Difference of reservoir forming conditions of different depressions and accumulation models of Neogene hydrocarbon in Bohai Bay Basin

Based on the study of source-reservoir-cap rock,trap,and transportation conditions of Neogene in Bohai Bay Basin,we discussed the accumulation conditions in different depressions,and established three hydrocarbon accumulation models. The study shows that it is common in the Neogene,Bohai Bay Basin: the hydrocarbon was mainly provided by the heterogeneous source rocks; the reservoir rocks developed well; fault was the migration pathway; hydrocarbon acculmulation time was late; and different depressions provided different hydrocarbon accumulation conditions. From the peripheral depressions to Bozhong depression,the source rock layers for the Neogene hydrocarbon become newer. The contribution of the new source rocks increases and the quality of the cap rock becomes better. Three modes of hydrocarbon migration were proposed,namely direct migration through source faults,secondary adjustment migration and mixed transportation. These models can be summarized as: source rock supply hydrocarbon-transported by source faults-trapped by structural features model in the drape structural belt, secondary source supply hydrocarbon-transported by shallow faults-trapped by faults model in the shallow concave belt,and mixed sources for hydrocarbon-multiple migration ways-trapped by unconformity model in the slope belt. The drape structural belt model is in favor of accumulating bulk Neogene hydrocarbon due to its corking accumulation conditions,while the shallow concave belt model and the slope belt model limited by the preservation and migration conditions are relatively less enriched in hydrocarbon.