Occurrence Characteristics of Solid Bitumen and Its Control Mechanism on Pore Structure in Marine Over-Mature Shales: A Case Study of the Wufeng–Longmaxi Formations in the Middle Yangtze Region

This study investigates the occurrence characteristics of solid bitumen and its control on pore structure in the overmature marine shales of the Wufeng–Longmaxi Formations in the Jianshi–Badong area, Middle Yangtze region. Total organic carbon analysis, Rock-Eval pyrolysis, organic petrography, organic matter reflectance measurements, petrographic thin-section observations, and scanning electron microscopy were integrated. The shale samples contain 0.10%–6.73% total organic carbon, with an average of 2.20%, and the organic-rich intervals are mainly concentrated in the middle to lower part of the Longmaxi Formation. The average equivalent vitrinite reflectance (EqRo), calculated from solid-bitumen reflectance, is 2.06%, indicating an early overmature stage. The original organic matter is inferred to have been dominated by Type II1 kerogen, which was extensively transformed into indigenous solid bitumen during thermal evolution. Four principal occurrence types of solid bitumen were identified: laminated, dispersed, massive, and mineral-associated. Organic-pore development differs among these occurrence types. Nanometer-scale pores are locally preserved in laminated and mineral-associated solid bitumen, whereas dispersed solid bitumen in clay-rich matrices generally shows poor pore development. The pore structure is jointly controlled by the occurrence mode of solid bitumen, rigid-mineral support, clay-mineral compaction, thermal maturity, and hydrocarbon generation and expulsion. Quartz- and pyrite-associated solid bitumen is comparatively favorable for pore preservation, whereas clay-associated solid bitumen is more susceptible to pore deformation and closure. These results clarify the petrographic control of solid-bitumen occurrence on pore preservation in overmature marine shales.