鄂尔多斯盆地苏里格庙与靖边天然气单体碳同位素特征及其成因

苏里格庙上古生界砂岩气藏天然气重烃含量高,干燥系数低,为湿气;靖边奥陶系风化壳气藏天然气重烃含量低,干燥系数高,属干气。苏里格庙天然气的正构烷烃碳同位素具有δ13 C₁<δ13 C₃ <δ13 C₂ <δ13 C4的规律,靖边天然气具有δ13 C₁<δ13 C₂ <δ13 C₃ 的规律。二者高碳数烷烃的同位素差别较大,尤其是苏里格庙的乙烷碳同位素比靖边平均重 3.6‰。根据天然气单体碳同位素及其含量变化,苏里格庙天然气属煤成气,靖边天然气属煤成气与油型气的混合气。苏里格庙和靖边天然气中均含少量CO₂ 气,其碳同位素表明苏里格庙除苏 5井为无机CO₂ 气外,均为有机CO₂ 气;靖边林 1、林 5井为无机CO₂ 气,陕参 1井为有机CO₂ 气。结合烷烃气和CO₂ 气,苏里格庙天然气可以分为含有机CO₂ 煤成气和含无机CO₂ 煤成气。碳同位素比值与Ro 关系分析认为,苏里格庙的天然气成熟度处于成熟到高成熟阶段,其相应气源岩的平均Ro 为 1.2 9%～ 1.39%,这一结果与该区二叠系煤岩的成熟度符合较好。

Characteristics of the Individual Carbon Isotope of Natural Gas from Suligemiao and Jingbian Gas Fields in Ordos Basin

Natural gas from the gas pool reservoired in Upper Paleozoic sandstone of Sulige miao area is wet gas that is characterized by high content of heavy hydrocarbon and low dry coefficient;otherwise,natural gas from Ordovician weathering crust g as pool in Jingbian area belongs to dry gas for its low content of heavy hydroar bon and high dry coefficient.The istotpic composition of Suligemiao gas has the tendency of δ13 C₁<δ13 C₄, but Ji ngbian gas tends to be δ13 C₁<δ13 C₃. The isotopi c difference of heavy carbon alkane is much larger between Suligemiao and Jinybi an natural gas.Particularly,the carbon isotope of ethane of Suligemiao gas is hi gher than that of Jingbian.The difference is equal to 3.6%.According to individu al carbon isotopes and compositions of natural gas,it can be seen that the Sulig emiao Permian gas is coalbed gas,and the Jingbian Ordovician gas is gaseous mixt ure.Average R o of Suligemiao gas and its source rock is equal to 1.29%～1.39% according to CH ₄ isotope values, which is consistent with R o of the Permian rock.A little of CO ₂ content is found in Suligemiao and Jing b ian natural gas.In terms of the carbon isotope values,it is believed that inorga nic CO ₂ exists in well Su5 in Suligemiao and well Lin1, Lin5 in Jingbian.The rest are all organic CO ₂.In combination with hydrocarbon gas and CO ₂,the n atural gas in Suligemiao could be divided into coal-formed gas with organic CO ₂ and inorganic CO ₂.The maturity of coal-formed gas in Suligemiao reaches middle to high stage and the corresponding R o is 1.29%～ 1.39%,which i s in line with the maturity in Permian coal.