Abstract: Abundant 1-alkyl一2，3，6-trimethyl aryl isoprenoids were detected in the aromatic fraction of the Paleozoic crude oil samples from Lunnan Oilfield，Tarim Basin，NW China．These compounds were considered to be derived from β-cleavage of diaromatic carotenoid moieties mainly found in the chlorobiaceae family of photosynthetic sulfur bacteria and regarded as a biomarker that indicates a saline and strongly reducing sedimentary environment．It can be inferred that these biomarkers indicate prolific bacteria growth in a stratified water column at and below the oxic-anoxic boundary layer in depositional environment，which implied that contributions of first-class source rocks in a stratified water column and highly reducing sedimentary environment were underestimated in present exploration strategy in the Tarim Basin．Especially，the distribution of C15·3，4，5-isomer was surveyed in these oil samples to pursue its geochemical significance or application．Furthermore，the ratio of C15-3，4，5 isomer to C13-2，3，6 isomer(C15-3，4，5／Q3-2，3，6)was proposed and correlated with some typical geochemical parameters(such as Pr／Ph，Ga／C31H，Ts／Tm)．The results in this paper show that the ratios of C15-3，4，5／C13-2，3，6 have a systematic increase with these increasing parameter values．This indicates that，apart from being a biomarker of having a specific precursor and indicating hypersaline and highly reducing sedimentary facies，1-alkyl-trimethyl aryl isoprenoids can also have a significance in the determination of organic matter maturity．The above-mentioned discussion can help us better understand their distribution，thermal maturity effect，diagenetic and catagenetic transformation of isorenieratene．