Isotopomer approaches to the detection of anaerobic oxidation of natural gas hydrocarbons

<p>Hydrocarbons are the main constituents of natural gas. Their chemical and isotope abundance is a window to biogeochemical processes occurring in the subsurface. Stable isotopes of natural gas hydrocarbons are traditionally measured through compound-specific isotope analysis (CSIA) where each hydrocarbon is separated before its isotope ratio is determined.</p><p>Recently a variety of methods have been developed to determine position-specific isotope composition of propane, the first hydrocarbon with two distinct isotopomers: central and terminal [1][2][3][4]. The relative abundance of propane isotopomers (e.g. &#916;¹³C_central = &#948;¹³C_central - &#948;¹³C_terminal) is a promising tool for tracing sources and sinks of hydrocarbons in natural gas reservoirs. In particular, anaerobic oxidation of propane starts with a fumarate addition at the central position, which is expected to lead to a specific enrichment of the central ¹³C-isotopomer of the remaining propane.</p><p>We measured &#916;¹³C_central values of propane throughout the course of its oxidation by bacteria BuS5 [5] and showed that the isotope fractionation is located mainly on the central position, which differs from the signature expected for thermogenic evolution [6]. The approach has been used to detect anaerobic oxidation of propane in several natural gas reservoirs: Southwest Ontario (Canada), Carnarvon Basin (Australia), Michigan (USA) [6], and more recently Tokamachi mud volcano in Japan [7]. In addition, isotopomers of n-butane and i-butane analysed using the same technique allows gaining insights into the mechanism of their microbial oxidation.</p><p>The isotopomer approach presented here can thus shed light on the fate of natural gas hydrocarbons. In combination with clumped isotope measurements of methane and ethane, the approach can provide unprecedented information regarding carbon cycling in the subsurface.</p><p>&#160;</p><p>[1] Gilbert et al., <strong>2016</strong> GCA v177, p205</p><p>[2] Piasecki et al., <strong>2016 </strong>GCA v188 p58</p><p>[3] Gao et al., <strong>2016</strong> Chem Geol. v435, p1</p><p>[4] Liu et al., <strong>2018</strong> Chem Geol. v491, p14</p><p>[5] Kniemeyer et al., <strong>2007</strong> Nature v449, p898</p><p>[6] Gilbert et al., <strong>2019</strong> PNAS v116, p6653</p><p>[7] Etiope et al., <strong>2011</strong> Appl. Geochem. v26, p348</p>