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 °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 Δ¹³CH₃D values (anomalies relative to a stochastic distribution of isotopes) appear to be relatively close to equilibrium at the experimental temperature, whereas Δ¹²CH₂D₂ values are 30–40‰ lower than expected for equilibrium. The large deficit in Δ¹²CH₂D₂ 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 Δ¹²CH₂D₂ deficits, of pyrolysis product methane. Finally, we propose a model for the isotope signatures of natural thermogenic methane where the non-equilibrium Δ¹²CH₂D₂ 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 Δ¹²CH₂D₂ to equilibrium through hydrogen exchange. Our findings demonstrate that anomalous depletion in Δ¹²CH₂D₂ is not a unique signature for microbial or putative abiotic methane, and specifically, it can be generated during pyrolytic chemistry.

甲烷块同位素分析是一种用于限制甲烷形成或平衡温度，或区分热源，微生物或“非生物”来源的甲烷的工具。地热计量学的应用是基于热力学平衡中多重取代的同位素的相对丰度的温度依赖性，而生物成因或“非生物成因”的分配则依赖于与合成相关的动力学同位素效应，这会干扰成簇的同位素丰度而偏离预期的平衡比例。但是，在热生成过程中或热生成气体的后代存储过程中的动力学过程可能会导致同位素失衡，混淆测温应用或导致微生物或非生物生成气体的“假阳性”识别。在包括非常规石油伴生气体在内的热气中和煤热解实验中都观察到了非平衡的团簇同位素组成。同位素不平衡可能是由气体迁移（包括萃取）过程中表达的动力学同位素效应引起的，或者是由不可逆的化学过程引起的，例如破坏烷基前体中的碳-碳键。在这项研究中，我们在400°C的正十八烷（C₁₈ H ₃₈）。我们表征了气体的化学性质，以及气体产物的化合物特定的碳氢同位素和甲烷团簇同位素组成。我们发现，Δ ¹³ CH ₃（相对于同位素的随机分布异常）d值显得相对接近平衡在实验温度，而Δ ^12倍CH ₂ d ₂ 的值是30-40‰低于预期平衡。大赤字Δ ¹² CH ₂ d ₂ 可以通过将受两个不同的动力学同位素效应影响的氢原子组装到甲烷分子（以前称为“组合效应”）中来进行解释。我们提出描述完整同位素体系，包括反常Δ动力学模型¹² CH ₂ d ₂热解产物的甲烷的赤字。最后，我们提出一种用于天然生热甲烷的同位素特征的模型，其中所述非平衡Δ ¹² CH ₂ d ₂组合物是catagenetic甲烷产量的发作的签名。我们的模型也描述了其中该签名消失，进一步成熟驱动Δ方式¹² CH ₂ d ₂通过氢交换达到平衡。我们的研究结果表明，在Δ反常耗尽¹² CH ₂ d ₂ 是不适合的微生物或推定非生物甲烷的唯一签名，并且具体地，它可以热解化学期间生成。