Revisiting Of Oil Source In The Cratonic Area Of The Tarim Basin

The Tarim Basin is one of the largest frontier basins in China.After decades of exploration and development,numerous of commercial oil accumulations preserved in Sinian,Cambrian to Paleogene have been revealed,yet the origins of the marine oils in cratonic areas are still unresolved.Based on a number of Paleozoic source rocks we restudy the geochemical characteristics of source rocks in the Tarim Basin.Meanwhile a total of 40 oil samples have been performed on comprehensive two–dimensional gas chromatography with time–of–flight mass spectrometry analyses,the quantitative and qualitative study on light hydrocarbons between C₅^C₁₃ are also accomplished in this paper so as to find new parameters indicating oil source in the Tarim Basin.The Lower Cambrian source rocks in the Tarim Basin,with Xidashan formation in the east and Yuertusi formation in the middle–west,shows excellent hydrocarbon generation potential;the good source rocks in the Middle Cambrian are mainly deposited in the Moheershan formation in the eastern Tarim Basin.The Ordovician source rocks are mainly composed of Lower–Middle Ordovician black shales,Middle–Upper Saergan–Yingan black shales in the Kalpin Uplift,and Upper Ordovician Calcareous mudstones in the northern Tabei Uplift,respectively.For Cambrian source rocks,the majority C₂₁/C₂₃ tricyclic terpane ratios are greater than 1.0.The relative abundance of homohopanes is low but the gammacerane is higher.The ratios of C₂₈/(C₂₇+C₂₈+C₂₉)steranes are greater than 25%.Few rearranged steranes are observed.Most Cambrian source rocks are of high abundance of triaromatic dinosteroids.The ratios of(C₂₆+C₂₇)/C28 triaromatic steroids and C₂₇/(C₂₆+C₂₇+C₂₈)ratios are great than 1.0 and 0.25,respectively.The stable carbon isotopes of soluble organic fractions are heavier than–31‰,however the values of kerogen maceral groups are abnormally light,with the lightest value up to–35.6‰.The isotopic reversal is observed which believed to be caused by high maturity.The Upper Ordovician Lianglitage formation source rocks are of great differences with Lower–Middle Ordovician Heituwa formation and Upper Ordovician of Yangjikan profile based on the distribution of homohopanes,the relative abundance of gammacerane and rearranged steranes,the ratios of C₂₈/(C₂₇+C₂₈+C₂₉)steranes,the distribution of triaromatic steroids and dinosteroids and the stable carbon isotopes of soluble organic fractions.A series of parameters indicates that the Lower–Middle Ordovician Heituwa formation and Upper Ordovician of Yangjikan profile show the same geochemical characteristics.In this paper,a new method using to calculate each C₂₆,C₂₇ and C₂₈ triaromatic steroid on traditional GC–MS analysis is established,and thus the relative composition of C₂₆,C₂₇and C₂₈ triaromatic steroid are also calculated.A ternary diagram of C₂₆,C₂₇ and C₂₈triaromatic steroid is also proposed and effectively used in oil–source correlation study in the cratonic area of the Tarim Basin.Besides,the ternary diagram can also be used to distinguishing different depositional environments.On the basis of oil–source analysis performed on typical marine oils,the type“A”oils,mainly composed of TD2,TZ26 and ZS1C,have a series geochemical characteristics followed by:C₂₁/C₂₃ tricyclic terpanes>1.0,Low abundance of steranes>25%,high relative abundance of triaromatic dinosteroids,(C₂₆+C₂₇)/C₂₈triaromatic steroids ratios>1.0,the ratios of C₂₆/C₂₈ 20S and C₂₇/C₂₈ 20R greater than0.34 and 0.50,respectively.C₂₇/(C₂₆+C₂₇+C₂₈)triaromatic steroids ratios>25%,high relative abundance of perylene with perylene/benzo[a]pyrene>0.17,and heavy stable carbon isotope values with values heavier than–31‰.All geochemical parameters suggest that oils of type“A”shows a good correlation with the Cambrian source rocks.Oils of type“B”,as represented by S46,shows an opposite geochemical characteristics with oils of type“A”.All indicators suggest that oils of type“B”are correlation with Upper Ordovician Lianglitage formation source rocks.After numerous attempts a reasonable and effective method analyzing light hydrocarbons between C₅^C₁₃ in crude oils has been established.A total of 40 marine crude oils collected from the Tarim Basin are also analysed.The Cambrian sourced oils show apparent loss on light hydrocarbons between C₅^C₁₃,as expressed by few isoalkanes and cycloalkanes detected.In contrast,the Ordovician oils show abnormal abundance of isoalkanes,cycloalkanes and alkylbenzene compounds.In oils sourced fromtheCambriansourcerocks,theratiosof?2,2–dimethylhexane+1,1,3–trimethylcyclopentane+2,3–dimethylhexane+1,1,2–trimeth ylcyclopentane?/heptaneand?cis–1–ethyl–3–methylcyclohexane+cis–1–ethyl–4–methylcyclohexane+1,3–dimethyloctane+isopropylcyclohexane+4,4–dimethyloctane?/?propylbenzene+1–methyl–3–ethylbenzene+1–methyl–4–ethylbenze ne+1,3,5–trimethylbenzene+1–methyl–2–ethylbenzene?are less than 0.25 and 0.35,respectively;while the ratios in Ordovician oils are greater than 0.43 and 0.35,respectively.Theratiosof?cis–1,4–dimethylhexane+3,3–dimethylhexane+trans–1,2–dimethylcyclopentane+1,1–dimethylcyclohexane+3,4–dimethylheptane+3,3–dimethylheptane?/octaneand1–methyl–3–propylbenzene/1,3–dimethyl–5–ethylbenzene are less than 0.1 and 1.0 in Cambrian sourced oils while the ratios in Ordovician oils are greater than 0.27 and 1.0,respectively.Three correlation diagrams are established on the basis of the above 4 parameters and can be used in oil–source study in the cratonic area of Tarim Basin.