Reservoir Characterization and Genetic Analysis Based on Shale Lithofacies Classification: A Case Study of the Paleogene Shahejie Formation, East China

The quality of shale oil reservoirs is a major factor determining shale oil production capacity. Research on shale oil reservoirs has primarily focused on lithology. However, there has been little research on lithofacies classification. Moreover, there is still a lack of research on potential reservoir differences between different lithofacies and their controlling factors. In this context, the present study aims to classify the lithofacies of shale oil reservoirs in the Paleogene Shahejie Formation of the Jiyang Depression using different methods, including rock core and thin section observations, scanning electron microscopy (SEM) analysis, and X-ray diffraction (XRD). In addition, the characteristics and genesis of the high-quality shale oil reservoirs were studied using three-dimensional micro-CT scanning, low-pressure nitrogen adsorption, high-pressure mercury injection, and core physical property testing. The results showed better physical properties of combined shale and lenticular crystal limestone (C1), continuous parallel planar calcareous mudstone and uncontinuous laminate mudstone (C2), and continuous parallel planar calcareous mudstone and laminate mudstone (C3) compared with those of the other lithofacies; C1 exhibited the best physical properties. These three combined lithofacies consisted mainly of interconnected pores with medium and large pore throats, as well as fractures; the pore size mainly ranged from nanometers to micrometers. The high-quality reservoir conditions in combined lithofacies are the result of both basic sedimentary lithofacies and diagenetic history. The results of the current study provide new ideas and a useful reference for future related studies on mud shale reservoirs.