Characterization of the Pore Structure and Influencing Factors of CH₄ Adsorption Capacity in Marine Shale Gas Reservoirs

To enhance the understanding of the gas-bearing characteristics of marine shale gas reservoirs, shale samples from well ZG3, southern Sichuan Basin, China, were selected as the focus of this study. A series of analyses, namely total organic carbon (TOC) content, large-field stitching scanning electron microscope technology (LFS-SEM), X-ray diffraction (XRD), gas adsorption (CO₂+N₂), and CH₄ adsorption experiment, were conducted to characterize the micropore structure of the shale comprehensively. Utilizing fractal theory, the study explored the relationships among the shale pore structure, organic pore content, heterogeneity, and CH₄ adsorption capacity. The findings indicate the following: (1) Shale samples in the study area displayed a relatively high content of TOC (Avg. 2.15%), with a mineral composition predominantly consisting of quartz and clay, averaging 42.4 and 34.5%, respectively. The average organic matter porosity is 7.19%. (2) CO₂+N₂ adsorption experiments yielded pore volume (PV) and specific surface area (SSA) distribution curves across the full range of pore sizes. The Total-PV was primarily attributed to mesopores and micropores, with average contribution rates of 51.2 and 45.7%, respectively, while macropores contributed only 3.1%. The Total-SSA was predominantly provided by micropores, accounting for an average contribution rate of 85.3%, with mesopores contributing 14.6% and macropores contributing a mere 0.07%. Using the FHH model, two types of fractal dimensions, namely <i>D</i> ₁ and <i>D</i> ₂, were determined with <i>P</i>/<i>P</i> ₀ = 0.5 as the boundary, yielding average <i>D</i> values of 2.6313 and 2.8798, respectively. The average CH₄ adsorption capacity was determined to be 3.57 cm³/g. Correlation analysis revealed that Langmuir adsorption volume (<i>V</i> _L) is positively correlated with micropore content, TOC content, quartz content, organic pore content, and fractal dimension, while showing a negative correlation with clay. These research findings hold significant implications for the efficient development of shale resources.