Differences and controlling factors of pores structure between coal and shale in Longtan Formation from western Guizhou Province, China

Abstract

Transitional facies with high-frequency cycles of coal-shale-sandstone assemblages are widely developed in the Upper Permian Longtan Formation in western Guizhou Province, exhibiting significant contrasts in pore structures between coal and shale reservoirs. A comparative study was conducted on the differences in pore structure between coal and adjacent shale using coal rock samples from six typical coal bearing gas wells in Guizhou, employing analytical techniques such as geological analysis, scanning electron microscopy (SEM), and low-temperature liquid nitrogen adsorption. The research results show that the specific surface area of coal is 44.2–168 m²/g, with a total pore volume of 0.024–0.065 cm³/g. These pores are primarily semi-closed and slit-shaped. The volume and specific surface area of micropores (<2 nm) have absolute advantages, and are positively correlated with the degree of thermal evolution, mainly micropores, as they are closely associated with the gas generation process. In contrast, macropores (>2 nm) exhibit strong heterogeneity, which is linked to differences in microscopic components. The specific surface area of shale is 43.2–66.6 m²/g, and the total pore volume is 0.032–0.059 cm³/g, mainly composed of inkbottle-shaped pores. The distribution of micropores and mesopores is relatively uniform, and the pore size distribution curve shows a bimodal patterns with peaks at approximately 3 nm and 30 nm. Despite structural differences, pores of <10 nm are the main contributors to the specific surface area in both coal and shale. The extractable asphalt has a significant impact on the pore space in coal, and pore volumes across all size ranges increase notably after extraction. The degree of thermal evolution and organic matter content of coal are the main influencing factors on pore structure, while the organic matter content and mineral type of shale are the main factors affecting pore structure, with thermal maturity playing a less significant role. These findings provide critical insights for the co-exploration of coalbed methane and shale gas in coal-measure systems in western Guizhou Province.