Characteristics of reservoir space and sweet spot evaluation of shale oil in the second member of Paleogene Funing Formation in Subei Basin: a case study of well QY1 in Qintong Sag

The second member of the Paleogene Funing Formation in the Subei Basin is a key production layer for conventional oil and an optimal target for the exploration and development of continental shale oil in East China. The analysis and testing of core samples from well QY1 in the Qintong Sag indicate that the shale features low total organic carbon (TOC) content, relatively low vitrinite reflectance (Ro), balanced mineral composition, and a complex pore network. Using petrological and geochemical methods, the lithofacies characteristics, reservoir space characteristics, oil-bearing and mobility characteristics, brittleness index, and compressibility characteristics of this shale oil reservoir were studied to identify production sweet spots. The second member of the Funing Formation is a mixed shale layered reservoir, with mineral composition mainly consisting of clay minerals, felsic minerals, and carbonate minerals. The average TOC value is 1.32%, and Ro ranges from 0.9% to 1.1%. The average porosity is 4% in the middle and lower parts and 2.2% in the upper part. Based on the abundance of organic matter, structural characteristics, and lithology, the shale of the second member of the Funing Formation can be divided into six lithofacies, with significant differences in reservoir properties. The development characteristics of the laminae are an important reason for the different reservoir space characteristics among different lithofacies. Except for low organic matter laminated/layered shale with poor calcite and dolomite, other lithofacies have good oil content. The high organic matter layered shale with rich calcite demonstrates the highest TOC content. The number of layers correlates well with oil and gas mobility, with the average OSI value decreasing from 202.62 mg/g in medium organic matter laminae shale with poor calcite and dolomite to 77.83 mg/g in high organic matter massive mudstone. High organic matter massive mudstone, due to the presence of a large amount of plastic minerals, has the worst fracturing effect among the six types of lithofacies. Medium organic matter laminae shale with poor calcite and dolomite is the optimal lithofacies, while medium organic matter layered shale with poor calcite and dolomite and medium organic matter layered shale with rich calcite and dolomite are slightly less favorable but can still be key targets for exploration and development. Based on the vertical distribution of the dominant lithofacies, sublayers ③ to ⑤ of submember Ⅰ and sublayers ② to ④ of submember Ⅱ of the second member of the Funing Formation are selected as geological sweet spots in this area.