Pore, fracture characteristics and diagenetic evolution of medium-maturity marine shales from the Silurian Longmaxi Formation, NE Sichuan Basin, China

Abstract The characteristics of pores and fractures, as well as the diagenetic evolution path, are of great significance for identifying the formation of shale reservoirs and the enrichment mechanism of unconventional hydrocarbons. Compared with mature marine shale, medium-maturity marine shale has the characteristics of high clay content (>30%), high carbonate content (>15%), and low quartz content (<40%). The thermal simulation experiment based on medium-maturity marine shale provides a theoretical basis for understanding the relationship between pore fracture genesis characteristics and diagenetic evolution. Low-temperature N 2 adsorption, field emission scanning electron microscopy, and micro-X-ray fluorescence experiments were conducted on marine shale samples of different maturity levels obtained after thermal simulation. The results indicate that (1) When R o < 1.2%, the pore type is inorganic pores, and the fracture type is bedding fractures. The size of inorganic pores rapidly decreases under compaction. (2) When R o is between 1.2 and 2.0%, the cementation and dissolution dominate the development of inorganic pores. Micro-fractures are formed at the edge of organic matter (OM), while the bedding fractures are enlarged. A large number of dissolution pores are formed inside mineral particles, and the development of inorganic pores is highest during this stage. (3) When R o exceeds 2.1%, the hydrocarbon generation process dominates the formation and maintenance of a large number of OM pores. The framework composed of microcrystalline quartz is beneficial for the preservation of OM pores. When R o reaches 2.80%, the interparticle pores between organic matter and rigid mineral particles are further increased.