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

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 (CH3D)-C-13 and (CH2D2)-C-12, to evaluate the formation conditions of methane within fluid inclusions in Alpine tectonic quartz fissures. We observe that this CH4 is relatively C-13 and H-2 enriched (delta C-13 = -26 to -39 parts per thousand; delta H-2 = -126 to -146 parts per thousand), 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 (T Delta(CH3D)-C-13: 120-300 degrees C; T Delta(CH2D2)-C-12 = 120-260 degrees 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). (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

The study investigates the formation conditions of methane in Alpine tectonic quartz fractures using stable isotope compositions, indicating the methane is formed through advanced catagenesis of organic matter. The research suggests that the methane was trapped beneath the Alps during Mid-Miocene tectonic nappe emplacement around 25 to 15 million years ago and later released into the atmosphere during tectonic uplift around 17 to 10 million years ago.