Classification of Source–Reservoir Systems and Shale Gas Occurrence Modes in the Upper Permian Dalong Formation, Western Hubei, Middle Yangtze Region

This study investigates the reservoir properties and source–reservoir system types of the Upper Permian Dalong Formation shale in Western Hubei (Middle Yangtze region) through core observations and systematic experiments, including total organic carbon (TOC) analysis, X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), low-pressure gas adsorption, and methane isothermal adsorption. The Dalong Formation shale is dominated by organic-rich siliceous facies, with average quartz and TOC content of approximately 60% and 6.16%, respectively. Organic matter (OM) pores are the most abundant pore type, and the average porosity is 2.75%. Mesopores account for ~69% of the total pore volume, whereas the specific surface area is mainly controlled by micropores (~60% on average). The mean methane adsorption capacity is 3.53 m3/t, and the TOC content is identified as the primary factor governing both micropore/mesopore development and the adsorption capacity. Based on the lithological assemblages, limestone-to-stratum ratio, and TOC content, four source–reservoir systems are identified: (1) pure shale, (2) laminated shale, (3) source-intercalated reservoir, and (4) reservoir-intercalated source. The pure shale system, developed in deep-water platform basin facies, exhibits the highest reservoir quality (average TOC: 8.29%; pore volume: 32.99 × 10−3 cm3/g; specific surface area: 44.16 m2/g; methane adsorption: 4.11 m3/t). In contrast, the reservoir-intercalated source system, occurring in platform margin slope facies, shows the poorest performance (TOC: 3.03%; pore volume: 7.46 × 10−3 cm3/g; methane adsorption: 2.72 m3/t), while the laminated shale and source-intercalated reservoir systems yield intermediate values. Accordingly, four distinct shale gas occurrence models are established. The pure shale system is identified as the most favorable exploration target, followed by the laminated shale and source-intercalated reservoir systems. This classification framework offers a practical reference for shale gas evaluation in analogous rift trough marine shale systems.