Micropore System of Terrestrial Clay-Rich Shale from Dongyuemiao Member of Jurassic Ziliujing Formation in the Fuxing Area, Eastern Sichuan Basin

Terrestrial shale reservoirs exhibit pronounced heterogeneity stemming from the diverse material composition, intricate pore structure, varying pore types, and the multiscale nature of pore spaces. This heterogeneity translates into significant regional variations in the physical and chemical properties within the reservoir, profoundly impacting the hydrocarbon enrichment, accumulation, and recovery processes. In this study, the pore system across the full-scale spectrum of terrestrial shale from the Dongyuemiao (DYM) Member is systematically investigated, leveraging advanced techniques, such as field emission scanning electron microscopy, gas adsorption, and high-pressure mercury intrusion porosimetry. Furthermore, the factors influencing the pore structure from the perspectives of geochemical characteristics, mineral composition, and fractal dimensions are examined. Results show the primary difference in pore volume (PV) postextraction occurs in micropores and mesopores. Samples with a lower clay mineral content and a more developed shell exhibit a more complex pore space network morphology. Notably, no correlation between total organic carbon and the pore structure is observed. Conversely, clay mineral content positively correlates with PV, specific surface area (SSA), while carbonate minerals exhibit inverse trend with these two pore structure parameters. The clay minerals also positively correlate with the slope values obtained from water spontaneous imbibition. This indicates that clay minerals enhance the pore connectivity for water, whereas carbonate minerals have an opposite effect. The fractal dimension (D) positively correlates with the micropore volume, mesopore volume, total PV, BET SSA, and porosity values. Specifically, chalky grained laminated argillaceous shale displays a relatively larger average D, accompanied by the highest PVmic, PVmes, and SSA values. This indicates that this type of shale has a strong heterogeneity in the micropore and mesopore surfaces. The formation and preservation of reservoir pores are significantly influenced by diagenetic processes, and strong compaction is the primary cause of densification in DYM shale. In addition, intense diagenetic alteration of clay minerals has a significant impact on the formation and preservation of inorganic mineral-related pores in shale. These insights provide a comprehensive understanding of pore structure heterogeneity in the DYM shale, and offer valuable implications for the prediction of fluid flow behavior and the optimization of hydrocarbon exploration and production strategies.