to the development of micropores (pore size less than 2 nm) and macropores. In contrast, the elevated content of siliceous and clay minerals foster the formation of macropores and mesopores, respectively. The gas content of the deep shales is positively correlated with the TOC content and the siliceous mineral content, so does the macropore volume to gas content. Shales with a high siliceous content manifest the optimal gas-bearing properties, establishing them as the favorable lithofacies for the enrichment of deep shale gas. Moderately shallow shales in the Da'an area primarily contain micropores and mesopores, with adsorbed gas predominating. In contrast, deep-seated shales in the area principally exhibit mesopores and macropores and the predominance of free gas. The well-developed macropores create favorable conditions for free gas preservation, thus increasing the gas content in the deep-seated shales.