Distinct Pore Surface and Pore Network Controls on Adsorbed Gas Storage and Free Gas Effectiveness in Longmaxi Marine Shales

Pore heterogeneity in marine shales critically controls gas storage and transport; yet, how pore surface fractal attributes and pore network geometry differentially regulate adsorbed and free gas among lithofacies remains unclear. This study investigates Longmaxi marine shales from the Weiyuan area, Sichuan Basin, using XRD, FE-SEM, low-pressure N2 adsorption, and high-pressure CH4 isothermal adsorption. According to mineral composition, samples are divided into four lithofacies: quartz–calcite (#QC), quartz–pyrite (#QP), quartz–dolomite (#QD), and quartz–illite (#QI). Results show that adsorbed and free gas are controlled by different pore structure descriptors. Adsorbed gas content decreases as #QC > #QP > #QI > #QD and is mainly governed by micropore development, specific surface area, and surface roughness/complexity. Free gas content follows #QP > #QC > #QI > #QD and is primarily controlled by pore network effectiveness: connectivity enlarges connected pore space, whereas the tortuosity factor increases flow resistance and restricts migration. The #QP lithofacies have the most favorable network conditions, with high connectivity and a low tortuosity factor, indicating the greatest free gas enrichment potential. These findings provide a lithofacies-specific framework for reservoir assessment and sweet spot identification in Longmaxi shale.