Geochemical Study of Natural CO₂Emissions in the French Massif Central: How to Predict Origin, Processes and Evolution of CO₂Leakage

Abstract

This study presents an overview of some results obtained within the French ANR (National Agency of Research) supported Géocarbone-Monitoring research program. The measurements were performed in Sainte-Marguerite, located in the French Massif Central. This site represents a natural laboratory for CO<sub>2<sub/>/fluid/rock interactions studies, as well as CO<sub>2<sub/> migration mechanisms towards the surface. The CO<sub>2<sub/> leaking character of the studied area also allows to test and validate measurements methods and verifications for the future CO<sub>2<sub/> geological storage sites. During these surveys, we analyzed soil CO<sub>2<sub/> fluxes and concentrations. We sampled and analyzed soil gases, and gas from carbo-gaseous bubbling springs. A one-month continuous monitoring was also tested, to record the concentration of CO<sub>2<sub/> both in atmosphere and in the soil at a single point. We also developed a new methodology to collect soil gas samples for noble gas abundances and isotopic analyses, as well as carbon isotopic ratios. Our geochemical results, combined with structural geology, show that the leaking CO<sub>2<sub/> has a very deep origin, partially mantle derived. The gas rises rapidly along normal and strike-slip active faults. CO<sub>2<sub/> soil concentrations (also showing a mantle derived component) and CO<sub>2<sub/> fluxes are spatially variable, and reach high values. The recorded atmospheric CO<sub>2<sub/> is not very high, despite the important CO<sub>2<sub/> degassing throughout the whole area.