Geochemistry characteristics and Re-Os isotopic dating of Jurassic oil sands in the northwestern margin of the Junggar Basin

Abstract

The study of hydrocarbon charging history in shallow basin margin is the key to the investigation of the relationship between hydrocarbon fluid and sandstone type uranium deposit. In this paper, the geochemistry, origin and charging history of hydrocarbon from the northwestern margin of the Junggar Basin were investigated by GC-MS, carbon isotope analysis and Re-Os dating. The results showed that the oil sand samples experienced at least 5 levels of biodegradation, which rendered it impossible to characterize hydrocarbon source correctly by sterane parameters. However, the terpane distribution spectra were relatively stable owing to terpane's strong resistance to degradation. In the terpane spectrum, the relative abundance of C20, C21 and C23 showed an increasing trend, and C23/H and G/H ratios were 0.75-1.34 and 0.27-0.65, respectively, in keeping with a lacustrine aquatic organism source formed in a brackish and anoxic environment. The consistent light carbon isotope composition (<-28‰) among different samples further indicated that hydrocarbons mainly sourced from the Permian Fengcheng Formation in the Mahu Depression, rather than from the Jiamuhe and Wuerhe Formations. Here, the oil sand age is dated by Re-Os isotope (155±51 Ma) for the first time. The age suggested that the northwest Jurassic strata experienced a large-scale hydrocarbon charging during the Late Jurassic-Early Cretaceous period. This event covered and protected the paleo-interlayer oxidation zone as well as paleo-uranium ore bodies formed in the middle-late Jurassic era, but it also increased the difficulty in contemporary uranium prospecting. © 2018, Editorial Office of Earth Science Frontiers. All right reserved.