胜利油田天然气氩同位素特征

Characteristics of Argon Isotopes in Natural Gases of Shengli Oilfield

对胜利油田２１个天然气样进行氩同位素组成分析，结合甲烷碳同位素组成特征和地质背景，阐明了天然气中４０Ａｒ／３６Ａｒ比值分布规律及其与源岩、储层的关系，进而讨论了天然气中空气氩和放射成因氩的分布规律，指出胜利油田存在着生物－热催化过渡带气、油田伴生气、煤型气和富ＣＯ２气。它们具有各自的氩、碳同位素组成和演化特征。第三系和中生界自生自储天然气的４０Ａｒ／３６Ａｒ比值分别为３３５和５６４。通过数学分析，首次确定古生界和前震旦系新生古储天然气的储层时代效应系数分别为１．２５和１．４７。并认为前震旦系储层中天然气来自第三系，而储于古生界的天然气是约７０％的第三系油型气和约３０％的中生界煤型气混合的结果。与岩浆作用有关的无机ＣＯ２和第三系有机成因烃类气混合形成平方王富ＣＯ２气藏。

Shengli Oilfield is located in the Southeastern Bohai bay Basin and tectonically belongs to the ji'yang Depression, oil and gas reservoirs are found in the strata of Archaeozoie, Palaeozoic,Mesozoic and Cenozoic. Studies revealed that the Carboniferous -Permian coal seams and Jurassic coae-bearting fotmations are the source rocks of coae-type gases, carbonate rocks that sedimented durng palaeozoic Era are the source rocks of high -CO2 -contained natrual gases. The isotopic composition of argon is a key parameter to identify the gases source and genetic type in this oilfield.Based on the argon isotopic analytical results of 21 gas samples,associatd with carbon isotopic compositon and geological background,the present paperdealt with the distribution law of 40Ar/36Ar and its relation with the source rocks and the age of reservoirs, then the author discussed the distribution law of the air argon and the radio activity-generated argon in natural gases,concluded that: The isotopic composition of argon is mainly related with the age of source rocks; There is a certain rertain reservoir-time effect in the gases that generated by new strata and traped in old ones Magamatic activity can result in the increas of argon isotopic ratio. Synthetical study of carbon and angon isotopes indicated that in the target area, gases in the Tertiary strata are bio- thermal catalysed tranzationat gas and thermal catalysed oil-type gas that traped by their own source rock ;There are self-origion and self-reservoir coal-type gases in Mesozoic strata. The gases in the granite-gneiss of pre-sinian were derived from Tertiary strata ;palaeozoic gases probably were the mixture of middle Tertiary oil-type gases and Mesozoic coal- type gases; The formation of high CO2 - contained gas were mainly related with magamatic activities ; The genetic types of biothermal catalysed transitional gas, oil - type gas, coaltype gas and high - CO2 - contained gas have the characteristics of contineously evolution of Meso-cenozoic basens in China and gas-generating sends, each type has its own argon and carbon isotopic composition and evolution feature. the average 40Ar/36Ar values of Trtiary and Mesozoic in situ sourced and reservoired gases are 335 and 564 respectively. According to the mathmatical analysis, the coefficient of reservoir -time effectswere primarily calculated for the gases that sourced from new strta and reservoired in old ones, 1. 25 and 147 for palaeozoic and pre-Sinian granite-gneiss respectivily. It is determined that the gas in pre-Sinian is the mixture of Tertiary. oil-type gas(70% )and Mesozic coal-type gas (30% ).High - CO2 - contained gases in Pingfangwang Gasfield are the mixture of Terteary organic hydrocarbons and inorganic CO2formed by the decomposition of carbonate rocks which resulted from magmatic activities.