Origin, Charging History, and Migration–Accumulation Patterns of Crude Oils in the Eocene Dainan Formation, Southeastern Gaoyou Sag, Subei Basin

Deep lacustrine rift basins present persistent challenges in hydrocarbon provenance determination, charging-history reconstruction, and migration pathway prediction within heterogeneous deep reservoirs. This study applies an integrated approach combining biomarker and light-hydrocarbon geochemistry, fluid-inclusion microthermometry, and two-dimensional migration modelling to the deep Eocene Dainan Formation in the southeastern Gaoyou Sag, Subei Basin. Based on 20 analytical entries from Dainan oils/reservoir samples and Funing Formation source-rock extracts, the results indicate mixed contributions from the E1f2 and E1f4 source intervals rather than a single-source origin. Maturity parameters place most oils within the early- to main-oil window, while hydrocarbon inclusions with dominantly blue fluorescence and homogenization temperatures of 85–110 °C are consistent with a dominant Sanduo-period charging episode. Integrated migration modelling identifies two accumulation styles: a northern fault-slope migration system and a southern Zhenwu fault-conduit system, with effective accumulation favoured by the coupling of active faults and connected sand bodies. The established genetic model offers a transferable framework for derisking exploration in analogous deep lacustrine rift basins.