Lithofacies classification and its controls on the pore structure distribution in Permian transitional shale in the northeastern Ordos Basin, China

Sea level changes more frequently during the deposition of transitional shale than marine shale, resulting in the strong heterogeneity and instability of the sedimentary environment and making transitional shale gas evaluation difficult. The appropriate identification and classification of transitional shale lithofacies types can improve evaluation of shale reservoirs and can provide the geological basis for the evaluation and potential of shale gas reservoirs during exploration and development. The lower Permian shale in the northeastern Ordos Basin is selected as the focus of this research. Based on X-ray diffraction (XRD), total organic content (TOC), vitrinite reflectance (R-o), nitrogen and carbon dioxide adsorption, and argon ion polishing scanning electron microscopy (AIP-SEM) data, the lower Permian transitional shale in the study area is divided into silty mudstone and muddy siltstone lithofacies. On this basis, the differences in the organic geochemical characteristics, pore development characteristics and influencing factors of the shale reservoir in the two lithofacies are compared. The results show that the mean values of w(TOC) and w(Ro) in the muddy siltstone lithofacies are 1.9% and 1.0%, respectively. Siliceous minerals dominate the composition of the rock, and the main types of reservoir spaces are mineral matrix pores, including interparticle (interP) pores and intraparticle (intraP) pores related to inorganic minerals. Macropores and mesopores are the main components of the pore volume, and account for 40.8% and 49.8% of the total volume, respectively. The pore structure is mainly affected by the TOC and siliceous mineral contents. In the silty mudstone lithofacies, the average values of w(TOC) and w(Ro) are 2.4% and 1.2%, respectively. The clay mineral content is relatively high. The main types of reservoir spaces are organic pores and interP pores. Additionally, the pore volumes of macropores, mesopores and micropores are almost the same, accounting for 24.9%, 44.6% and 30.4% of the total pore volume, respectively. The pore structure is mainly affected by the organic matter content and clay mineral content.