Geogenic greenhouse gas emissions and shale weathering: lessons learned from an alpine "eternal flame"

Natural hydrocarbon gas seeps have been reported over a wide range of geological settings, and their contribution to atmospheric concentrations of potent greenhouse gases (CO₂, CH₄) may be considerable. Potential gas sources in sedimentary environments include organic-rich source rocks such as peats, coalbeds, and shale clays. The origins of natural methane seepages occurring in the French Subalpine Chains are poorly constrained and only little information is available on the mechanisms behind the gas release by weathering of organic-rich shales, notably when overprinted by recent tectonism. The investigated site, where outcropping Callovian-Oxfordian black shales emit methane, higher alkanes and CO₂ through macro- and micro-seeps, shows the complexity of these mechanisms. We report geochemical, isotopic and petrographic data from the Meylan-Rochasson eternal flame site, from in-situ measurements, gas, pore water and sediment sampling and degassing experiments, and propose a conceptual model covering all stages of reservoir evolution from early diagenesis to exhumation and weathering. Rock degassing experiments under helium allowed extracting alkanes and CO₂ dissolved in pore waters. Isotopic signatures from below 20-40 cm are comparable to the macro-seep, indicating a common thermogenic origin of alkanes through maturation of organic matter at ∼150 °C. Secondary methane oxidation processes affect both point- and diffuse emissions of thermogenic methane. In the pore waters of outcropping shales and colluvium thermogenic gas mix with a microbial methane component. This superposition of weathering, oxidation of OM and microbially driven processes needs to be considered in future studies investigating the importance of direct natural gas emission from shales worldwide.