Coupled methane clumped isotope and propane position-specific isotope analyses indicate significant methane contribution from biodegradation of hydrocarbons in terrestrial methane seeps located at central Japan

Terrestrial hydrocarbon seeps are widely distributed in oil/gas field. To constrain the sources and post-generation processes occurring in these seeping gases, various geochemical approaches, such as chemical and stable isotope composition (δ¹³C and δD) of hydrocarbons have been extensively used. However, the interpretation can be ambiguous due to the overlap of signatures when using these approaches only¹. Some recently developed analytical techniques, such as methane clumped isotope analysis (Δ¹³CH₃D and Δ¹²CH₂D₂)^2-4 and propane position-specific isotope analysis (PSIA)⁵ may provide new clues to improve our understanding of the origin and fate of hydrocarbons.

In this study, we focus on gas samples from different gas seeps and mud volcanos in central Japan collected from 2019 to 2022, where hydrocarbons were considered mainly originating from thermal cracking of organic matter⁶. Gas compositions, bulk stable isotopes of hydrocarbons and associated CO₂, clumped isotopes of methane, PSIA of propane and other geochemical parameters have been studied. Coupled methane clumped isotope signatures and propane PSIA information provide direct evidence of secondary microbial methane formation associated with biodegradation of non-methane hydrocarbons. The contribution of secondary microbial methane in different seeps/mud volcanos and its temporal changes are also discussed by a mixing model integrating all these isotope information, which provides valuable constraints on methane sources in terrestrial seeps.

References: [1] Milkov and Etiope, 2018, Org. Geochem.; [2] Stolper et al., 2014, Geochim. Cosmochim. Acta.; [3] Stolper et al., 2014, Science; [4] Young et al., 2017, Geochim. Cosmochim. Acta.; [5] Gilbert et al., 2019, Proc. Natl. Acad. Sci. U.S.A.; [6] Etiope et al., 2011, Appl. Geochem.