Abstract: The Archean buried hills are the major field for obtaining discoveries in deep formations in Bohai Sea area in recent years. Based on 3D seismic data, mud logging, wireline logging and laboratory test data, hydrocarbon accumulation conditions of the Archaean buried hills in Bohai Sea area are analyzed, and enrichment patterns of key buried hills are summarized. The study results show that the Archean buried hills in Bohai Sea area had unique hydrocarbon accumulation conditions. Jointly controlled by multi-episode compression uplift and fault block tilting during the Indosinian–Himalayan periods, multiple types of buried hill traps were formed in Bohai Sea area. The oil and gas reservoirs were characterized by the upper source rock and lower reservoir type, which were mainly composed of dark mudstone source rocks in the third member of the Cenozoic Shahejie Formation (Sha 3 member) , the first and second members of Shahejie Formation (Sha 1+2 members),and the third member of Dongying Formation (Dong 3 member), as well as the large- and medium-sized buried hill reservoirs around the hydrocarbon-rich sags, showing significant source control effects. Influenced by multi-stage compression, shear, and extensional activities in the Mesozoic–Cenozoic, complex fracture networks were developed in multi-type buried hills, which was the key factor for the formation of large-scale reservoirs after superimposed by later fluid dissolution. The rapid subsidence in Bohai Sea area during the Oligocene–Neogene resulted in overpressure in the thick mudstone in Shahejie–Dongying formations, providing an effective regional cap rock for the Archean buried hills. Based on this understanding, two hydrocarbon enrichment patterns are established, i.e., “overpressure hydrocarbon charging, and gas after oil” pattern below source rock for the exposed type buried hills and “high-intensity hydrocarbon charging in a narrow window, and composite pathway for hydrocarbon migration” pattern beside source rock for the covered types buried hills, respectively. In addition, the key factors for the large-scale hydrocarbon accumulation in the Archean buried hills included the high-intensity hydrocarbon charging and strong reservoir transformation. The Archean buried hills covered by the Mesozoic/Paleozoic in Bozhong Sag is a major exploration field in the future, providing exploration orientation of deep buried hills in Bohai Sea area.

Key words: Bohai Sea area, deep to ultra-deep formations, Archean buried hill, hydrocarbon accumulation condition, enrichment pattern