Geochemical Characteristics and Conditions for Hydrocarbon Generation of Jurassic Immature Source Rocks from Heshentuoluogai Basin, Northwestern Xinjiang
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摘要: 新疆西北部和什托洛盖盆地勘探程度较低，侏罗系煤系烃源岩较为发育，选取了和什托洛盖盆地和布克河凹陷与白杨河凹陷的四条侏罗系野外地质剖面，对侏罗系烃源岩进行了系统的地球化学实验室分析测试，确定了该盆地生烃潜力。研究表明，侏罗系八道湾组、三工河组以及西山窑组煤系烃源岩有机碳含量、氯仿沥青“A”、总烃以及生烃潜力值均达到中等—好的烃源岩有机质丰度标准。煤系烃源岩有机质显微组分镜质体较为富集，壳质组相对含量达到20%，为孢子体、角质体、树脂体以及木栓质体，泥岩富集部分藻类体，荧光特征较强，惰质体不发育，母质类型为Ⅱ2型，部分为Ⅲ型，煤为Ⅲ型，煤中氯仿沥青“A”与族组分同位素偏重，暗色泥岩略低，煤系烃源岩族组分非烃的同位素值均明显偏轻，芳烃同位素略重，受控于煤系烃源岩中多环芳烃较为发育，更为富集δ13C，非烃组分N、S、O化合物富集δ12C。实测Ro值较低，烃源岩成熟度偏低，但盆地模拟结果表明研究区凹陷中心区源岩已进入生烃门限。研究区低熟煤系烃源岩分子标志化合物也存在其特殊性，正构烷烃以高碳数的分布为主，煤的Pr/Ph值明显高于泥岩，无β胡萝卜烷、孕甾烷与升孕甾烷分布、伽玛蜡烷指数低，表明侏罗系煤以湖沼相的氧化环境为主，而西山窑组泥岩主要以弱氧化环境沉积特征为主，另外萜烷系列化合物几乎无倍半萜、二环倍半萜以及三环萜烷的分布，藿烷系列存在ββ生物构型分布，为热演化程度较低的标志，分子标志化合物参数的相关性表明，弱氧化的湖沼相的沉积水体有利于此类化合物的分布，煤抽提物中规则甾烷C27、C28、C29呈反“L”型分布，主要以高等植物的陆源输入为主，而侏罗系泥岩存在“V”型分布，有低等水生藻类的贡献，与富氢组分中存在藻类体有较好的的匹配关系，具有更有利的生烃条件。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 infield 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, (S1+S2). 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 richhydrogen 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 Ⅱ2, 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 nonhydrocarbon 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 12C is accumulated in nonhydrocarbon containing NSO compounds, but 13C 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, nalkanes 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 βcarotene, 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 C27, C28, C29 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.
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