甲烷及其同系物δ13C值反序排列特征的数值模拟与非生物成因天然气藏探讨

本文采用数值模拟的方法对天然气中甲烷及其同系物的δ13C排序特征进行了研究,结果表明:两种具正序分布特征的生物成因天然气混合后可产生具反序分布特征的天然气.相应地,两种具反序分布特征的非生物成因天然气混合后也可产生具正序分布特征的天然气.对于前者而言,作为混合的两个端元天然气,它们必须具有不同的成因或来源,或它们是明显不同的演化阶段的产物.关于松辽盆地昌德气藏若干口天然气井甲、乙、丙、丁烷碳同位素排序特征的讨论表明,用两种生物成因天然气混合的观点很难解释这种反序排列.

The Possibility and Explanation of Complete Reverse Isotopi c Distribution of Light Hydrocarbons in Natural Gas

Carbon isotope distribution among the C₁ to C₄ hydrocarbon is one of the most important criteria to identify the genesis of natural gas.Generally,bio genic natural gases possess a “normal distribution ”in carbon isotopes of C 1 to C₄ hydrocarbons and abiogenic natual gases a “reversal distribution”in carbon isotopes of C₁ to C₄ hydrocarbons.Some scholars consider that “isot opic reversals”can result from the mixture of two kinds of biogenic natural gas with “normal distribution” in carbon isotope.However,prevenient numerical sim ulation did not confim the viewpoint.By means of numerical simulation,the possib ility of “isotopic reversals” in C₁ to C₄ hydrocarbons resulted from the mixture of two kinds of biogenic natural gas was discussed in this paper. Computation in this article shows that a complete reverse carbon isotopic distri bution among C₁ to C₄ hydrocarbons (i.e. δ13C methane >δ13 C propane >δ13C n-butane) can be produced by the mixture of two kinds of biogenic gas with a normal carbon isotopic distribution (i.e. δ13C m ethane <δ13C ethane <δ13 propane <δ13C n-butane). While a normal carbon isotopic distribution in natural gas can also be produced by two e ndmembers with reverse distributions. As for the first case, the two endmembers should be of different source, different formation, or of different evolutional stage. It is unreasonable to explain the isotopic reverse distribution of the gas sampl es from several wells of the Changde pool, Songliao Basin by the mixture model, because the isotopically higher endmember needed to form this real gas is not fo und in Songliao Basin. So a better explanation to the isotopic reversal in this gas pool is to consider it to be abiogenic.