Enrichment Characteristics And Accumulation Models Of Neogene Hydrocarbon In Bohai Bay Basin

Geological data，geophysics information，geochemical datum and previous achievementswere all applied to research the accumulation conditions and enrichment characteristics ofNeogene hydrocarbon in Bohai Bay Basin. And then we analyzed the hydrocarbonaccumulation characteristics and compared its difference in the typical area of differentenrichment patterns，established effective accumulation models and discussed the maincontrolling factors about hydrocarbon enrichment of Neogene.The study of the Cenozoic tectonic-sedimentatary characteristics in Bohai Bay Basinshows that from the outskirts to the center of the basin，the settlement and extension rate ofbasement and the intensity of fault activity decreased gradually during the rifting stage andincreased during the depression stage，and the depression evolutionary types changed fromearly break-early decline and inherited developmental play to late break-late decline play. Theformation thickness of Paleogene became thinner and the lithology changed from sandstone,siltstone to mudstone，while Neogene is obviously thicker than Paleogene and the lithologychanged from conglomerate rock and sandstone to siltstone. The hydrocarbon accumulationconditions also changed regularly from the outskirts to the center of the basin. The sourcerock series become shallower and the quality of cap rock becomes better along this direction，while reservoir is distributed all over the basin. There are three modes of hydrocarbonmigration namely direct migration by source faults，secondary faults adjustment migrationand mixed transportation.The oil and gas enrichment degree increases from the outskirts to the center of the basin，while hydrocarbon enrichment layer becomes newer，and the enrichment zones change fromthe slope belt, shallow concave belt to drape belt. The hydrocarbon accumulation types ofdifferent depression are in accordance with its evolutionary types. The enrichment styles ofNeogene can be divided into3categories which can be further divided into6types，which are style of drape structural belt containing high salient style and low uplift style，style of slopebelt containing the style controlled by fault and overlap zone style and style of shallowconcave containing central uplift belt style and common shallow concave style. A greatquantity of hydrocarbon is easy to be accumulated in drape structural belt for its advantageoushydrocarbon accumulation condition，however，style of slope belt is restricted by thepreservation conditions and style of shallow concave is restricted by the migration pathways，which are not conducive to accumulating large-scale oil and gas of Neogene.Based on the study above，three kinds of accumulation modes are established，the firstone is accumulation mode of drape structural belt with original source for hydrocarbons，direct migration by source faults，and trap controlled by structure；the second one isaccumulation mode of shallow concave belt with secondary source for hydrocarbons，migration by shallow faults，and trap controlled by fault；the third one is accumulation modeof slope belt with mixed source for hydrocarbons，multiple migration，and trap controlled byunconformity. Main controlling factors on hydrocarbon accumulations include：the oil and gasdistribution in general is controlled by the depression evolutionary type，the key factor forhydrocarbon migration and accumulation is fault activity，hydrocarbon enrichment layer isaffected by the difference of reservoir structure，the oil and gas accumulation is guaranteed bythe quality of cap rock.