Characteristics and origin of oil and gas in the Nanpu sag, Bohai Bay Basin, China: Insights from oil-source correlation and source rock effects

ABSTRACT The Nanpu sag is a typical petroliferous area in the Bohai Bay Basin. Crude oil density and viscosity, natural gas components and biomarker chemistry of crude oil, and shale sample extracts were analyzed to determine the characteristics and the origin of oil and gas and the effects of source rock on oil and gas distribution. The discovered heavy oil present in the shallow layers of the Liuzan, Gaoshangpu, and Laoyemiao regions are a product of biodegradation and oxidation. The crude oil samples from the offshore locations are low density and viscosity, indicating good preservation conditions. Biomarker compositions indicate that the crude oils can be divided into four types: A, B, C, and D. Both type A and D oils were sourced from the third member of the Shahejie Formation. Type A oil is primarily distributed in the Liuzan and Gaoshangpu regions, whereas type D oil is chiefly distributed in the nos. 1, 2, 5, and Beipu structural belts (SBs). Type B oil, derived from the third member of the Dongying Formation, is primarily distributed in the Laoyemiao, Gaonan, and Liunan SBs. The type C oil is primarily derived from the first member of the Shahejie Formation and is distributed in the Beipu and nos. 1–4 SBs. The hydrocarbons are primarily distributed in the regions where the thick source rocks interact with faults. The discovered natural gas in the Nanpu sag is primarily observed as dissolved gas in oil and free gas.