Fault reactivation in No.4 structural zone and its control on oil and gas accumulation in Nanpu sag,Bohai Bay Basin,China

Based on seismic and drilling data,the reactivation mechanism of the pre-existing basement F4 strike-slip faultin Nanpu sag and its controlling effect on hydrocarbon accumulation difference are systematically studied.By defining fault activation stages,back-stripping fault throw and physical modeling,it is found that the Nanpu No.4 structural zone formed by the Cenozoic reactivity of the F4 fault grew from south to north,with strike-slip deformation dominated in the south and extensional deformation dominated in the north.Faults in the No.4 structural zone and those in the adjacent No.2 and No.3 structural zones were different fault systems,which grew separately,contacted and connected,and finally interwove under the action of unified stress field.By constructing the identification chart of deformation mechanisms of reactivation of pre-existing faults,it is concluded that during the sedimentary period of the Paleogene Shahejie Formation,F4 fault was reactivated by strike-slip faulting,and during the sedimentary period of Paleogene Dongying Formation and Neogene Guantao-Minghuazhen formations,it was reactivated by oblique extension.The controlling effects of Cenozoic reactivation of F4 fault on hydrocarbon accumulation include:(1)As the oil-source fault,it controlled the vertical cross-layer migration of oil and gas.(2)It gave rise to strike-slip transfer zone to control the distribution of sand bodies.(3)It grew upward and interacted with faults in the neighboring area,controlling the formation of two types of traps,and was favorable for oil and gas accumulation.