Differential Hydrocarbon Accumulation Model in the Southwestern Bozhong Sag, Bohai Bay Basin, Eastern China

ABSTRACT In recent years, significant progress has been made in the deep exploration of the Bozhong Sag within the Bohai Bay Basin, eastern China, leading to the discovery of metamorphic oil and gas reservoirs such as Bozhong 19‐A, Bozhong 21‐B and Bozhong 13‐C. Globally, the oil and gas source and accumulation mechanisms of deep large reservoirs, due to strong subsurface heterogeneity and complex structural conditions, remain critical issues to be addressed in understanding the distribution patterns of deep hydrocarbon reservoirs. The study was based on a number of different analyses. These included geochemical analyses of hydrocarbons, the hydrocarbon generation history of sub‐depressions, fluid inclusion identification and quantitative grain fluorescence measurements. The aim was to clarify the hydrocarbon sources and accumulation models across different structural belts in the southwestern Bozhong Sag. Results show that the hydrocarbon source rocks for the Bozhong 19‐A, Bozhong 21‐B and Bozhong 13‐C structures all originate from the Shahejie Formation, though the primary hydrocarbon supply sub‐sags vary. The study area has undergone three stages of hydrocarbon charging: an early stage of crude oil charging (12–5.1 Ma), a middle stage of highly mature oil and gas charging (5.1 Ma) and a late stage of natural gas charging (0 Ma). The dynamic hydrocarbon accumulation process in the deep layers of the southwestern Bozhong Sag is characterised by ‘early‐stage source control, mid‐stage adjustment and late‐stage formation’. The deep hydrocarbon accumulation model is governed by the maturity of the source area: oil reservoirs or highly volatile oil reservoirs form near source areas of moderate maturity, whereas condensate gas reservoirs or gas reservoirs develop near source areas of higher maturity. The findings enhance the understanding of deep hydrocarbon accumulation processes and provide valuable insights for the exploration and prediction of deep reservoirs under similar geological conditions worldwide.