Oil-source identification for the mixed oils derived from multiple source rocks in the Cainan Oilfield, Junggar Basin, Northwest China. Part I: Fundamental geochemical features of source rocks

Four hydrocarbon source rocks, in age of Carboniferous, Permian, Triassic, and Jurassic developed in the east of the Junggar Basin, Northwest China. The geochemical study shows that Permian source rocks, which contain high abundances of organic matter with good genetic potential, are the best hydrocarbon source rocks, and then, the Triassic source rocks in this area. The organic matter in Jurassic and Carboniferous source rocks mainly compose of type Ⅲ kerogen and their abundance varies greatly, for which the genetic potential is lower than that of Permian and Triassic rocks. These four source rock units in the Fukang Sag all reach a mature high mature stage and have generated large amounts of oil. Permian extracts are isotopically lighter, and have low Pr/Ph ratios, are rich in β carotane, tricyclic terpenes, gammacerane and C 28 and C 29 steranes, contain almost no diasteranes and have a low content of C 27 steranes. Triassic extracts are characteristically rich in Ts, C 29 Ts and diahopanes, but have low gammacerane contents. The carbon isotopic composition of Jurassic extracts is noticeably heavier than that of Permian and Triassic extracts. Jurassic source rocks have Pr/Ph ratios generally 3.0, and are rich in pentacyclic hopanes and C 29 steranes, but poor in tricyclic terpenes, gammacerane, and C 27 and C 28 steranes.