Intramolecular isotope distributions, including isotope clumping and position specific fractionation, can provide proxies for the formation temperature and formation and destruction pathways of molecules. In this study, we explore the position-specific hydrogen isotope distribution in propane. We analyzed propane samples from 10 different petroleum systems with high-resolution molecular mass spectrometry. Our results show that the hydrogen isotope fractionation between central and terminal positions of natural propanes ranges from −102‰ to +205‰, a much larger range than that expected for thermodynamic equilibrium at their source and reservoir temperatures (36–63‰). Based on these findings, we propose that the hydrogen isotope structure of catagenic propane is largely controlled by irreversible processes, expressing kinetic isotope effects (KIEs). Kinetic control on hydrogen isotope composition of the products of thermal cracking is supported by a hydrous pyrolysis experiment using the Woodford Shale as substrate, in which we observed isotopic disequilibrium in the early stage of pyrolysis. We make a more general prediction of KIE signatures associated with kerogen cracking by simulating this chemistry in a kinetic Monte Carlo model for different types of kerogens. In contrast, unconventional shale fluids or hot conventional reservoirs contain propane with an isotopic structure close to equilibrium, presumably reflecting internal and/or heterogeneous exchange during high temperature storage (ca. 100–150 °C). In relatively cold (<100 °C) conventional gas accumulations, propane can discharge from its source to a colder reservoir, rapidly enough to preserve disequilibrium signatures even if the source rock thermal maturity is high. These findings imply that long times at elevated temperatures are required to equilibrate the hydrogen isotopic structure of propane in natural gas host rocks and reservoirs. We further defined the kinetics of propane equilibration through hydrogen isotope exchange experiments under hydrous conditions; these experiments show that hydrogen in propane is exchangeable over laboratory timescales when exposed to clay minerals such as kaolinite. This implies rather rapid transfer of propane from sources to cold reservoirs in some of the conventional petroleum systems. Propane is also susceptible to microbial degradation in both oxic and anoxic environments. Biodegradation of propane in the Hadrian and Diana Hoover oil fields (Gulf of Mexico) results in strong increases in central-terminal hydrogen isotope fractionation. This reflects preferential attack on the central position, consistent with previous studies.

中文翻译：

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天然丙烷中氢同位素的特定位置分布：热裂解、平衡和生物降解的影响

分子内同位素分布，包括同位素聚集和特定位置的分馏，可以为形成温度以及分子的形成和破坏途径提供代理。在这项研究中，我们探索了丙烷中特定位置的氢同位素分布。我们使用高分辨率分子质谱法分析了来自 10 个不同石油系统的丙烷样品。我们的结果表明，天然丙烷的中心和末端位置之间的氢同位素分馏范围从 -102‰ 到 +205‰，比其源和储层温度（36-63‰）的热力学平衡预期范围大得多。基于这些发现，我们提出，致畸丙烷的氢同位素结构在很大程度上受不可逆过程控制，表现出动力学同位素效应 (KIE)。以伍德福德页岩为底物的含水热解实验支持对热裂解产物氢同位素组成的动力学控制，在该实验中，我们在热解早期观察到同位素不平衡。我们通过在不同类型干酪根的动力学蒙特卡罗模型中模拟这种化学反应，对与干酪根裂解相关的 KIE 特征进行更一般的预测。相比之下，非常规页岩流体或热常规储层含有同位素结构接近平衡的丙烷，可能反映了高温储存（约 100–150 °C）期间的内部和/或非均相交换。在相对寒冷 (<100 °C) 的常规气藏中，丙烷可以从其源头排放到较冷的储层，即使烃源岩热成熟度很高，也足够快以保持不平衡特征。这些发现意味着需要长时间在高温下平衡天然气主岩和储层中丙烷的氢同位素结构。我们通过含水条件下的氢同位素交换实验进一步定义了丙烷平衡的动力学；这些实验表明，当暴露于高岭石等粘土矿物时，丙烷中的氢可以在实验室时间尺度内进行交换。这意味着丙烷从源头到一些常规石油系统中的冷库相当快速地转移。丙烷在好氧和缺氧环境中也容易被微生物降解。Hadrian 和 Diana Hoover 油田（墨西哥湾）中丙烷的生物降解导致中央末端氢同位素分馏的强烈增加。这反映了对中心位置的优先攻击，与之前的研究一致。