Oil-source correlation of mixed oils derived from multiple source rocks in the Cainan Oilfield, Junggar Basin, Northwest China. Part III: Geological and geochemical analysis of possible oil sources in the Cainan Oilfield

The crude oils in the Cainan Oilfield are mixtures of hydrocarbons from Permian, Triassic and Jurassic source rocks, based on their distribution in the reservoirs, and their carbon isotopic and biomarker compositions. A 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 matching calculation with 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. The oils with similar features to the Cainan oils, occur only above the Jurassic Sangonghe Formation around the Fukang sag. Hence they are derived from the Fukang sag.