有机质碳同位素热力学分馏与天然气碳同位素组成

Carbon Isotopic Thermodynamic Fractionation of Organic Matter and Carbon Isotopic Composition of Natural Gases

干酪根中不同结构和官能团具有不同的碳同位素组成,这种差异可以用有机质碳同位素热力学同位素因子(β因子)进行预测。煤岩模拟实验产物中,δ13CCO2相对烃类气体而言明显偏重,这与干酪根中羧基的β13C较大有关。含水实验产物的δ13CCO2轻于无水实验产物的δ13CCO2是由于含水实验中所增加CO2的碳同位素组成相对较轻造成的,含水实验增加的CO2产率是由β13C相对较小的部分亚甲基碳通过断裂、氢转移、以及与水反应转变而来。模拟实验低温阶段(≤300℃),甲氧基中的甲基断裂可能的甲烷形成的主要方式。而甲氧基的β13C大于甲基,所以低温阶段出现了甲烷碳同位素组成先较重后变轻的现象。

Different structures and functional groups in organic matter have different carbon isotopic compositions.This kind of difference can be predicted by carbon isotopic thermodynamic factor（β-factor） of organic matter.In the simulation experiments,the carbon isotopic composition of CO2 is relatively heavy,which is related to that the thermodynamic carbon isotopic factor（β^13C） of carboxyl is relatively large in kerogen.In hydrous experiments,part of methylene finally changes into CO2 by way of cleavage,hydrogen transfer and reaction with water.Because the β^13C of methylene is relatively small,correspondingly the carbon isotopic composition of the increased CO2 is relatively light in hydrous experiments,so the carbon isotopic composition of CO2 in the two series of hydrous experiments is slightly lighter than that in anhydrous experiments.This phenomenon proves that water medium is involved in the chemical reaction of hydrocarbon generation.In the three series of experiments,the carbon isotopic composition of methane is relatively heavy in the low temperature phase（≤300℃）,which may be related to that the carbon isotopic composition of methane precursor is heavier in this early-mature stage,such as methoxy group.The thermodynamic carbon isotopic factor of methoxy is relatively heavy,which causes that the isotopic composition of methane is relatively heavy in the low evolution stage.While the theory of isotope kinetic fractionation is developing at a high speed at present,we still can not neglect the isotope thermodynamic fractionation,which will be a great help to us in the further study on the mechanism of hydrocarbon generation.