海相干酪根与原油裂解气甲烷生成及碳同位素分馏的差异研究

利用封闭金管高压釜体系对海相原油和成熟干酪根进行了热解生气实验,获取了两类裂解气的组分和甲烷碳同位素数据,对比研究了两类母质在生气机理上的差异,并借助碳同位素分馏动力学参数讨论了甲烷碳同位素分馏的异同点.结果表明,原油裂解气富含C2-5重烃,其后期裂解是甲烷的重要来源;而干酪根裂解气中C2-5的含量较低,其后期裂解对干酪根甲烷气的贡献较小.这是两类甲烷气体生成的最大差异之一.两类裂解气甲烷碳同位素都有随着热解温度增高,碳同位素值先变轻再变重的特点,但原油裂解气甲烷碳同位素的最小值对应的温度较高;在相同热解温度下,干酪根裂解气甲烷碳同位素值要重于原油裂解气甲烷碳同位素值,这与后者前系物经过多次碳同位素分馏有关.因此,生气机理的差异是造成同位素分馏差异的根本原因,两类甲烷气体碳同位素分馏动力学参数的差异也是有成因意义的.

Comparison of gas generation and carbon isotope fractionation of methane from marine kerogen- and crude oil-cracking gases

Typical kerogen and crude oil samples were pyrolyzed using sealed gold tube system to compare their differences in methane generation and carbon isotope fractionation. The results show that the kerogen cracking methane mainly originates from direct cracking of kerogen and the cracking of C_(2-5) components contributes little to the methane generation, whereas most part of oil cracking methane is derived from cracking of C_(2-5) components as formed during early cracking of crude oil. The carbon isotope fractionation of methane both from kerogen-and crude oil-cracking gases displays a similar evolution trend, i.e. decreasing in early stages and increasing in later stages. But the carbon isotopes of methane from oil cracking gases are much lighter than those from kerogen cracking gases at similar pyrolysis temperatures, which is probably caused by the more carbon isotope fractionation of methane precursors of oil cracking methane than that of kerogen methane precursors. Their kinetic parameters of methane carbon isotope fractionation are obtained and discussed for their genetic significances. The result indicates that the difference in methane precursors leads to their distinct carbon isotope fractionation.