Gas concentration and flow rate measurements as part of methane baseline assessment: Case of the Fontaine Ardente gas seep, Isère, France

Abstract

Methane and CO2 gas emissions from a gas seep located in the French Alps, documented over two millenaries, have been quantified along with gas emissions from the miniseepage area contouring the main vent. Several tons per year of both gas phases are released in the atmosphere from the main seep (18 tons of CH4 and 5.5 tons of CO2) which has been modified by an old borehole. Diffuse seepage brings additional CH4 (1 ton per year) and CO2 (2.3 tons per year) which are emitted from a small area of 240 m2. Secondary CH4 oxidation processes are likely to occur near the surface. Biologically produced CO2 is also found in the soil in the seepage area and the biological component becomes predominant as CO2 concentrations drop lower than 2% vol. Soil gas CH4+CO2 enrichments are oriented along N45 and N170 pathways that represent the main structural directions of the area. Soil flux anomalies are more scattered because surface clay formations have been strongly reworked at surface. Variety of methods (IRGA, micro-GC, accumulation chamber flux measurements, IR-camera) were tested and are discussed in connection with environmental impact monitoring and the role of sampling conditions, with emphasis on shale gas and carbon storage baseline assessments The role of potential leakage pathways represented by the old borehole structure is also discussed as an analogue with leaky shale gas wells.