Characteristics And Origin Of Petroleum In Mahu Sag,Junggar Basin

Mahu Sag in the northwestern margin of Junggar Basin is one of the most hydrocarbon-rich sags in the Junggar Basin that has been proven.It has good petroleum geological conditions and great potential for petroleum exploration.This paper systematically analyzed the properties of petroleum and source rocks in Mahu Sag,and combine with the biomarker parameters to clarify the genesis of petroleum.The reservoirs discovered in Mahu Sag are mainly distributed in the Permian,Triassic and Jurassic.Oils API is distributed at 29°?48°,mainly light-to-medium oils,and oils carbon isotope distributed at-30.5‰?-27.5‰.The Triassic oils has a higher density,a lower ratio of pristane and phytane,and a higher content of carotenes and gamma waxes,and the tricyclic terpenes C20,C21,C23 are mainly ascending distribution.Oils quality of the Permian oils is lighter than that of the Triassic and the maturity is higher.Oils properties between the fault zone and the Mahu sag is obviously different,that becomes lighter from the fault zone-slope zone-the center of Mahu sag,conforming to the law of differential accumulation of petroleum migration.Natural gas has the characteristics of organic gas,dry gas and wet gas coexist.According to the carbon isotope distribution of hydrocarbons,natural gas is dominated by oil-type gas.Three sets of source rocks are developed in the Permian in Mahu Sag,from bottom to top are the Jiamuhe Formation,Fengcheng Formation and Xiawuerhe Formation.The Permian Fengcheng Formation is the most dominant source rocks in Mahu Sag,with high organic matter abundance,mainly saprolite type,and oil-dipping source rocks.The organic phase is mainly C-phase and D/E-phase source rocks,and the carbon isotope distribution of kerogen ranges from-31.2‰?-24.8‰;The source rocks of Jiamuhe Formation and Xiawuerhe Formation are of poor types,the initial hydrogen index is less than 200mg/g TOC,mainly F-phase source rocks,the carbon isotope distribution of kerogen ranges from-23.6‰ to-19.2‰,which are humic source rocks and do not have the ability to generate oils.The source rocks of the Lower Wuerhe Formation are currently in the stage of maturity evolution,the Fengcheng Formation are currently in the stage of high maturity evolution,and the Jiamuhe Formation is in the stage of high maturity to overmaturity evolution due to its deep burial depth.Combining the oils-source rocks comparison and the indication of petroleum properties on the source kitchen,it is believed that the oils in each block and different formations of Mahu Sag come from the source rocks of the Fengcheng Formation,but the physical properties and geochemical properties are different.The heavier petroleum produced in the early stage of source rocks maturity have a longer migration distance to the sag slope belt and the northwestern marginal fault zone.The lighter petroleum produced in the middle and late stages have a closer accmulation to the center of the hydrocarbon generation sag.