Keywords: 
• Tarim Basin /
• marine oils /
• biomarker /
• tricyclic terpane series /
• stable carbon isotope

Abstract: The crude oils from Ordovician reservoir in TZ52 and TZ162 well have been analyzed, the results showed that there are some evident differences in the distributions and compositions of their biomarker from two kinds of the known marine oils in the middle zone of Tarim Basin. For example, it is firstly reported that its trycyclic terpane series is of special steplike model with C19>C20>C21>C23>C24>C25>C26 in relative abundance, C24 tetracyclic terpane is abnormally abundant and it relative abundance is much more than C26 tricyclic terpanes, which occurs in those source rocks and the crude oils deposited under fresh water swamp and shallow lacustrine facies. As for their triterpane series, the abundance of gammacerane is much less than C31 homohopanes, but the distributions of their sterane and hopane series are very similar to the marine oils derived from middle to middle-upper Ordovician source rocks such as with abnormally abundant pregnane and homopregnane, relatively lower C28 sterane in C27-29 steranes and more abundant C29 norhopane secondary to C30 hopane. In addition, their stable carbon isotope of whole oil is very light and their δ13Ｃ values are less than -30‰ like other marine oils, showing that this kind of crude oil is marine. Based on the biomarker data in the source rocks with abundant benthic marco-algae from 4-5 member of Lianglitage formation of upper Ordovician in TZ12 well, it is found that their distribution and the relative composition of its C19-26 tricyclic terpane, C24 tetracyclic terpane and gammacerane is very similar to this kind of crude oils, showing that there is a close relationship between this kind of novel crude oils and the source rocks. Therefore, it should pay much more attention to the source rocks and their exploration potential from Lianglitage Formation of upper Ordovician in Tarim Basin.