Characteristics of Argon Isotopes in Natural Gases of Shengli Oilfield

Shengli Oilfield is located in the Southeastern Bohai Bay Basin and tectonically belongs to the jiyang Depression, oil and gas reservoirs are found in the strata of Archaeozoic, Palaeozoic,Mesozoic and Cenozoic. Studies revealed that the Carboniferous-Permian coal seams and Jurassic core-beating formations are the source rocks of coae-type gases, carbonate rocks that sedimented during Palaeozoic Era are the source rocks of high-CO₂-contained natrual gases.The isotopic compition of argon is a key parameter to identify the gases source and genetic type认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 of⁴⁰Ar/³⁶Ar and ' its relation with the source rocks and the age of reservoirs,then the author discussed the distribution law of the sir 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 argon isotopes indicated that in the target area,gases in the Tertiary strata are bio-thermal catalysed tranzational 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-coritained 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 basins in China and gas-generating series, 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 effects were primarily calculated for the gases that sourced from new sttta and reservoired in old ones,1. 25 and 147 for Palaeozoic and pre-Sinian granite-gneiss respectivlly. 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 Tertiary organic hydrocarbons and inorganic CO₂formed by the decomposition of carbonate rocks which resulted from magmatic activities.