Clumped Isotopes in Source-rock Methane as an Improved Geothermometer for Reconstructing the Thermal History of Sedimentary Sequences

Thermogenic methane is generated by the thermal breakdown of organic matter during the deep burial of sedimentary sequences. The two doubly substituted (or “clumped”) methane isotopologues, ¹³CH₃D and ¹²CH₂D_2, can be used as a geothermometer to reconstruct the gas’s formation temperatures; however, the exact mechanisms controlling methane clumped-isotopic compositions remain unclear⁽¹⁾.

Recent field-based clumped-isotope studies of thermogenic methane primarily relied on samples taken from natural gas reservoirs and oil fields^{(2, 3)}. In such settings, reservoir mixing and other kinetic isotope effects (e.g., from leakage or extraction) can distort primary equilibrium signals, thereby biasing the derived clumped-isotope temperatures⁽¹⁾.

Here, we present a new approach to methane clumped-isotope thermometry, which involves the direct recovery and analysis of methane from occluded porosity in source rocks. Because the gas analyzed formed and remained in situ, the clumped-isotope signal obtained is less likely to be altered by post-generation processes than that of reservoir gases, potentially enhancing the reliability of these geothermometers.

Source-rock methane is recovered and purified through acid digestion, amine-based CO₂ (and H₂S) removal, and cryogen-aided gas chromatographic separation prior to methane bulk and clumped isotope analysis by quantum-cascade laser absorption spectroscopy (QCLAS)⁽⁴⁾.

Preliminary methane clumped-isotope data were acquired from bituminous Triassic source rocks (limestones and dolomites) collected along a Southern Alps transect (northern Italy), for which independently-constrained burial temperatures range from less than 100°C to over 250°C⁽⁵⁾. The obtained ∆¹³CH₃D and ∆¹²CH₂D₂ values are consistent with methane formation under thermodynamic equilibrium conditions. However, the inferred formation temperatures are generally lower than expected peak burial temperatures at the respective locations along the transect. We benchmark these temperatures against carbonate clumped-isotope thermometry and established thermal-maturity proxies, including vitrinite/solid-bitumen reflectance and pyrolysis Rock-Eval T_max. Interpreting these constraints within the well-studied thermal history of the Southern Alps allows us to further evaluate the utility of clumped isotopes in source-rock methane as an improved geothermometer for reconstructing the thermal evolution of sedimentary basins.

 

References

(1) Stolper, D.A., Lawson, M., Formolo, M.J., et al. (2018). The utility of methane clumped isotopes to constrain the origins of methane in natural gas accumulations. In: Lawson, M., Formolo, M.J., & Eiler, J.M. (eds). From Source to Seep: Geochemical Applications in Hydrocarbon Systems. Geological Society, London, Special Publications, 468, 23–52.

(2) Stolper, D., Lawson, M., Davis, C., et al. (2014). Formation temperatures of thermogenic and biogenic methane. Science, 344, 1500–1503.

(3) Young, E.D., Kohl, I.E., Sherwood Lollar, B., et al. (2017). The relative abundances of resolved ¹²CH₂D₂ and ¹³CH₃D and mechanisms controlling isotopic bond ordering in abiotic and biotic methane gases. Geochimica et Cosmochimica Acta, 203, 235–264.

(4) Zhang, N., Prokhorov, I., Kueter, N, et al. (2025). Rapid high-sensitivity analysis of methane clumped isotopes (Δ¹³CH₃D and Δ¹²CH₂D₂) using mid-infrared laser spectroscopy. Analytical Chemistry, 97, 1291–1299.

(5) Fantoni, R. & Scotti, P. (2003). Thermal record of the Mesozoic extensional tectonics in the Southern Alps. Atti Ticinensi di Scienze della Terra, 9, 96–101.