A New Classification Method of Shale Reservoirs Based on the Relationship between Pore Volume and Surface Area: Shahejie Formation, Jiyang Depression, and Bohai Bay Basin

The differentiation of fluid occurrence can occur due to the coexistence of pores and surfaces, that is, the adsorbed and free states. However, on account of the unclear relation between the pore and surface, the evaluation and classification of shale are limited from the perspective of the reservoir. In this paper, a suite of shale samples were collected from the Shahejie Formation in the Jiyang Depression for nitrogen gas adsorption, high-pressure mercury intrusion, and field emission scanning electron microscopy. We analyzed the relationship between the pore volume (PV) and specific surface area (SSA), classified the shale reservoir based on the relationship, and discussed the characteristics and significances. In this study, the shale reservoir can be divided into the pore type, surface-pore type, and surface type according to the relationships between the PV and SSA. The pore type has the smallest SSA, the largest average pore size, more mesopores and macropores, and fewer micropores. The surface-pore type has a modest average pore size and SSA, more meso- and micropores, and fewer macropores. The surface type has the smallest average pore size, the largest SSA, more micropores, and fewer meso- and macropores. The fluid in the surface type is mainly the adsorbed state, while the fluid in the pore type is mainly in the free state, and the fluid in the surface-pore type is indicative of the coexistence of the adsorbed and free state.