Towards a Better Knowledge of Natural Methane Releases in the French Alps: A Field Approach

Abstract

We report investigations performed at some hydrocarbon gas seeps located in the French Subalpine Chains in zones of outcropping Jurassic black shales, increasing the reported number of such occurrences in this part of the Alps. We present the characteristics of each of the seeps, based on soil flux measurements and soil gas measurements. Gases emitted are CH4-rich (87–94%) with the exception of one site (78.5% CH4 + 8.2% CO2) where an active landslide may induce dilution by atmospheric air. CO2 is generally measured at low levels (<1.6%). Concentrations in C2H6 are more variable, from less than 1% to more than 2.3%. Gas is emitted over areas of various sizes. The smallest gas emission area measures only 60×20 cm, characterized by a strong hydrocarbon flux (release of about 100 kg of CH4 per year). At a second site, hydrocarbon emissions are measured over a surface of 12 m2. For this site, methane emission is evaluated at 235 kg per year and CO2 emission is 600 kg per year, 210 kg being related to gas seepage. At the third site, hydrocarbons are released over a 60 m2 area but strong gas venting is restricted to localized seeps. Methane emission is evaluated at 5.1 tons per year and CO2 emission at 1.58 tons per year, out of which 0.53 tons are attributed to gas seepage. Several historical locations remain uninvestigated at present, and numerous others may still be unknown. We outline strategies to search for such unrecorded sites. Considering the topography of the potential alpine and perialpine emission areas, the possibilities to detect gas emissions appear of the size recorded so far seem to be restricted to ground-based methods or to methods offering the possibility to point orthogonally to the soil towards the seep maximum. If such sites are to be investigated in the future in the frame of Environmental Baseline Assessment (EBA), even establishing appropriate monitoring protocols will be challenging.

A gas-flow funnel system to quantify advective gas emission rates from the subsurface

“…Especially, the estimation of greenhouse gas emission rates from the subsurface into the atmosphere is an important part of climaterelated research activities and associated efforts concerning the global carbon cycle. In this regard, research focuses on gas emissions from subsurface reservoirs of carbon dioxide (CO 2 ) from geological sources or (leaking) carbon capture and storage (CCS) sites (Bigi et al 2014;Dixon and Romanak 2015;Holloway et al 2007;Jones et al 2014;Jung et al 2015;Kis et al 2017); on methane emissions from melting permafrost or related to volcanic activity (Etiope and Klusman 2002;Gal et al 2019;Kang et al 2014); and on sulfur dioxide (SO 2 ) and hydrogen sulfide (H 2 S) emissions which might also considerably contribute to volcanic exhalations (O'Dwyer et al 2003;Symonds et al 2018;Werner et al 2013). The monitoring of radon fluxes can serve investigations related to earthquakes and/or volcanic eruptions in volcanic areas, and the detection of tectonic fault structures (Schuetze et al 2012;Steinitz et al 2007;Zafrir et al 2016).…”