油型气乙烷碳同位素演化规律与成因

通过分析四川盆地和北美Appalachian、Fort Worth等盆地页岩气和常规油型气乙烷碳同位素特征和成因，得出随着成熟度的增加，乙烷碳同位素经历3个演化阶段：初次生气阶段正常增加，与甲烷碳同位素具有线性相关；二次裂解阶段同位素值减少，与乙烷含量的倒数具有线性关系，符合二端元气混合模型；湿气裂解阶段碳同位素值增加，与乙烷含量的自然对数呈线性关系，符合瑞利分馏模型。不同阶段页岩气碳同位素倒转的成因不同，瑞利分馏阶段碳同位素分馏速率乙烷>丙烷，并且甲烷碳同位素发生次生变化是造成过成熟阶段页岩气碳同位素反序的原因。富¹²C的乙烷是四川盆地页岩气高产井的重要地球化学特征。对于成熟度R_o>2.2%的天然气，乙烷碳同位素用于气源鉴别需要用瑞利分馏的公式校正。

Evolution law and genesis of ethane carbon isotope of oil type gas

Based on analyzing the characteristics and genesis of ethane carbon isotope of shale gas and conventional oil type gas in Sichuan Basin, and the Appalachian Basin, Fort Worth Basin, and other basins in North America, it is believed that ethane carbon isotope will experiences 3 stages with increasing maturity. In the first gas generation stage, it increases normally with a linear correlation with methane carbon isotope; it decreases in the secondary cracking stage with a linear relation with the reciprocal of ethane content, which is in line with the two end-member gas mixed model; it increases in the wet gas cracking stage with a linear relation with the natural logarithm of ethane content, which is in accordance with the Rayleigh fractional model. The genesis for carbon isotope reversal of shale gas varies in different stages. The fractionation rate of ethane carbon isotope is greater than that of propane carbon isotope in the Rayleigh fractional stage, and the secondary change of methane carbon isotope is a reason for the carbon isotope reverse order of shale gas in the overmature stage. ¹²C-rich ethane is an important geochemical characteristic of high-productivity shale gas well in Sichuan Basin. For natural gas with R_o>2.2%, ethane carbon isotope can be used to identify gas source after the correction using Rayleigh fractional formula.