Clumped isotope analysis of methane revolutionizes petroleum geochemistry

Abstract:

The isotopic characterization of geological gas deposits is crucial for the exploration of resources for global energy production. Methane (CH₄) is a primary constituent of natural gas reservoirs and in addition to its commercial importance it also plays a major role in the global carbon cycle: It is involved in various fluxes within atmospheric, microbial, hydrothermal and magmatic systems, and it is a potential factor in the anthropogenic acceleration of climate change. The importance and versatility of methane fuels the development of methods to decipher its origins, sources and sinks, formation conditions, and transport paths.

The commonly measured molecule average (or ‘bulk’) isotope compositions of methane, δ¹³C and δD, are determined by conventional low-resolution IRMS. They are valuable tools to aid identification of methane’s origins but are often ambiguous with respect to provenance and formation conditions. Also, they do not record gas formation temperatures.

The doubly-substituted (‘clumped isotope’) signatures, Δ¹³CH₃D and Δ¹²CH₂D₂, add additional independent compositional dimensions. They refine the forensic identification of methane sources, they aid in recognition of methane’s formation mechanisms, and they can be utilized as a natural geothermometer in cases where the gas formed in internal thermodynamic equilibrium.

The interference-free analysis of ¹³CH₃D and ¹²CH₂D₂ is, however, impossible with conventional low-resolution IRMS, because both isotopologues are afflicted with multiple isobaric interferences arising from inevitable ionization adducts and isotopologues. HR-IRMS overcomes this limitation by routinely achieving mass resolving powers sufficient to fully resolve ¹³CH₃D and ¹²CH₂D₂ from their interfering adducts (see figure).

We demonstrate that both bulk and clumped isotope signatures can be directly determined with high precision using the Thermo Scientific™ Ultra™ HR-IRMS. We illustrate the scientific usability of combined bulk and clumped measurements of methane and highlight the gain of information in comparison to utilizing classical stable isotope signatures alone. We furthermore explore how HR-IRMS analyses of non-traditional clumped isotopes in other natural gas components (e.g. H₂ and N₂) can extend the petroleum geochemists toolbox.