次生型负碳同位素系列成因

Origins of secondary negative carbon isotopic series in natural gas

烷烃气碳同位素系列类型有3种：随烷烃气分子碳数递增，δ^13C值依次递增称为正碳同位素系列，是有机成因原生烷烃气的一个特征；随烷烃气分子碳数递增，δ^13C值依次递减称为负碳同位素系列；不按以上2种规律而出现不规则增减则称为碳同位素倒转。负碳同位素系列又分为原生型和次生型2种。原生型负碳同位素系列是无机成因的；次生型负碳同位素系列是正碳同位素系列经次生改造来的，出现在过成熟的页岩气和煤成气中。关于次生型负碳同位素系列成因的观点繁多，包括：二次裂解、扩散、过渡金属和水介质在250-300℃范围内发生氧化还原作用导致乙烷和丙烷瑞利分馏等。详细研究对比后发现不论页岩气或者煤成气，次生型负碳同位素系列仅出现在过成熟页岩或源岩区，在成熟和高成熟页岩或者源岩区未见次生型负碳同位素系列，由此得出过成熟或者高温（＞200℃）是次生型负碳同位素系列的主要控制因素，在此主控因素下可由二次裂解、扩散或者乙烷和丙烷瑞利分馏的一种或几种方式促使次生型负碳同位素系列的形成。

The carbon isotopic series of alkane gases could be divided into three types：（1）positive carbon isotopic series：δ^13C values increase with increasing carbon numbers among the C1-C4 alkanes,which is a typical characteristic for primary alkane gases;（2）negative carbon isotopic series：δ^13C values decrease with increasing carbon numbers among the C1-C4 alkanes;（3）partial carbon isotopic reversal,which has no increasing or decreasing relationship between the δ^13C values and carbon numbers.Negative carbon isotopic series could be further subdivided into primary and secondary origins.The former is a typical characteristic of abiogenic gases while the later is resulted from the secondary alteration imposed on biogenic gases,which is commonly observed in over-mature shale gas or coal-derived gas.Previous works have proposed several possible explanations for negative carbon isotopic series of secondary origins such as secondary cracking,diffusion,the Rayleigh fractionation of ethane and propane through redox reaction with the participation of transition metal and water at 250-300℃.After a comparative study,the authors found that negative carbon isotopic series of secondary origin for both shale gas and coal-derived gas appeared in the area where source rocks（shales）are at over-mature stage while it was not observed in the area where source rocks（shales）are only at mature-high mature stage.As a result,extremely high maturity（〉200℃）is the main controlling factor for negative carbon isotopic series of secondary origin.Within this maturity interval,secondary cracking,diffusion and Rayleigh fractionation of ethane and propane could happen separately or together.