Pore structure characteristics and main controlling factors of the Longmaxi shales in northeastern Chongqing

To investigate the pore characteristics and controlling factors of the Longmaxi shales in northeastern Chongqing, this study samples 12 shales from the Longmaxi Formation and utilizes physical experiments, including field emission scanning electron microscopy (FE-SEM), liquid nitrogen adsorption and carbon dioxide adsorption, to characterize the pore structures. The pore structure parameters are then compared with those of shales from different tectonic zones in the Sichuan Basin. The results indicate that the pores of shales in the Longmaxi Formation in northeastern Chongqing are predominantly comprised of organic matter pores, inorganic pores and micro-fractures. Compared to the marine shales with similar maturity in southern Sichuan, these samples exhibit relatively fewer and smaller organic matter pores but more developed inorganic pores and micro-fractures. Based on the pore-fracture characteristics, inorganic pores can be classified into three types: intergranular pores, interlayer pores in clay minerals, and intragranular pores. Micro-fractures are categorized as organic matter-related micro-fractures and inorganic mineral-related micro-fractures. The influence of TOC content on pore structure occurs in two stages: when TOC content is less than 5.93%, it exhibits a strong positive correlation with micropore volume and specific surface area; when TOC content is more than 5.93%, it shows a negative correlation with these factors, and no correlation with mesoporous parameters. In addition, there is no significant correlation between the inorganic mineral (clay and brittle mineral) content and pore parameters. The SEM images also show that the tectonic compression reduces pore volume and pore connectivity, leading to the conversion of free gas into adsorbed gas in shale reservoirs, thus improving the gas storage capacity of the shales. The research results provide a theoretical basis for shale gas exploration in complex structural areas.