Main controlling factors of shale oil enrichment in second member of Paleogene Funing Formation in Gaoyou Sag of Subei Basin

The second member of the Paleogene Funing Formation in the Gaoyou Sag is a key area for shale oil exploration within the Subei Basin. The successful implementation of eight horizontal shale oil wells evidences its significant potential and promising prospects of exploration. Addressing challenges such as variations in well productivity and unclear understanding of the main controlling factors of shale oil enrichment, this study utilized core data, logging data, analysis testing data, and production dynamics to investigate the geological characteristics and main controlling factors of shale oil enrichment in the second member of the Funing Formation. The results reveal that the mudstone and shale in this member predominantly consist of felsic-argillaceous mixed rocks and felsic-calcilutite/dolomitic mixed rocks, with moderate organic matter abundance and mainly TypeⅠand TypeⅡ₁ organic matter. From bottom to top, the organic matter type gradually transitions from humus to sapropelic, with an overall increase in organic matter abundance, providing favorable conditions for shale oil formation. A comprehensive evaluation of lithofacies, oil generation potential, oil content, reservoir characteristics, and compressibility identified three favorable exploration layers in the second member of the Funing Formation: Ⅴ-4 to Ⅴ-9, Ⅳ-2 to Ⅳ-7, and Ⅱ to Ⅲ.Shale oil enrichment in the second member of the Funing Formation is characterized by the following factors: (1) Favorable lithofacies combinations, particularly mixed rocks interbedded with dolomite bands, form the material basis for shale oil enrichment, resulting in high shale oil productivity. (2) A complex pore and fracture system is central to shale oil enrichment, with laminar fractures and cracksserving as the main pathways for shale oil flow, effectively connecting matrix pores and enhancing shale oil conductivity. (3) Higher maturity and favorable preservation conditions are crucial for high and stable shale oil production, with vitrinite reflectance (R_o) between 0.8% and 0.9%, indicating relatively higher retained oil content. Wells located away from long-term active faults exhibit relatively higher free hydrocarbon (S₁) content.