The Origin And Accumulation Mechanism Of Deep Condensates In The Bozhong 19-6,Bohai Bay Basin

Over serveal decadeds of petroleum exploration in Bohai Bay Basin proves that a lot of oil is mainly contained in middle-shallow reservoirs,with the extremely low reserves of natural gas.In recent years,a large condensate pool has been discovered in Bozhong19-6 structural belt in the southwest of Bozhong sag in Bohai Sea area,with proven reserves > 100 billion m3.The tortuous exploration history of deep petroleum reveals the differential enrichment of deep oil and gas,which indicates that the necessary research of origins and preservation mechanisms of deep oil and gas is required to reval the distribution of deep oil and gas.On the basis of abundant geological,geochemical and geophysical data,this paper systematically analyzes the origins of oil and gas in BZ19-6 area,the thermal evolution history of source rock,the charging history of oil and gas,and the pressure evolution of regional cap rock.Based on these studies,the accumulation processes and main controlling factors of this large condensate field are summriezed to reveal the accmulatition mode of deep gas reservoir in a tectonically active region.The results show that the crude oil in the Southwest part of the Bozhong depression is mature.According to the maturity parameters of saturated hydrocarbons,aromatic hydrocarbons,and light hydrocarbons,the estimated Ro values of BZ19-6 buried hill condensate range from 1.1% to 1.3%,which is obviously higher than that of the oil in the nearby areas.Nevertheless,the relatively low concentrations of methyldiamantane in high mature BZ19-6 condensates indicates limited secondary cracking of reservoir oil.Based on the sourcerelated biomarkers of crude oil,the deep crude oils are mainly sourced by the the third member of Shahejie formation(Es3)source rock which was formed in the weak reduction weak oxidation transitional sedimentary environment,with the characteristics of mixed organic matter source.The chemical and carbon isotopic compositions of natural gas in BZ19-6 buried hill indicate that it is mianly derived from kerogen cracking gas with a corresponding Ro range of 0.8%-1.3%.The carbon isotope of ethane and light hydrocarbons reveal that the significant contribution of Es3 source rock to gas accumulation in the deep layer.Namely,the condensate reservoir in the study area is characterized by similar source of oil and gas.Finally,the formation mechanism and controlling factors of BZ19-6 buried hill gas reservoir are invenstigaed through hydrocarbon generation of source rock,hydrocarbon charging history,and pressure evolution of cap rock.The results show that BZ19-6 buried hill reservoir has experienced early oil and late gas continuous charging,with early charge of crude oil since the deposition of Minghuazhen Formation and gas charging from the deposition of the Upper part of the Minghuazhen Formation.The late gas charge generally matches with the occurrence of strong overpressure evolution of cap rock.High intensity and rapid gas generation,near source strong charging,overpressure dynamic sealing and late hydrocarbon generation and accumulation are the the key geological conditions for dynamic enrichment of deep gas.The dynamic hydrocarbon accumulation model of BZ19-6 can be summarized as near source continuous chaging of early oil and late gas,matured with increasing burial coupled with strong overpressure in the late-stage.