鄂尔多斯盆地伊盟隆起上古生界天然气成因及气源

鄂尔多斯盆地伊盟隆起上古生界天然气的成因及来源一直存在争议。通过天然气碳同位素和轻烃综合分析，δ¹³C₂值普遍大于-25.4‰，烷烃气的碳同位素为正碳同位素系列，甲基环己烷指数（MCH）大于50%，具有煤成气的特征。伊盟隆起上古生界烃源岩以煤为主，什股壕地区煤层厚度较大，厚度为6～10 m，镜质体反射率（R_o）为0.8%～1.2%，烃源岩进入了大量生烃阶段，本区烃源岩对天然气近源成藏具有一定的贡献。伊盟隆起天然气甲烷含量平均为94.5%，高于十里加汗地区平均值91.2%；天然气δ¹³C₁值也明显高于十里加汗地区，平均数值相差3‰。由此认为天然气侧向运移过程中发生了成分色层效应和碳同位素分馏作用，伊盟隆起上古生界部分天然气可能来自泊尔江海子断裂以南地区。

Origin and source of natural gas in the Upper Paleozoic of the Yimeng Uplift,Ordos Basin

No consensus has been reached on the origin and source of natural gas in the Upper Paleozoic of Yimeng Uplift, Ordos Basin.Based on a comprehensive analysis of carbon isotope and light hydrocarbon, the δ¹³C₂ values are gene-rally more than -25.4‰, alkane carbon isotope values change in normal order and the relative content of methylcyclohexane is more than 50%, which show that natural gas is coal-derived gas.The Upper Paleozoic source rocks in Yimeng Uplift are dominated by coals.In Shiguhao area, the thickness of coalbeds ranges from 6 to 10 m.The source rocks with a vitrinite reflectance from 0.8% to 1.2% reached the major stage of hydrocarbon generation.So the local source rocks have an important contribution to near-source gas accumulation.Average methane content of Yimeng Uplift is 94.5%, which is higher than that of Shilijiahan area (91.2%); δ¹³C value was alos significantly heavier than that of Shilijiah area with the difference value of 3‰.It is believed that component differentiation and carbon isotope fractionation occurred in natural gas migration process.So some natural gas in the Upper Paleozoic of Yimeng Uplift may come from the south of Boerjianghaizi fault.