Rock Types and Reservoir Characteristics of Shahejie Formation Marl in Shulu Sag, Jizhong Depression, Bohai Bay Basin

Due to the complicated lithology in the E-S3 Member of the Shahejie Formation in the Shulu sag, Jizhong depression, it is difficult to classify the rock types and characterize the reservoirs at the marl intervals. In this paper, a four-element classification method has been proposed, and seven rock types have been identified by analyzing the mineral composition. The primary rock types are medium-high organic carbonate rocks and medium-high organic shaly-siliceous carbonate rocks. With the methods of field emission scanning electron microscopy, high-pressure mercury intrusion, nitrogen adsorption, and nano-CT, four types of reservoir spaces have been identified, including intra-granular pores, intergranular pores (inter-crystalline pores), organic pores, and micro-fractures. By combining the method of high-pressure mercury intrusion with the method of the nitrogen adsorption, the porosity of the marl has been measured, ranging from 0.73% to 5.39%. The distribution of the pore sizes is bimodal, and the pore types are dominated by micron pores. Through this study, it has been concluded that the sag area to the east of Well ST1H is the favorable area for the development of self-sourced and self-reservoired shale oil. According to the results of geochemical and reservoir analysis, the III Oil Group may have sweet spot layers.