High-Quality Reservoir Space Types and Controlling Factors of Deep Shale Gas Reservoirs in the Wufeng-Longmaxi Formation, Sichuan Basin

Reservoir space types and controlling factors influence the reservoir quality and gas content of deep shale. The deep shales of the Wufeng-Longmaxi formations in the Sichuan Basin and periphery are taken as the research object, and SEM, RXD analysis, and porosity testing are used to summarize and compare the characteristics of reservoir space types and discuss the main controlling factors and their influence on gas content and porosity. The results indicate that (1) the main reservoir space types of deep shale are organic matter (OM) pores, followed by inorganic pores and microfractures. The Weiyuan, Yongchuan, Jiangdong, and Wulong areas with the development of OM pores are better than that of the Dongxi, Dingshan, and Nanchuan areas. (2) The Weiyuan, Dingshan, and Yongchuan areas are mainly of intercrystalline, intragranular, dissolution, and interlayer pores. The Jiangdong, Nanchuan, and Wulong areas primarily feature intercrystalline pores and intergranular pores, while the Dongxi area is dominated by intergranular pores, with minor interlayer and intragranular pores. Inorganic pore sizes are larger in the Weiyuan, Dingshan, Dongxi, and Wulong areas compared to the Yongchuan, Jiangdong, and Nanchuan areas. (3) In the Weiyuan, Dongxi, and Nanchuan areas, the main fracture types are interlayer, grain margin, and bedding fractures. The Yongchuan and Jiangdong areas are dominated by structural fractures, as well as interlayer, grain margin, and marginal shrinkage fractures. The Dingshan area primarily features grain margin fractures, while microfractures are less developed in the Wulong area. (4) Biogenic silica promotes the preservation of OM pores. OM pores are well-developed in structure stable areas, while inorganic pores and microfractures are more prevalent in highly deformed regions. (5) Sustained overpressure promotes the formation of high-quality deep shale reservoirs and shale gas enrichment, while sustained low pressure hinders reservoir formation and shale gas enrichment. Early overpressure and late rapid pressure relief are conducive to reservoir formation but unfavorable to shale gas enrichment.