Micropore structure characteristics of Wufeng-Longmaxi Formation black shale along Qilongcun section in northwest Guizhou

Pore types and structures are of great significance for shale gas resources evaluation. In order to study the pore structure features and evolution regularity of shale, the authors used field emission-scanning electron microscopy (FE-SEM) and nitrogen adsorption method to study the pore characteristics and structures of black shale samples from the Late Ordovician-Lower Silurian Wufeng-Longmaxi Formation of the marine shale gas reservoir in northwest Guizhou. The results show that the shale has various kinds of reservoir space, mainly comprising intragranular pores, intergranular pores, organic pores and microfractures. Low temperature nitrogen adsorption experiment results show that the pores in the shale are mainly mesopores in size, and mainly include ink bottle-like and parallel plate-like pores in structure. The authors investigated the main factors influencing the shale pore development, organic carbon content, degree of thermal evolution and content of mineral composition and found that their actions were not conducted singly but restricted each other in influencing the development of shale pores. Quartz content is correlated positively with micro-and mesopores, but has no evident relationship with macropores. Clay content has no obvious relationship with micro-and mesopores, but has negative relationship with macropores. The organic matter pore is at its peak of development, has an important contribution to shale pores, and increases with the increase of maturity.