Geological Characteristics and Main Controlling Factors of Shale Oil Enrichment: Insights from the Paleogene Funing Formation, Subei Basin

Lacustrine shale oil has become an important target for increasing unconventional oil and gas reserves and production in China. However, compared with shallow to medium-depth shale oil systems, ultra-deep shale oil systems have experienced more complex burial evolution, diagenetic alteration, and fluid activities, and their large-scale enrichment mechanisms remain poorly understood. This study focuses on the ultra-deep shale oil system of the fourth member of the Paleogene Funing Formation (E1f4) in the Gaoyou Sag, Subei Basin. Core observations, mineralogical analyses, organic geochemical tests, scanning electron microscopy (SEM), pore structure characterization, and laser confocal microscopy were integrated to investigate shale lithofacies, organic matter characteristics, and shale oil occurrence mechanisms, and to identify the key factors controlling shale oil enrichment. The results show that five major lithofacies are developed in the E1f4, including laminated calcareous–dolomitic shale and laminated clay-rich mixed shale. Among them, carbonate-rich laminated lithofacies exhibit the highest organic matter abundance and the best reservoir quality. Laser confocal microscopy further reveals that heavy hydrocarbons are mainly retained within organic-rich clay laminae, whereas light hydrocarbons preferentially migrate into and accumulate within adjacent carbonate-rich and felsic laminae, forming an efficient source–reservoir association at the microscale. The integrated results indicate that abundant algal organic matter and thick high-quality source rocks provided sufficient hydrocarbon-generation potential. Differential source–reservoir associations controlled by laminated structures, together with multi-scale pore–fracture networks, promoted short-distance hydrocarbon migration and effective accumulation. In addition, thick mudstone roof and floor seals, a weak faulting background, and an overpressure system ensured long-term hydrocarbon preservation. This study reveals the enrichment mechanism of ultra-deep mixed shale oil in faulted lacustrine basins of eastern China, improves the understanding of lacustrine shale oil accumulation, and provides a theoretical basis for sweet spot evaluation and favorable area selection in similar basins.