Geochemistry of clumped isotopologues of CH₄ within fluid inclusions in Alpine tectonic quartz fissures

Abstract

Petroleum-bearing fluid inclusions are small encapsulations of oil and gas that persist in the rock record long after their parent fluids have moved on. The chemical and isotopic compositions of inclusions offer a unique way to investigate hydrocarbon formation pathways in deep Earth environments and in ancient geological times. Here we use stable isotope compositions of methane, including clumped isotopologues ¹³CH₃D and ¹²CH₂D₂, to evaluate the formation conditions of methane within fluid inclusions in Alpine tectonic quartz fissures. We observe that this CH₄ is relatively ¹³C and ²H enriched (δ¹³C = −26 to −39‰; δ²H = −126 to −146‰), consistent with the formation by advanced catagenesis of organic matter, and has clumped isotope compositions indicating intramolecular isotopic equilibrium at catagenetic to low-grade (sub-greenschist) metamorphic temperatures (TΔ¹³CH₃D: 120-300 °C; TΔ¹²CH₂D=120-260 °C). Our findings, combined with an analysis of the geographic distributions of inclusion-bearing fissures and fluid inclusion PVT behaviors, provide direct evidence of reservoirs of thermogenic methane trapped beneath the Alps during Mid-Miocene tectonic nappe emplacement (c.a. 25 to 15 Ma). The gas was sequestered in a concentrated form that likely favored the accumulation of up to 50,000-150,000 Megatons (Mt) of CH₄ across the Helvetic domain in Switzerland. Following the subsequent tectonic uplift during the period 17 to 10 Ma, we suggest that the trapped methane-rich fluids migrated to shallower depths along faults, leading to the release of Alpine metamorphic methane into near-surface aquifers and the atmosphere at rates of 0.01 to 0.03 Mt⋅y⁻¹.

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