Molecular Nitrogen Genesis in Natural Gases and Relationship with Gas Accumulation History in Tarim Basin
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摘要: 塔里木盆地塔北和塔中地区的海相腐泥型天然气,N2含量较高,尤其是湿气,N2含量分布在10.1%～36.2%,而干气的N2含量则低于10%,即湿气的N2含量高于干气的N2含量。同是下古生界寒武—奥陶系来源的海相腐泥型天然气,为什么湿气和干气的氮气含量相差如此之大?根据与氮气相伴生的烃类气体、非烃气体及稀有气体的组份及同位素特征,认为塔里木盆地的中高氮天然气属于有机成因,来源于下古生界海相烃源岩。文章还提出塔北和塔中地区湿气和干气N2含量差异与源岩的演化程度和圈闭的捕获条件有关。Abstract: Molecular nitrogen (N2 )content of marine saporpelic ytpe gases derived from Lower Paleozoic source rock is high, ranging from 1.61～36.2 percent of molecular nitrogen. N 2 content of wet gases ranges from 10.1% to 36.2%, while dry gases less than 10%, it is to say that N 2 content of wet gases is higher than that of dry gases.Why is there large difference of N 2 content between wet gases and dry gases derived from the same type source rock? What relationship it has with gas migration and accumulation ? Based on the composition and isotope geochemical characteristics of associated gases and non hydrocarbon gases,as well as rare gas in nature gas,N 2 genesis and origin are ascertained in this paper. The study shows that natural gases of middle high N 2 content belong to organic genesis,which is originated from lower Paleozoic marine carbonate source rock. It is indicates that N 2 content difference between wet and dry gas has close relationship to source rock, maturity and natural gas accumulation history.For the same type source rock, N 2 content in gas is controlled by source rock maturity and entrapment conditions.Because of the difference of nitrogen existence form in organic matter of carbonate and mudstone or shale source rock,nitrogen is mainly combined in kerogen by chemical bond for carbonate source rock,but for mudstone or shale source rock,besides chemical bond,nitrogen is bonded in clay mineral by ion substitution form during organic matter maturation.Thermal simulation shows that nitrogen release mechanism is different for the different source rock.N 2 release capacity reachs peak at the generation peak for carbonate source rock, N 2 content of gases decreasing at the high super maturity stage.While for mudstone or shale source rock, N 2 release capacity reachs peak at the high super stage.Therefore,N 2 content of natural gas entrapped at different maturity stage of source different, N 2 content variation in natural gases can be used to reveal natural gas accumulation history, and also to evaluate trap effectiveness. Lower N 2 content of dry gas in Tabei area demonstrates that oil and gas containing higher N 2 content at the generation peak have been lost,and that natural gas captured in the present traps is mainly originated from high super maturity source rock,The above conclusion is supported by heavy methane carbon isotope and large dryness of the gas,and also supported by geologic conditions of Tabei area. Higher N 2 content and lighter methane carbon isotope of wet gas in Tazhong area shows that gas maturity is lower than that of dry gas in most part of Tabei area,natural gas with high N 2 content formed by source rock at the generation peak is captured by trap.
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