Determination Of Oil Sources In The Western Slope Of Mahu Sag,Junggar Basin On The Basis Of Compound Specific Carbon Isotopes And Molecular Parameters And Concentrations

Junggar Basin is one of the major oil producing provinces in the northwestern China.It is also a major target for oil exploration in China.The discovery of giant oil province of conglomerates within the Lower Baikouquan Formation and Lower Wuerhe Formation in the Mahu Sag is the most important breakthrough in oil exploration in this basin.This discovery demonstrates that abundant oil resources exist in the Mahu Sag.On the basis of geological settings,oil charges for reservoirs in the central area(western slope)of the sag can be derived from source rocks within both the Lower Permian Fengcheng Formation and Middle Permian Lower Wuerhe Formation.It is very important to determine oil sources for reservoirs in the central area of the sag on the basis of oil geochemistry for oil exploration and oil potential evaluation in this sag.In this study,24 crude oils and 10 oil-bearing reservoir rocks were collected.Sequential extraction was performed on the reservoir rocks to obtain free oil,adsorbed oil and inclusion oil.The crude oils and oil components from the reservoir rocks by sequential extraction were further analyzed by GC,GC-MS and GC-IRMS to document the oil sources and oil migration and accumulation. 24 oils from the western slope of Mahu Sag,Junggar Basin were classified into two groups on the basis of carbon isotope distribution patterns of individual n-alkanes,and parameters and concentrations of saturate compounds,aromatic compounds,dibenzothiophene(DBT),methyl dibenzothiophenes(MDBTs),dibenzofuran(DBF)and methyl dibenzofurans(MDBFs).Group I oils have no reversal trend for carbon isotopes of n-alkanes with carbon number while Group II oils have this reversal trend with various extents.Compared with Group II oils,Group I oils have(1)relatively higher Pr/n-C₁₇and Ph/n-C₁₈ratios and lower Pr/Ph ratio,(2)relatively higher ratio of gammacerane/(gammacerane+C₃₀hopane),(3)relatively higher ratios of tricyclic terpane/(tricyclic terpane+C₃₀hopane),TA(I)/(TA(I)+TA(II)triaromatic steranes and 4-MDBT/1-MDBT,and(4)relatively higher concentrations of DBT and MDBTs,lower concentrations of DBF and MBDFs and higher ratios of DBT/(DBT+DBF)?(MDBTs)/(MBDTs+MDBFs).In particular,these two groups of oils can be clearly distinguished on the basis of distribution pattern of carbon isotopes of n-alkanes,concentration of MDBFs and ratios of DBT/(DBT+DBF)and(MDBTs)/(MBDTs+MDBFs).Group I oils were derived from source rocks within the Lower Permian Fengcheng Formation while Group II oils are mixtures of oils from source rocks within the Fengcheng Formation and Middle Permian Lower Wuerhe Formation.Group I oils are mainly distributed in reservoirs within relatively older and deeper strata compared with Group II oils.These two oil groups have different migration routes and accumulation mechanisms.Oil components from the ten reservoir rocks contain relatively higher content of pentacyclic terpanes compared with tricyclic terpanes with very low contents of Ts,C₂₉Ts and C₃₀diahopane but very high content of gammacerane.Oil components from reservoir rock AH012 have higher ratios of tricyclic terpane/(tricyclic terpane+C₃₀hopane)and Ts/(Tm+Ts),demonstrating lower maturities compared with those from the other reservoir rocks.Oil components from all the ten reservoir rocks have similar distribution pattern for carbon isotopes of individual n-alkanes demonstrating a common oil source from source rocks within the Fengcheng Formation.