The Cainan Oilfield: A Typical Mixed Crude Oil of Three-endmember

There are four generative hydrocarbon source rocks of Carboniferous, Permian, Triassic, and Jurassic age in the east of the Junggar Basin. This geochemical study has shown that Permian source rocks, which contain high abundance of organic matter with good genetic potential, are the best hydrocarbon source rocks in this area. Triassic source rocks are shown to be next best in these aspects. The abundance of mainly type I organic matter in Jurassic and Carboniferous source rocks varies greatly, and their genetic potential is lower than that of Permian and Triassic rocks. These four source rock units in the Fukang sag all reach a mature to high mature stage and have generated large amounts of oil. Using carbon isotopic and biomarker data, the crude oils in this area can be divided into five groups. The first group of crude oils is derived from the Permian, the second group from the Triassic, and the third group from the Jurassic source rocks. The geochemical properties of the fourth group of oils are intermediate between the properties of typical Permian, Triassic and Jurassic oils. The fifth group of crude oils is particularly isotopically heavy, and originate from Carboniferous source rocks. The crude oils in the Cainan field belong to the fourth group of oil, and 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 around 60%. 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 60% Permian oil, 20% Triassic oil and 20% Jurassic oils indicates that the mixture has similar geochemical features. In summary, the crude oils in the Cainan oilfield are typical mixed oil, and mainly derived from Jurassic source rocks in the Fukarig Sag.