Geochemical characteristics and origin of the crude oils and condensates from Yakela Faulted-Uplift, Tarim Basin

• The oils from Yakela Faulted-Uplift were distinguished into three genetic groups.
• Marine and terrestrial oil both occurred in Yakela Faulted-Uplift.
• Yakela Faulted-Uplift is a borderline which separates the carton marine oil province from terrestrial oil province.

Yakela Fault-Uplift is one of the most petroliferous belts of Tarim Basin in China. In this regard, the geochemistry and origin of the representative oils from different fields on the belt were investigated and compared with hydrocarbons from adjacent Kuqa Foreland Depression and Tahe Oilfield using GC, GC–MS and stable carbon isotope techniques. Three oil groups can be distinguished in terms of their molecular and stable carbon isotope composition. The oils from Yakela and Donghetang fields (group A) share similar molecular compositions and δ13C values with Tahe oils. They are characterized by high concentrations of nC7 and branched alkanes, resulting in elevated heptane values and nC7/MCC6 ratios. Mango’s light hydrocarbon parameter K2 values are low, as are Pr/Ph, (C19+C20+C21)/(C23+C24) tricyclic terpanes and C24 tetracyclic/C23 tricyclic terpane ratios. High abundances of tricyclic terpanes, fluorine and methylated dibenzothiophenes are present, also relatively negative δ13C values. The geochemical characteristics indicate that they originated from marine source rocks in cratonic area. The condensates from Dalaoba (group B1) and Luntai fields (group B2) show most similarity with Kuqa oils in molecular compositions, distinctly different from group A. However, difference also apparently presents between Dalaoba and Luntai oils, the latter with more abundant C19–C20 tricyclic terpanes and positive δ13C values. The δ13C composition indicates that Dalaoba oils originated from Mesozoic lacustrine shale source rocks in the Kuqa Depression, while Luntai oils are generated from Mesozoic coal. The results show that Yakela Faulted-Uplift separates the cratonic marine oil province from non-marine oil province in north.