天然气中烷烃气碳同位素研究的意义

Significance of the study on carbon isotopes of alkane gases

天然气中烷烃气的碳同位素值蕴含丰富的科学信息,为研究其重要理论及实践意义,分析、总结了国内外学者对烷烃气中单组分(甲烷、乙烷)碳同位素值的研究成果。结果认为:依据δ13 C1—Ro回归方程能对勘探目的层天然气的类型或成熟度作出推断;煤成气的δ13 C2基本上重于-28.0‰,油型气的δ13 C2基本上轻于-28.5‰,而介于-28.0‰～-28.5‰之间是上述两类气的共存区,且以煤成气为主。此外,还重点讨论了烷烃气碳同位素系列所反映的油气地质和地球化学信息,认为具有正碳同位素系列的烷烃气属于有机成因气,负碳同位素系列的烷烃气基本上属于无机成因气;但在沉积盆地中个别出现的负碳同位素系列是由于正碳同位素系列次生改造(扩散分馏、相态转换分馏)所致,其烷烃气不是无机成因的。

Much information can be obtained from carbon isotopes of alkane gases in natural gas.In order to study their theoretical and practical significance,this paper analyzes and summarizes the available research results on carbon isotope values of single component like methane or ethane of alkane gases.The following conclusions are obtained.The δ13C1-Ro regression equation can be used to determine the type or maturity degree of natural gas in the targeted pay zone.The δ13C2 values of coal-derived gas are mostly greater than-28.0‰,while those of oil-type gas are commonly less than-28.5‰,and both types of gas might occur in the interval between-28.0‰ and-28.5‰ with predominant coal-derived gas.Additionally,this paper focuses on the hydrocarbon geological and geochemical information reflected from carbon isotopes of alkane gases.It is concluded that alkane gases with positive carbon isotope values are of organic origin,while those with negative carbon isotope values are of basically inorganic origin.However,some alkane gases with negative carbon isotope values may occur in sedimentary basins,which are the results of secondary alteration(diffusive fractionation,phase-transformation fractionation) of carbon isotopes with positive values,and thus are not of inorganic origin.