Accumulation mechanisms and modes of Neogene hydrocarbons in Jizhong Depression

Based on the study of distributional rules and sources of Neogene hydrocarbons,characters of fault activities and fault sealing properties,we recognized two kinds of migration and accumulation modes for Neogene hydrocarbons in Jizhong Depression,Bohai Bay Basin,namely a far-source stepped lateral migration pattern in slope zones and a near-source vertical migration pattern in fault terrace belts.The vertical migration pattern in fault terrace belts occurs typically in eastern step-fault zones in Raoyang Sag,where Neogene reservoirs are distributed adjacent to kitchen subsags and nearby strongly active faults of source rocks,and the accumulation degree and layer system of Neogene hydrocarbons are determined by the intensity and duration of fault activities during the main hydrocarbon accumulation period.While the far-source stepped lateral migration pattern dominates the Neogene hydrocarbon occurrence in the Wen'an slope belt,Baxian Sag,where Neogene reservoirs are far from kitchen subsags,the vertical migration of hydrocarbons is not strong due to a weak tectonic movement of the late stage,and the migration pattern and accumulation direction of Neogene hydrocarbons are determined by the lateral sealing ability of faults.