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.
Highlights
The occurrence of natural Earth degassing—and hydrocarbon degassing—is known since very ancient times
We focus on the quantification of free gas leaks at the soil/atmosphere interface [10] where the influence of external parameters on the measurements of gas emissions needs to be taken into account [21]
If gas emissions were reduced by the effect of high-pressure conditions, this effect would have concerned the complete monitoring period so that a hypothetical variability of the gas emissivity linked to pressure effects would be very low
Summary
The occurrence of natural Earth degassing—and hydrocarbon degassing—is known since very ancient times. The growing interest for natural hydrocarbon seepage areas has two main angles: firstly, to better understand the production mechanisms of the hydrocarbon gas phases [4] and to determine if the gas is produced by thermal or biological processes from organic matter [5, 6] or by abiotic reactions involving hydrogen [7]; secondly, to better describe the degassing patterns [8], to identify gas migration pathways, and to quantify the amounts of gases released into the atmosphere [4, 9] and the related impacts onto the global atmospheric methane budget [10] These investigations are in relation with environmental concerns related to the increasing share of unconventional gas exploitation [11] and to natural and man-induced gas emissions from hydrocarbon fields [12, 13] with a special focus on the integrity of gas and oil wells [14], including abandoned wells [15]. We focus on the quantification of free gas leaks at the soil/atmosphere interface [10] where the influence of external parameters (temperature, soil properties, humidity, and so on) on the measurements of gas emissions needs to be taken into account [21]
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