- 1Institute for Marine and Atmospheric research (IMAU), Utrecht University, Netherlands (a.r.gilbert@uu.nl)
- 2Earth and Planetary Sciences Department, Institute of Science Tokyo, Japan
- 3JAMSTEC, Yokosuka, Japan
- 4Swiss Federal Laboratories for Materials Science and Technology (Empa), Dübendorf, Switzerland
Microbial methane production represents an important source of methane on Earth. In oil and gas reservoirs, microbial methane can be formed from secondary methanogenesis, i.e., from C2+ hydrocarbons biodegradation, either directly [1] or indirectly from the biodegradation products [2]. Despite its global significance [2], secondary methanogenesis is arguably challenging to detect, mainly because methane isotopic signature overlaps with that of the existing thermogenic methane in the reservoir, and is thus inferred only from indirect proxies such as high 13C content of propane and CO2.
Here, we combine methane clumped isotopes with propane position-specific isotope analysis (PSIA) of 19 samples from mud volcanoes and gas seepages located in Tokamachi area (Niigata, Japan). Previous studies have shown that both propane and CO2 in Tokamachi natural gas samples are 13C-enriched, consistent with biodegradation-associated methanogenesis [3]. Propane 13C-PSIA shows a clear biodegradation trend where δ13C of the central position of propane is specifically enriched as the relative amount of propane decreases [4]. Interestingly, the extent of propane biodegradation, as indicated by the difference between the two positions of propane (∆Central = δCentral - δTerminal), correlates with ∆13CH3D and ∆CH2D2 of methane, both of which tending towards equilibrium values at high biodegradation rates. A simple models shows that ca. 20% of methane present in the subusrface is produced directly or indirectly from hydrocarbons anaerobic biodegradation. This study emphasizes the importance of using multiple indicators to tackle hydrocarbons cycling in the subsurface, in particular methanogenesis associated with hydrocarbons biodegradation.
References:
[1] Zhou et al. 2022 Nature v. 601, 257
[2] Milkov 2011 Org. Geochem. v. 42, 184
[3] Etiope et al. 2011 Appl. Geochem., v. 26, 348
[4] Gilbert et al. 2019 Proc. Natl. Acad. Sci., v. 116, 6653
How to cite: Gilbert, A., Jajalla, M., Nakagawa, M., Taguchi, K., and Zhang, N.: Quantification of secondary methanogenesis from multiple isotopologue proxies: a case study in Tokamachi mud volcano, Japan, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17015, https://doi.org/10.5194/egusphere-egu25-17015, 2025.
