鄂尔多斯盆地奥陶系盐下马四段高含硫天然气地球化学特征及成因

Geochemical characteristics and genesis of high sulfur natural gas in the fourth member of Majiagou Formation in the Ordovician sub-salt reservoirs,Ordos Basin

鄂尔多斯盆地盐下天然气勘探取得重大突破,在马四段多次钻获高含硫化氢天然气,地质和地球化学特征表明其成因和来源尚不明确。通过系统采集马四段天然气、乌拉力克组页岩气和上古生界致密砂岩气样品,测定天然气的组分和碳、氢同位素组成,结合盆地中东部地质实际开展天然气成因和气源研究。结果显示,马四段天然气是以甲烷为主的干气,干燥系数平均值为0.97,乙烷平均含量为1.07%,丙烷平均含量为0.32%,其余重烃气平均含量0.38%;硫化氢平均浓度为124774×10-6,是TSR作用的产物,明显区别于上古生界致密砂岩气和乌拉力克组页岩气,研究区内由西向东硫化氢含量逐渐降低;马四段天然气中甲烷碳同位素组成介于-44.8‰~-36.4‰,平均值为-40.3‰;乙烷碳同位素组成介于-27.7‰~-19.1‰,平均值为-24.3‰;甲烷氢同位素组成介于-172‰~-140‰,平均值为-162‰;乙烷氢同位素组成介于-140‰~-112‰,平均值为-119‰;马四段天然气具有较轻的甲烷碳同位素组成和较重的甲烷氢同位素组成,指示马四段天然气具有与上古生界煤成气不同的气源。以甲烷碳、氢同位素组成及组分作为高含硫天然气的主要判识参数,识别出马四段天然气具有“自生自储”的特征,气源岩为马家沟组含膏层系发育的海相烃源岩;TSR促进了烃类向非烃的转化,重烃气向甲烷的转化,也会导致烃类碳同位素组成的变化;部分气藏乙烷碳同位素组成已经明显重于干酪根碳同位素组成,超出了热演化的影响范围,凸显TSR作用对乙烷碳同位素影响较大。基于马家沟组盐下天然气的形成演化地质背景,初步估算TSR能导致乙烷碳同位素组成变重5‰~10‰。

A major breakthrough has been made in the exploration of subsalt natural gas in the Ordos Basin.High-sulfur natural gas content has been gradually drilled in the fourth member of Majiagou Formation.The geological and geochemical characteristics indicate that its genesis and source are still unclear.Natural gas samples from the fourth member of Majiagou Formation,shale gas from Wulalik Formation and Upper Paleozoic tight sandstone gas were collected systematically.The composition and carbon-hydrogen isotopes composition of natural gas were determined,and the genesis and source of natural gas were studied in combination with the geological background in the central and eastern parts of the basin.The results showed that the natural gas in the fourth member of Majiagou Formation is dry gas dominated by methane.The average drying coefficient was 0.97,the average ethane content was 1.07%,the average content of propane was 0.32%,and the average content of heavy hydrocarbon gas was 0.38%.The average concentration of hydrogen sulfide is 124774×10-6,which is the product of TSR,which is obviously different from the Upper Paleozoic tight sandstone gas and the shale gas of Wulalik Formation.The content of hydrogen sulfide gradually decreases from west to east in the research area.The carbon isotopic composition of methane in the natural gas of the fourth member of Majiagou Formation ranges from-44.8‰to-36.4‰,with an average value of-40.3‰.The carbon isotopic composition of ethane ranges from-27.7‰to-19.1‰,with an average value of-24.3‰.The hydrogen isotopic composition of methane ranges from-172‰to-140‰,with an average value of-162‰.The isotopic composition of ethane hydrogen ranges from-140‰to-112‰,with an average of-119‰.The natural gas in the fourth member of Majiagou Formation has a light carbon isotope composition of methane and a heavy hydrogen isotope composition of methane,indicating that the natural gas in the fourth member of Majiagou Formation has a different gas source from the Upper Paleozoic coal-type gas.Using the carbon-hydrogen isotopes composition of methane as the main identification parameters of high sulfur natural gas,it is identified that natural gas in the fourth member of Majiagou Formation is characterized by“self-generation and self-accumulation”.The gas source rock is a marine source rock developed in the gypsum-bearing strata of the Majiagou Formation.TSR promotes the conversion of hydrocarbons to non-hydrocarbons,the conversion of heavy hydrocarbon gas to methane,and also leads to changes in the carbon isotopic composition of hydrocarbons.The carbon isotope composition of ethane in some gas reservoirs is significantly heavier than that of kerogen,which is beyond the influence range of thermal evolution,highlighting the great influence of TSR on ethane carbon isotope.Based on the geological background of the formation and evolution of the Majiagou Formation subsalt natural gas,it is estimated that TSR can cause a change in the carbon isotope composition of ethane from 5‰to 10‰.