准噶尔盆地西北缘超覆带侏罗系油砂地球化学特征及Re-Os同位素定年

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

盆地边缘浅部的油气充注史研究是探讨油气流体和砂岩型铀矿关系面临的关键难题。对准噶尔盆地西北缘超覆带侏罗系油砂提取物进行饱和烃GC-MS分析、碳同位素测试及Re-Os同位素定年。结果表明,样品普遍遭受了至少5级的生物降解,部分甾烷化合物参数已不能客观反映油源特征。而萜烷抗降解能力强,其分布谱图比较相似,C20、C21、C23三环萜烷的相对丰度呈"上升型";C23/H比值为0.75~1.34;G/H比值为0.27~0.65;以上指示了石油的母质以湖相低等水生生物输入为主,形成于半咸水、偏还原的沉积环境。样品的氯仿沥青"A"及族组分的碳同位素组成较轻且几乎一致,均低于-28‰,进一步表明该区侏罗系中的石油具有相同来源,初始来源于玛湖坳陷二叠系风城组,而与佳木河组和乌尔禾组的关系不大。油砂Re-Os同位素测年首次获得了等时线年龄为(155±51)Ma,表明西北缘侏罗系在晚侏罗世—早白垩世发生了一期大规模的油气充注,进而掩盖和保护了中、下侏罗统于中、晚侏罗世阶段形成的古层间氧化带型铀矿体,并增加了找矿难度。

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 bydrocarbon 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 （22o, C~ and Cz3 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%o） 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.