Geochemical Characteristics and Conditions for Hydrocarbon Generation of Jurassic Immature Source Rocks from Heshentuoluogai Basin, Northwestern Xinjiang

Key words: 
• hydrocarbon generation potential /
• organic maceral /
• isotopes /
• biomarkers /
• Jurassic source rock /
• Heshentuoluogai Basin

Abstract: Heshituoluogai basin is located on the northwestern of Jungar basin, and the distribution of the dark mudstone is extensive. According to the geochemical analysis of the Jurassic mudstones form the four infield sections in Hebukehe and Baiyanghe depression, the generated hydrocarbon conditions have been described. The results indicate that the organic richness is high including TOC, chloroform bitumen “A”, total hydrocarbon, (S1+S2). Through the analysis of the vitrinite reflectance of the source rock, Ro is less than 0.65 showing the immature of the source rock, but the results of the basin mod indicate that the maturity of the depression extend the threshold the generated hydrocarbon. In the organic components of Jurassic source rock, the vitrinite is concentrated and especially in the coal, but devoid of intertinite. The relative abundance of richhydrogen exinite is about 20%, including sporophyte, cutinite, resinite, and suberinite commonly, and alginate is accumulated in mudstone, which reflect the strong fluorescent characteristics. So the organic type of mudstone is Ⅱ2, partially Ⅲ. From the analysis of the carbon isotopes of the chloroform bitumen “A” and group compositions, the carbon isotope of coal is heavier than mudstone. The isotope value of nonhydrocarbon in group compositions is lighter, but the aromatic hydrocarbon′s is heavier, which is controlled by the extensive distribution of polycyclic aromatic hydrocarbons in Jurassic source rocks, especially in immature source rocks, suggesting that 12C is accumulated in nonhydrocarbon containing NSO compounds, but 13C is aggregated in polycyclic aromatic hydrocarbons. Also the distribution of molecular biomarkers from the extracts of immature source rocks is particular. In total ion currency profile, nalkanes are distributed by high carbon numbers mainly and hopanoid compounds exit. A Pr/Ph ratio of coal is higher than mudstone. Correlated with the characteristics of the devoid of βcarotene, pregnanes and low gammacerane index, Jurassic source rock is mainly deposited in oxygenated and lacustrine conditions, but xishanyao Formation mudstone is deposited in poor oxic or reductive conditions. Additionally sesquiterpenoid, bicyclic sesquiterpenes and tricyclic terpanes are not exiting in the terpanoids. The regular C27, C28, C29 steranes of coal occur as opposite “L” pattern indicating that the organic matter of Jurassic source rock is originated from the lacustrine high plants, but in Xishanyao formation mudstone, the sterane is distributed by “V” pattern showing the algae input, which is correlated with the distribution of the organic maceral, suggesting the better generation conditions.