To elucidate the reservoir characteristics of organic-rich siliceous shale of the Upper Permian Dalong Formation in western Hubei, this study focused on the drilling cores of Well ED-2. Various techniques, including a mineral composition analysis, an organic carbon content analysis, a vitrinite reflectance measurement, a total porosity determination, field emission scanning electron microscopy (FE-SEM), and low-pressure CO₂ and N₂ physical adsorption tests, were employed to analyze the mineralogy, organic geochemistry, total porosity, and pore structure characteristics. Additionally, the factors influencing the reservoir performance of the Dalong Formation shale were investigated. The results indicated that the Dalong Formation’s shale was characterized as an organic-rich siliceous shale. Organic matter was mainly of sapropelic type, with a relatively high thermal evolution degree and Ro ranging from 2.59% to 2.76%. The total porosity of the Dalong Formation’s siliceous shale was low, indicating poor reservoir properties. Organic matter pores were highly developed, mainly the ones formed after the hydrocarbon generation of solid asphalt. Micropores and mesopores were the dominant pore types in the shale, with macropores being significantly less abundant. The study further revealed that the pore volume and specific surface area exhibited a significantly positive correlation with total organic carbon (TOC) content and clay minerals, while demonstrating a weak negative correlation with quartz content. The comprehensive analysis revealed that there were two factors contributing to the poor physical properties of organic-rich siliceous shale in the Dalong Formation. Firstly, in siliceous shale with a high quartz content, the siliceous component was partly derived from the siliceous cementation of hydrothermal fluids. This process led to the formation of secondary quartz that filled intergranular pores, resulting in a decrease in macropore volume, total porosity, and a weak negative correlation with quartz content. Secondly, in siliceous shale with a relatively high clay mineral content, the organic matter was subjected to stronger compaction due to the relatively low content of brittle minerals. This compaction caused the destruction of most macropores, leaving behind primarily micropores and mesopores. Consequently, the average pore size decreased, leading to poor physical properties.

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.