Characteristics of Pore Structure and the Evolution Process in Terrestrial Shale Reservoirs: A Case Study of the Shahejie Formation in Dongpu Depression, Bohai Bay Basin

Microscopic pore characteristics of shale are very important for exploring the oil occurrence and content. However, previous studies on heterogeneous terrestrial shale are lacking. We choose the Shahejie formation (SF) of the Dongpu Depression in the Bohai Bay Basin as a case study. First, the essential properties of microscopic pores were characterized on the basis of core observation, X-ray diffraction, scanning electron microscopy, high-pressure mercury pressure and low-temperature gas adsorption measurements, and computerized tomography scanning. Second, a classification scheme of pore size was established based on fluid invasion change characteristics. Finally, we discuss the influencing factors and evolutionary patterns of pores. The results show that (1) the pore sizes can be divided into type I (150 nm). The proportion of type II is relatively high, and fluids are difficult to inject into type I; (2) the shale reservoir space is significantly controlled by the organic matter content and mineral composition, and SF shale includes inorganic pores, organic pores and fractures, and the connectivity is generally poor; (3) the structural of the shale can be a good indicator of the reservoir sweet spot. The laminated shale usually has higher organic matter content and carbonate minerals, which results in more dissolution pores, intergranular pores, and fractures; (4) the pore evolution process can be divided into four stages: the development of inorganic primary pores, the formation of dissolution pores and organic pores, the reduction of porosity, and the pore structure tends to be stabilized. SF shale has moderate pore development conditions and currently is in the stage in which dissolution pores and intergranular pores predominate. The classification scheme we have established is more suitable for shale oil, and the evolution model will be helpful to overcome the heterogeneity of terrestrial shale, and the pore structure can be predicted quickly and initially based on the geological conditions of the shale.