Mixed oils derived from multiple source rocks in the Cainan oilfield, Junggar Basin, Northwest China. Part II: artificial mixing experiments on typical crude oils and quantitative oil-source correlation

Abstract

There are five groups of crude oils in the East Junggar Basin, four of which are derived from single sources (viz. Permian, Triassic, Jurassic, and Carboniferous). The crude oils in the Cainan field are of a special type in that they have little correlation with any of the four known source rocks in the Basin. Based on their distribution in the reservoir formations, and their carbon isotopic and biomarker compositions, it is concluded that these crude oils are mixtures of hydrocarbons from Permian, Triassic and Jurassic source rocks. A mixing calculation using whole-oil stable carbon isotope ratios of typical end-member crude oils suggests that the contribution of Jurassic source rocks to the Cainan crude oils averages 50–65%. A second calculation based on absolute concentrations of biomarkers indicates that the Jurassic contribution averages 62%, while those of the Triassic and Permian source rocks average 15 and 23%, respectively. Artificial laboratory mixing of three typical Permian, Triassic and Jurassic oils indicates that the average contribution of the Jurassic source rocks is 65%, whereas those of the Triassic and Permian rocks are around 15 and 20%, respectively. In summary, using whole-oil stable carbon isotope ratios and biomarker parameters, it can be concluded that the Jurassic source makes a contribution of 55–70%, while the Triassic and Permian sources each contribute 10–30%. Therefore the crude oils in the Cainan oilfield are mainly derived from Jurassic source rocks in the Fukang Sag.