Clumped isotope effects of thermogenic methane formation: Insights from pyrolysis of hydrocarbons
Clumped isotope effects of thermogenic methane formation: Insights from pyrolysis of hydrocarbons
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DOI：
10.1016/j.gca.2021.03.009
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发表时间：
2021-05-02
影响因子：
5
通讯作者：
Eiler, John
中科院分区：
文献类型：
--
作者：
Dong, Guannan;Xie, Hao;Eiler, John
甲烷聚集同位素分析是一种用于限制甲烷的形成或平衡温度，或区分甲烷的产热、微生物或“非生物”来源的工具。地热应用是基于温度依赖性的相对丰度的多取代同位素在热力学平衡，而分配的biogenicity或“abigenicity”依赖于动力学同位素效应与合成，这扰乱聚集的同位素丰度远离预期的平衡比例。然而，产热过程中或产热气体的生成后储存过程中的动力学过程可能导致同位素不平衡，混淆测温应用或导致微生物或非生物成因气体的“假阳性”鉴定。在非常规的石油伴生气和煤热解实验中观察到了非平衡的成团同位素组成。同位素不平衡可能是由气体迁移（包括提取）过程中表现出的动力学同位素效应或不可逆的化学过程（如烷基前体中碳-碳键的断裂）引起的。在这项研究中，我们进行了控制热解实验，在400摄氏度的正十八烷（C18 H38）。我们的特点是气体化学，和化合物特定的碳和氢同位素和甲烷聚集的同位素组成的气体产品。我们发现，Delta 13 CH 3D值（相对于同位素随机分布的异常）似乎在实验温度下相对接近平衡，而Delta 12 CH 2D 2值比平衡预期低30-40%。δ 12 CH 2D 2的巨大缺陷可以通过将受两种不同动力学同位素效应影响的氢原子组装到甲烷分子中来解释，以前称为组合效应。我们提出了一个动力学模型，描述了完整的同位素系统学，包括异常三角洲12 CH 2D 2赤字，热解产物甲烷。最后，我们提出了一个模型的同位素签名的天然热甲烷的非平衡三角洲12 CH 2D 2组成是一个签名的开始的后生甲烷生产。我们的模型还描述了这种特征消失的方式，因为进一步的成熟通过氢交换驱动Delta 12 CH 2D 2达到平衡。我们的研究结果表明，在三角洲12 CH 2D 2的异常耗尽是不是一个独特的签名微生物或假定的非生物甲烷，具体来说，它可以在热解化学过程中产生。(C)2021爱思唯尔有限公司保留所有权利。
Methane clumped isotope analysis is a tool used to constrain the formation or equilibration temperatures of methane, or to differentiate methane of thermogenic, microbial or 'abiotic' origins. Geothermometry applications are based on the temperature dependence of relative abundances of multiply-substituted isotopologues in thermodynamic equilibrium, whereas assignments of biogenicity or 'abiogenicity' rely on kinetic isotope effects associated with synthesis, which disturb clumped isotope abundances away from expected equilibrium proportions. However, kinetic processes in thermogenesis or during post-generation storage of thermogenic gas may cause isotopic disequilibrium, confounding thermometry applications or leading to 'false positive' identifications of microbial or abiogenic gases. Non-equilibrated clumped isotope compositions have been observed in thermogenic gases including unconventional oil-associated gases and from coal pyrolysis experiments. The isotopic disequilibria might be caused by kinetic isotope effects expressed during gas migration (including extraction), or by irreversible chemical processes, such as breaking carbon-carbon bonds in an alkyl precursor. In this study, we performed controlled pyrolysis experiments at 400 degrees C on n-octadecane (C18H38). We characterized the gas chemistry, and compound-specific carbon and hydrogen isotope and methane clumped isotope compositions of the gas products. We found that Delta 13CH3D values (anomalies relative to a stochastic distribution of isotopes) appear to be relatively close to equilibrium at the experimental temperature, whereas Delta 12CH2D2 values are 30-40% lower than expected for equilibrium. The large deficit in Delta 12CH2D2 can be explained by assembling hydrogen atoms affected by two distinct kinetic isotope effects into a methane molecule, previously referred to as a ,combinatorial effect,. We present a kinetic model that describes the full isotopic systematics, including anomalous Delta 12CH2D2 deficits, of pyrolysis product methane. Finally, we propose a model for the isotope signatures of natural thermogenic methane where the non-equilibrium Delta 12CH2D2 composition is a signature of the onset of catagenetic methane production. Our model also describes ways in which this signature disappears as further maturation drives Delta 12CH2D2 to equilibrium through hydrogen exchange. Our findings demonstrate that anomalous depletion in Delta 12CH2D2 is not a unique signature for microbial or putative abiotic methane, and specifically, it can be generated during pyrolytic chemistry. (C) 2021 Elsevier Ltd. All rights reserved.