Abstract The Gulf of Mexico (GOM) produces 5% of total U.S. dry gas production ( USEIA, 2016 ). Despite this, the proportion of microbial and thermogenic methane in discovered and producing fields from this area is still not well understood. Understanding the relative contributions of these sources in subsurface environments is important to understanding how and where economically substantial amounts of methane form. In addition, this information will help identify sources of environmental emissions of hydrocarbons to the atmosphere. We apply stable isotopes including methane clumped-isotope measurements to solution and associated gases from several producing fields in the U.S. Gulf of Mexico to estimate the proportions, properties and origins of microbial and thermogenic endmembers. Clumped isotopes of methane are unique indicators of whether methane is at thermodynamic isotopic equilibrium or affected by kinetic processes. The clumped methane thermometer can provide insights into formation temperatures and/or into kinetic processes such as microbial methanogenesis, early catagenetic processes, mixing, combinatorial processes, and diffusion. In this data set, we find that some fluids have clumped isotope methane apparent temperatures consistent with the methane component being produced solely by the thermogenic breakdown of larger organic molecules at substantially greater temperatures than those reached in shallow reservoirs. A portion of these reservoirs with hot clumped isotope methane temperatures are consistent with exhibiting a kinetic isotope effect. Other reservoirs have clumped isotope methane apparent temperatures, and other isotopic and molecular proportions, consistent with mixtures of microbial and thermogenic methane. We show that in certain cases the evidence is most consistent with formation of the microbial methane in the current reservoir. However, in other cases the methane is produced at significantly shallower depths and is then transported to greater depths as a result of post generation burial of methane bearing sedimentary sequences to the current reservoir conditions. For the first time, we show that methane of an unambiguously purely microbial origin (i.e. those that do not contain obvious contributions of thermogenic methane) is dominantly generated at temperatures less than 60 °C, despite burial to greater depths. This finding suggests that, while microorganisms are able to generate methane at temperatures up to 105 °C under laboratory conditions (Brock, 1985), in the Gulf of Mexico, microbial methane is dominantly produced in the 20–60 °C window.

中文翻译：

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识别墨西哥湾深水水库中的产热和微生物甲烷

摘要 墨西哥湾 (GOM) 生产美国干气总产量的 5% (USEIA, 2016)。尽管如此，该地区已发现和生产领域中微生物和产热甲烷的比例仍不清楚。了解这些来源在地下环境中的相对贡献对于了解经济上大量甲烷的形成方式和位置非常重要。此外，这些信息将有助于确定碳氢化合物排放到大气中的环境来源。我们对来自美国墨西哥湾几个产油田的溶液和伴生气体应用包括甲烷团块同位素测量在内的稳定同位素，以估计微生物和产热端元的比例、特性和起源。甲烷的聚集同位素是甲烷是否处于热力学同位素平衡或受动力学过程影响的独特指标。团块甲烷温度计可以深入了解地层温度和/或动力学过程，例如微生物产甲烷、早期后生过程、混合、组合过程和扩散。在该数据集中，我们发现一些流体具有聚集的同位素甲烷表观温度，与仅由较大有机分子在比浅层储层中达到的温度高得多的温度下的热分解产生的甲烷成分一致。这些具有热成块同位素甲烷温度的储层的一部分与表现出动力学同位素效应一致。其他储层聚集了同位素甲烷表观温度，和其他同位素和分子比例，与微生物和产热甲烷的混合物一致。我们表明，在某些情况下，证据与当前储层中微生物甲烷的形成最为一致。然而，在其他情况下，甲烷在明显更浅的深度产生，然后由于含甲烷沉积序列的后期埋藏到当前储层条件而被输送到更深的深度。我们第一次表明，尽管埋藏更深，但明确纯微生物来源的甲烷（即那些不包含明显产热甲烷贡献的甲烷）主要在低于 60 °C 的温度下产生。这一发现表明，