Bin Cheng, a b *, Hua Liu a, Xian Wu c, Shen Wang a and Zepeng Wu a

a Shandong Provincial Key Laboratory of Deep Oil and Gas, China University of Petroleum (East China), Qingdao 266580, China.

b Shandong Provincial Key Laboratory of Reservoir Geology, China University of Petroleum (East China), Qingdao 266580, China.

c Northwest Oilfield Company, SINOPEC, Urumqi, Xinjiang 830000, China.

* Corresponding author:

Key words: Biomarkers, δ13C, crude oils, petroleum geochemistry, Cambrian, Ordovician, source rocks, oil-source correlation, Tuoputai. Tarim Basin, Tabei Uplift, China.

Molecular and stable carbon isotope compositions of 46 Ordovician crude oil samples from wells in the Tuoputai region of the northern Tarim Basin were investigated using GC–MS, MRM GC–MS and IRMS to determine their genetic relationships and to identify possible source rocks. Thirty-three source rock samples from outcrops and cores were also investigated. The oil samples varied from light to heavy crudes and showed very narrow δ13C value ranges for the whole oil, saturated and aromatic fractions. The majority of the oils displayed very similar molecular compositions with relatively high concentrations of n-alkanes and isoprenoids and low concentrations of terpenoids and steroids. Comparison of the compositions of these crude oils strongly suggested their genetic affinity, while maturity parameters indicated maturity variations from the peak to the late oil generation stages. The samples also showed the characteristics of mixtures of biodegraded and fresh oil charges.

Bitumen extracts from Cambrian and Ordovician source rocks were studied in detail. The oil compositions suggested a marine marl source deposited in anoxic, hypersaline conditions with significant bacterial and algal organic matter inputs. The distributions of C26–C28 triaromatic steroids, tricyclic terpanes and regular steranes appear to have been greatly influenced by thermal maturation, making them unreliable for correlating the oils and the source rocks. In contrast, dinosteranes and triaromatic dinosteroids seem not to have been affected by maturation and were more useful for correlation studies. They indicated that there was no or little genetic relationship between the Cambrian – Lower Ordovician source rocks and the oils, but in general suggested a possible Middle – Upper Ordovician source for the oil accumulations in the Tuoputai field. However, the occurrence of triaromatic dinosteroids in oil from well TP28XCX may also suggest a minor contribution from Cambrian – Lower Ordovician source rocks.

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