Full-scale pore structure and its impact on methane adsorption in deep coal reservoirs of the Zijinshan area, Eastern Ordos Basin

As a critical supplement to unconventional natural gas resources, deep coalbed methane (CBM) requires a comprehensive understanding of reservoir pore structure and adsorption behavior. In this study, medium-rank coals from the Zijinshan area on the eastern margin of the Ordos Basin were analyzed using high-pressure mercury intrusion (HPMI), low-temperature nitrogen adsorption (LT-N2), low-pressure CO2 adsorption (LP-CO2), and scanning electron microscopy (SEM) to characterize full-scale pore size distributions. Results show that the pore system displays a stepwise distribution, with micropores (<2 nm) accounting for 68.77% of total pore volume and 97.76% of specific surface area, making them the primary sites for methane adsorption. Based on multi-scale pore data, Pearson correlation analysis and partial least squares regression (PLSR) were employed to determine the dominant controls on adsorption capacity. Total pore volume, micropores and mesopores structures, volatile matter, and fixed carbon were identified as major influencing factors. Fixed carbon enhances micropores development, while volatile matter contributes to the formation of pyrolysis-related pores and improved pore connectivity. Moreover, the high-temperature and high-pressure conditions typical of deep coal seams accelerate thermal evolution and compaction, further promoting the prevalence of micropores. These findings suggest that methane adsorption in deep coal reservoirs is controlled by the coupled effects of geological conditions, organic matter composition, and pore structure. This work provides insights into the nonlinear relationship between multi-scale pore systems and adsorption behavior, supporting efficient CBM exploitation under the dual-carbon strategy.