Natural gas seeps in the French Alps: Sources and pathways
Abstract
Natural gas emanations are part of the geochemical baseline to take into account when assessing global greenhouse gas emissions and potential impacts of conventional and unconventional gas exploration and exploitation on groundwater. Examples of such natural gas macro-seeps are known in several parts of the world (Etiope et al., 2009). Only a limited number of them have been characterized for their gas and isotopic compositions. Such analyses can provide essential information for baseline studies, providing insight in the sources (biogenic vs. ther-mogenic or modified thermogenic) and pathways of such seeps and may allow for distinction of natural seeps from stray gas leakage associated with human activities. Here, we report gas concentrations and multi-isotope data (δ 13 C and δ 2 H of methane and ethane, δ 13 C and δ 18 O of CO 2 , 3 He/ 4 He ratio) of two gas seeps in the French subalpine chains, both in a similar geological and structural position within Middle Jurassic claystones along the eastern border of the large synclinal structures of the Vercors and the Chartreuse massifs (Moss, 1992). The " ardent fountain " (fontaine ardente) of Le Gua, 30 km south of Grenoble has most likely the longest continuous written record of existence of any individual natural gas seep, mentioned explicitly as early as the first quarter of the 5 th century (Augustin of Hippo (St. Augustin), approx. 426) This natural seep was described in the past as a " wet seep " associated with a spring, whereas the second investigated seep, Rochasson near Meylan north of Grenoble, is a dry seep.
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