Improved understanding of the origin and accumulation of hydrocarbons from multiple source rocks in the Lishui Sag: Insights from statistical methods, gold tube pyrolysis and basin modeling

Hydrocarbon source correlations and the determination of hydrocarbon accumulation models in petroliferous basins with multiple potential source rocks are significant for hydrocarbon exploration. In this study, on the basis of gas chromatography and gas chromatography-mass spectrometry experiments, the oil origin was determined by a multiparameter oil-source correlation based on statistical methods. Through gold tube pyrolysis, fluid inclusion analysis, laser Raman analysis of inclusions and basin modeling, this study deeply explores the origin and accumulation of oil and gas in the Lishui Sag, East China Sea Basin. The oil in the west sub-sag is a mixture from source rocks of the Yueguifeng and Lingfeng formations (Danian-Selandian), while the oil in the east sub-sag is derived from source rocks of the Yueguifeng Formation (Danian). The hydrocarbon gas reservoir of the LA well block in the middle of the sag contains mainly oil-associated gas derives from the Yueguifeng Formation (Danian). The 3D basin modeling shows that the oil and gas generated by source rocks of the Yueguifeng and Lingfeng formations (Danian-Selandian) migrated upward into the reservoirs in the Mingyuefeng Formation (Thanetian). Due to the poor preservation conditions in the northern part of the study area, a large amount of oil and gas escaped and failed to accumulate. Controlled by fluid potential and faults, hydrocarbons in the Lishui Sag mainly migrated from the source kitchen to the slope zone. The stable carbon isotope data combined with the 3He/4He ratios show that the CO2 in the Lishui Sag are mantle-derived inorganic gas related to volcanic activity. The filling of CO2, caused the loss and reaccumulation of early filled oil and gas, which impacted the distribution of oil and gas reservoirs in the Lishui Sag. Therefore, the migration and accumulation mechanism of CO2 should be fully considered in oil and gas exploration in the Lishui Sag.