Tracing the origin of methane using clumped isotope geochemistry

Gazetas, Orestis (2025) Tracing the origin of methane using clumped isotope geochemistry. PhD thesis, University of Glasgow.

Abstract

Methane (CH4) is the main component of natural gas and the second most abundant greenhouse gas produced by natural and anthropogenic sources. Stable isotopic compositions of carbon and hydrogen (δ13C and δD) have improved our understanding on the production and sink pathways of methane in the atmosphere, subsurface, freshwater and marine environments. However, the overlapping of δ13C and δD values between diverse sources, produce ambiguous results difficult to interpret. Due to technological advancements, the clumped isotopic compositions Δ13CH3D and Δ12CH2D2 can be measured using HR-IRMS, providing additional insights into the formation temperature and origin of methane. This PhD thesis includes a description of the work carried out for testing, establishing, and validating methods for the purification of methane, the preparation of the equilibrated aliquots, and the measurement of bulk and clumped isotopologues at the Clumped Isotope Laboratory in SUERC. In this thesis, those methods were applied to understand the methane formation in a world-class helium deposit in Virginia gas field (Witwatersrand basin). The results suggest a dominant microbial source of the gas with smaller but ultimately significant contribution from abiotic sources. The data suggest negligible at best contributions from thermogenic sources. The large Δ13CH3D values (>7‰) and the trend identified were consistent with a scenario for anaerobic oxidation of methane by methanotrophs, with no more than ~1.6% CH4 consumption. Additionally, this PhD thesis includes work on calcite veins associated with a fracture network, in order to identify any potential relationship between calcites and methane in the Virginia gas field. The origin of carbon was assessed in the calcites through measuring the δ13C, and has been suggested to be a mixture of marine sources and carbon deriving from the oxidation of organic matter. The δ18O of the calcites and the precipitation temperatures of the calcites T47, indicate a mixture of two precipitating fluids; one related to hydrothermal sources and one consistent with a palaeometeoric origin. The T47 of the calcites veins indicate that the calcite precipitation has preceded the formation of the dominant microbial CH4, as the precipitation temperatures are in average greater than the limit of life at 122oC. Overall, no clear relationship between the calcites and methane is identified, however I discuss the different scenarios.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Colleges/Schools: College of Science and Engineering > Scottish Universities Environmental Research Centre
• Supervisor's Name: Clog, Dr. Matthieu, Stuart, Professor Finlay and Kohl, Dr. Issaku
• Date of Award: 2025
• Depositing User: Theses Team
• Unique ID: glathesis:2025-85591
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 14 Nov 2025 14:11
• Last Modified: 14 Nov 2025 14:12
• Thesis DOI: 10.5525/gla.thesis.85591
• URI: https://theses.gla.ac.uk/id/eprint/85591