Oil-source correlation of mixed oils derived from multiple source rocks in the Cainan Oilfield, Junggar Basin, Northwest China. Part II : Geochemical characteristics, typing and oil sources of typical crude oils

Using carbon isotopic and biomarker data, the crude oils in the studied area can be divided into five groups. The first is derived from the Permian source rocks, with δ13 C values generally less than - 29‰, Pr/Ph ratios less than 2, rich in β-carotane, tricyclic terpanes, gammacerane and C28 and C29 steranes, contain almost no diasteranes and a low content of C27 steranes. The second is from the Triassic source rocks, which is similar to the first group in carbon isotopic composition, but is characteristically rich in Ts, C29 Ts and diahopanes, and low gammacerane contents. The third group is generated from the Jurassic source rocks, and characterized with δ13C values ranging from - 28‰ to - 25‰, Pr/Ph ratios generally greater than 4.0, rich in pentacyclic hopanes and C29 steranes, but poor in tricyclic terpanes, gammacerane, and C27 and C28 steranes. The geochemical properties of the fourth group of oils are intermediate between the properties of typical Permian, Triassic and Jurassic oils, and which have little correlation with any of the four known source rocks in the basin. The fifth group of crude oils is isotopically heavy, and originated from Carboniferous source rocks.