Pore Structure and Controlling Factors of Continental Shale in the Xiagou Formation, Qingxi Sag, Jiuxi Basin

To clarify the development characteristics and controlling factors of nanomicron pores in lacustrine shale, this study examines the Cretaceous Xiagou Formation shale reservoir in the Qingxi Sag of the Jiuxi Basin. The rock composition, pore types, and pore structure of continental shale were analyzed using whole-rock X-ray diffraction, laser confocal microscopy, nitrogen adsorption, nuclear magnetic resonance, and high-pressure mercury injection. The different pore reservoir properties were compared, and the factors influencing reservoir development were investigated. The findings indicate that the Xiagou Formation shale comprises a mixture of carbonate and terrigenous clasts, with the carbonate primarily consisting of micritic microcrystalline dolomite. The shale contains various pore types, including nanometer-scale intercrystalline pores, organic matter pores, clay mineral pores, micrometre-scale brittle mineral intergranular pores, millimeter-scale brittle mineral intergranular dissolution pores, and fractures. Organic matter pores exhibit a dispersed distribution and significant heterogeneity. Based on pore size distribution, continental shale pores are classified into three types: (1) isolated nanopores, (2) relatively concentrated nanopores, and (3) nanomicron multiscale composite pores, with the third type exhibiting the most favorable reservoir properties. The study concludes that high-quality reservoir pores in continental shale are primarily influenced by dolomitization and the dissolution of brittle mineral pores by organic acids.