Spatial characteristics and geological implications of shale reservoirs of different lithotypes in the third member of Shahejie Formation in Qikou Sag, Bohai Bay Basin

Mudstone and shale formations developed in the first sub-member of the third member of the Shahejie Formation (Es31 Member) in the Qikou Sag, Bohai Bay Basin due to continuous lacustrine sedimentation. Atomic force microscopy, nitrogen adsorption, high-pressure mercury intrusion porosimetry, and field emission scanning electron microscopy were used to quantify the pore structure of the three main lithofacies in the study area. The results show that the average porosity and permeability of the mudstone are 3.54% and 0.00087 md, respectively. The pore structure is complex, and only the laminated shale has a high correlation between porosity and permeability. The laminated shale contains intergranular and intercrystalline pores, whereas the bedded shale primarily has intergranular dissolution pores and intergranular pores. The pore diameters of the two shales are predominantly at the submicron level. The pores of the laminated shale are located in and between the felsic laminae, and the pore connectivity is high. This lithofacies type is believed to be the most promising location for continental shale oil exploration in desert regions. The bedded shale is dominated by isolated pores with a large pore radius and large fluctuation. In contrast, to the laminated shale, the bedded shale has a high content of brittle minerals and low bedding density and is more likely to produce high-angle hydraulic fractures. The rock facies should be the focus of future shale oil evaluations in desert regions.