天然气形成过程中碳同位素分馏机理——来自热模拟实验的地球化学证据

Mechanisms of carbon isotope fractionation in the process of natural gas generation: Geochemical evidence from thermal simulation experiment

选取鄂尔多斯盆地低成熟煤样品开展封闭体系黄金管热模拟实验,分析产物特征,从实验角度探讨天然气碳同位素分馏机理,分析天然气碳同位素组成地球化学"异常"特征。鄂尔多斯盆地低成熟煤2℃/h(慢速)和20℃/h(快速)升温的烷烃气最大产率分别为302.74 mL/g和230.16 mL/g;低成熟煤快速升温和慢速升温的δ13C1值分别为-34.8‰^-23.6‰和-35.5‰^-24.0‰,δ13C2值分别为-28.0‰^-9.0‰和-28.9‰^-8.3‰,δ13C3值分别为-25.8‰^-14.7‰和-26.4‰^-13.2‰。实验产物中烷烃气在快速升温550℃出现了明显的碳同位素组成系列部分倒转,其他温度都表现为正碳同位素组成系列。升温系列中δ13C1表现为先变轻后变重的演化规律,δ13C1值的非单调性变化是由于早期CH4来源并非单一所致,可能是有机质的非均质性或者早期富集12CH4和富集13CH4活化能差值的变化形成的同位素分馏效应所致。重烃气碳同位素值的反转既可以发生在高过成熟的页岩气(油型气)中,也可以发生在煤成气中。结合甲苯热模拟实验,明确烷烃气在高过成熟阶段重烃气碳同位素值可发生反转和倒转现象。芳香烃脱甲基以及甲基链接所产生的同位素分馏效应可能是高过成熟烷烃气碳同位素组成反转、倒转的重要原因。

Low maturity coal samples were taken from the Ordos Basin to conduct gold tube thermal simulation experiment in a closed system, and the characteristics of the products were analyzed to find out the fractionation mechanism of carbon isotopes and the causes of abnormal carbon isotopic compositions of natural gas. At the heating rates of 2 ℃/h(slow) and 20 ℃/h(fast), the low maturity coal samples of the Ordos Basin had the maximum yields of alkane gas of 302.74 mL/g and 230.16 mL/g, the δ13 C1 ranges of-34.8‰ to-23.6‰ and-35.5‰ to-24.0‰;δ13 C2 ranges of-28.0‰ to-9.0‰ and-28.9‰ to-8.3‰;and δ13 C3 ranges of-25.8‰ to-14.7‰ and-26.4‰ to-13.2‰, respectively. Alkane gas in the thermal simulation products of fast temperature rise process showed obvious partial reversal of carbon isotope series at 550°C, and at other temperatures showed positive carbon isotope series. In the two heating processes, the δ13 C1 turned lighter first and then heavier, and the non-monotonic variation of the δ13 C1 values is because the early CH4 is from different parent materials resulted from heterogeneity of organic matter or the carbon isotope fractionation formed by activation energy difference of early enriched 12 CH4 and late enriched 13 CH4. The reversal of carbon isotope values of heavy hydrocarbon gas can occur not only in high to over mature shale gas(oil-type gas), but also in coal-derived gas. Through thermal simulation experiment of toluene, it is confirmed that the carbon isotope value of heavy hydrocarbon gas can be reversed and inversed at high to over mature stage. The isotope fractionation effect caused by demethylation and methyl linkage of aromatic hydrocarbons may be an important reason for carbon isotope inversion and reversal of alkane gas at the high to over mature stage.