Pore characteristics and genesis analysis of shallow, deep, and ultra-deep marine shale reservoirs: insights from the Longmaxi Formation shale

다국어 초록 (Multilingual Abstract)

The study of shale reservoirs is a crucial aspect of shale gas exploration and development. With China’s continued breakthroughsin deep and ultra-deep shale gas, the traditional view that reservoir quality deteriorates with increasing depth facessignificant challenges. Currently, there remains a limited understanding of the pore development characteristics and controllingfactors in shallow, deep, and ultra-deep shales, which affects the effectiveness of shale gas production. Taking theLongmaxi Formation shale in southern Sichuan Basin as a case study, this study conducted a series of experimental analyses,including TOC, XRD, low-temperature N₂ adsorption, CO₂ adsorption, SEM, and large-view FE-SEM splicing technology(Maps-SEM). The results reveal that: (1) the porosity of the Longmaxi Formation shale in southern Sichuan ranges from3.37% to 7.49%. The pore types are primarily organic matter pores and interparticle pores of clay minerals, with the porevolume mainly contributed by mesopores (50.8% contribution) and micropores (32.97% contribution), while macroporescontribute the least to pore volume at 16.22%. (2) The differences in porosity of shallow, deep, and ultra-deep marine shalesare not significant, but as burial depth increases, the proportion of macropores in the shale reservoir decreases, while theproportions of mesopores and micropores increase. (3) The porosity of the Longmaxi Formation shale is influenced by reservoirpressure, TOC, and mineral composition. The pressure coefficient of shallow shale is at normal to weak overpressure,with pores primarily influenced by normal compaction, while the pressure coefficient of deep to ultra-deep shale reservoirsis generally greater than 1.8, with overpressure offsetting part of the stress from the overlying rock layers, which is a majorreason for the relatively high porosity of deep to ultra-deep shales.