Mixed oils derived from multiple source rocks in the Cainan oilfield, Junggar Basin, Northwest China. Part I: genetic potential of source rocks, features of biomarkers and oil sources of typical crude oils

Abstract

There are four generative hydrocarbon source rocks of Carboniferous, Permian, Triassic, and Jurassic age in the east of the Junggar Basin. Geochemical study of these has shown that Permian source rocks, which contain high abundances 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 III organic matter in Jurassic and Carboniferous source rocks varies greatly and the genetic potential of these rocks is lower than that of Permian and Triassic rocks. These four source rock units in the Fukang sag all reach a mature to highly mature stage of thermal generation, and have generated large amounts of oil. Permian extracts are isotopically light with δ 13C ratios generally less than −29‰. They have low Pr/Ph ratios, are rich in β-carotane, tricyclic terpanes, gammacerane and C 28 and C 29 steranes, contain almost no diasteranes and have a low content of C 27 steranes. The carbon isotopic composition of the Triassic extracts is similar to that of the Permian, but they are characteristically rich in Ts, C 29Ts and diahopanes, and have low gammacerane contents. The carbon isotopic composition of Jurassic extracts is noticeably heavier than that of Permian and Triassic extracts, with δ 13C being generally greater than −28‰. Jurassic source rocks have Pr/Ph ratios generally >3.0, and are rich in pentacyclic hopanes and C 29 steranes, but poor in tricyclic terpanes, gammacerane and C 27 and C 28 steranes. Using carbon isotopic and biomarker data, the crude oils in this area can be divided into five groups. The first group of crude oils (A) is derived from the Permian, the second group (B) from the Triassic, and the third group (C) from the Jurassic source rocks. The geochemical properties of the fourth group (D) of oils are intermediate between the properties of typical Permian, Triassic and Jurassic oils and they are interpreted as mixed oils derived from multiple source rocks, including the Permian, Triassic and Jurassic. The fifth group of crude oils (E) is particularly isotopically heavy and originates from Carboniferous source rocks.