Abstract We have reassessed the stable carbon isotope systematics of methane (C1), ethane (C2), and propane (C3) in more than 500 natural gas samples from low-permeability hydrocarbon reservoirs around the world, with the purpose of providing new tools for thermal maturity assessment of natural gas that is increasingly produced from such reservoir types world-wide. A low-permeability reservoir resembles a semi-closed system and we found that the stable carbon isotope distribution in C1-C3 alkanes differs partially from what has been previously observed in natural gas from conventional hydrocarbon accumulations (e.g. C isotope distributions dominated by Rayleigh distillation and kinetic isotope effects). In a low-permeability reservoir, isotope exchange may play a more prominant role in the carbon isotope distribution, driving the system towards an even isotopic distribution (EID) of 6‰ between methane and ethane, and ethane and propane, at Ro of 1.5%. At higher maturity, ethane and propane depleted in 13C are formed as a consequence of thermal cracking of wet-gas components and possibly free radical decomposition/polymerization reactions, which leads to the occurrence of isotope reversals (δ13CC2

中文翻译：

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低渗透油气藏甲烷、乙烷和丙烷的稳定碳同位素系统学

摘要 我们重新评估了全球 500 多个低渗透油气藏天然气样品中甲烷 (C1)、乙烷 (C2) 和丙烷 (C3) 的稳定碳同位素系统学，旨在为世界范围内越来越多地从此类储层类型中生产的天然气的热成熟度评估。低渗透储层类似于一个半封闭系统，我们发现 C1-C3 烷烃中的稳定碳同位素分布与之前在常规烃类聚集的天然气中观察到的部分不同（例如，由瑞利蒸馏和动力学同位素效应）。在低渗透性储层中，同位素交换可能在碳同位素分布中发挥更突出的作用，推动系统在甲烷和乙烷以及乙烷和丙烷之间实现 6‰ 的均匀同位素分布 (EID)，Ro 为 1.5%。在更高的成熟度下，由于湿气组分的热裂解和可能的自由基分解/聚合反应，会形成 13C 耗尽的乙烷和丙烷，这导致同位素逆转（δ13CC2