| The Wufeng-Longmaxi Formation shale gas reservoirs in the Sichuan Basin are a key force for increasing natural gas reserves in China. As exploration and development shift toward ultra-deep layers with burial depths exceeding 4,500?m, the factors affecting the microscopic storage properties of shale become more complex, making characterization challenging and restricting shale gas exploration and development. This study selected ultra-deep, organic-rich siliceous shale core samples from the Wufeng-Longmaxi Formation DYS5 well in southeastern Sichuan (burial depth approximately 4,840?m). The samples were scanned using focused ion beam scanning electron microscopy (FIB-SEM), and three-dimensional pore structures and equivalent pore network models were extracted through Avizo 3D reconstruction. Quantitative characterization was conducted for parameters such as pore and throat volumes, coordination numbers, and tortuosity.The results indicate that under high-temperature and high-pressure conditions, the ultra-deep siliceous shale of the Wufeng-Longmaxi Formation in southeastern Sichuan still develops a small proportion of highly coordinated macropores (volume fraction 55.379%, number fraction 3.099%). These reservoirs are characterized by “micro–mesopore dominance” (10–80?nm) and highly developed isolated pores (coordination number 0 accounts for 94%), collectively resulting in “high storage, low permeability” properties. The preservation of effective pore space in these reservoirs is constrained by multiple interacting factors. Under intense tectonic compression, the rigid compaction-resistant framework formed by biogenic quartz, the pore-enhancing effect of hydrocarbon generation during organic matter thermal evolution, and the reduction of effective confining pressure due to fluid overpressure jointly dominate pore preservation. The findings of this study provide a microscopic geological basis for the exploration and development of ultra-deep shale gas.
Key words：Pore structure; Connectivity; Digital core; Pore network model; Longmaxi Formation; Ultra-deep shale; Southeastern Sichuan Basin |