Investigating the mechanisms that determine the bulk and position-specific carbon isotopic distributions of propane in natural gas will help elucidate its formation and evolution. We used octadecane and squalane as model compounds that give simple precursors for gas production in gold-tube isothermal pyrolysis experiments to study the variations in intermolecular and intramolecular distributions of isotopes in the generated gaseous hydrocarbons. The δC values of the products and inverses of their carbon numbers (1/ where = C–C) showed a negative linear relationship versus maturity, indicating that they formed by homolytic cleavage of C–C bonds and that the precursors showed homogeneous distributions of carbon isotopes. However, the significant difference in the kinetic isotopic effects (KIEs) of the gas generated by cracking of two model compounds under the same experimental conditions is not readily explained by single homolytic bond cleavage. The results indicate that besides the KIE of C–C bond cleavage, the bulk and position-specific carbon isotopic compositions of propane are related to the chemical and isotopic structures of the precursors and the propane transformation ratio. Based on the sites of C–C bond cleavage, two isotopic fractionation patterns (i.e., normal propyl and isopropyl models) may explain the bulk and position-specific carbon isotopic distributions of the generated propane. According to the propyl model, propane originates from (normal) propyl structures (CHCHCH*) in the precursor via C–C bond cleavage at a terminal site of a propyl group, while the isopropyl model involves propane derived from isopropyl structures (CHCH*CH) in the precursor, with C–C bond cleavage at the central site. Simulations show that these distributions for propane cracked from octadecane closely follow the propyl model, whereas propane generated from squalane showed mixed contributions from both models, and its bulk and position-specific carbon isotopic distributions depend on the proportions of propyl and isopropyl structures in the precursors. Variations of propane’s position-specific carbon isotopic distributions during the main stage of propane generation indicate that free radical reactions are the main pathway for thermogenic propane formation, resulting in similar KIEs for propane terminal and central carbons. Therefore, the position-specific carbon isotopic distribution of propane can provide evidence of its formation mechanism and shows potential for revealing the origin and evolution of natural gas.

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模型化合物的热解实验探索天然气中丙烷的碳同位素分馏

研究决定天然气中丙烷的体积和特定位置碳同位素分布的机制将有助于阐明其形成和演化。我们使用十八烷和角鲨烷作为模型化合物，为金管等温热解实验中的气体生产提供简单的前体，以研究生成的气态碳氢化合物中同位素的分子间和分子内分布的变化。产物的δ13C值及其碳数倒数（1/其中= C-C）与成熟度呈负线性关系，表明它们是由C-C键均裂形成的，并且前体表现出均匀的碳分布同位素。然而，在相同实验条件下两种模型化合物裂解产生的气体的动力学同位素效应（KIE）的显着差异很难用单均裂键断裂来解释。结果表明，除了C-C键断裂的KIE之外，丙烷的本体和特定位置碳同位素组成还与前体的化学和同位素结构以及丙烷转化率有关。基于 C-C 键断裂的位点，两种同位素分馏模式（即正丙基和异丙基模型）可以解释所生成的丙烷的本体和特定位置碳同位素分布。根据丙基模型，丙烷源自前体中的（正常）丙基结构（CHCHCH*），通过丙基末端的 C-C 键断裂，而异丙基模型则涉及源自异丙基结构（CHCH*CH）的丙烷。 ）在前体中，C-C键在中心位点断裂。模拟表明，从十八烷裂解得到的丙烷的这些分布紧密遵循丙基模型，而从角鲨烷生成的丙烷显示出两种模型的混合贡献，并且其体积和特定位置碳同位素分布取决于前体中丙基和异丙基结构的比例。丙烷生成主要阶段丙烷特定位置碳同位素分布的变化表明自由基反应是产热丙烷形成的主要途径，导致丙烷末端和中心碳具有相似的KIE。因此，丙烷特定位置的碳同位素分布可以为其形成机制提供证据，并显示出揭示天然气起源和演化的潜力。