Abstract
<title>Abstract</title> <p>Understanding the timing and mechanisms of hydrocarbon accumulation is critical for evaluating petroleum system evolution and identifying favorable exploration targets. The Paleogene clastic reservoirs in the Gaoyou Sag represent a key exploration target in the Subei Basin. However, the hydrocarbon accumulation processes and mechanisms remain poorly understood. This study integrates petrographic observations, fluid inclusion microthermometry, burial and thermal history modeling, and source rock evaluation to investigate the timing, stages, and mechanisms of hydrocarbon accumulation. The fluorescence characteristics and homogenization temperature distributions of fluid inclusions were systematically analyzed to constrain hydrocarbon charging histories. Basin modeling was conducted to reconstruct burial and thermal evolution. The results indicate that the Paleogene clastic reservoirs contain abundant hydrocarbon inclusions (HCIs) and coexisting aqueous fluid inclusions (AFIs). Oil inclusions commonly occur within intragranular fractures and quartz overgrowths, displaying irregular and elongated morphologies with yellow-green to blue fluorescence colors, suggesting charging by medium-to high-maturity oils. Integrated fluid inclusion analyses and basin modeling reveal two major hydrocarbon accumulation stages in the Gaoyou Sag, including an early stage at approximately 47–33 Ma and a late stage at approximately 29–19 Ma. The timing of hydrocarbon charging varies among structural zones, with earlier charging occurring in the deep sag center than in northern slope areas. These results provide new insights into the evolution of the petroleum system and the exploration potential of Paleogene clastic reservoirs in the Gaoyou Sag and may serve as a reference for hydrocarbon accumulation studies in analogous continental rift lacustrine basins.</p>