What can be learned from natural analogue studies in view of CO2 leakage issues in Carbon Capture and Storage applications? Geochemical case study of Sainte-Marguerite area (French Massif Central)

Natural analogues studies have received much interest over past years through the CO2 capture and storage applications. In this paper we focus on one natural analogue in leakage situation in order to describe the nature of the gas leakage, its interaction with surrounding rocks and aquifers and its behaviour over time. Soil gas and water monitoring techniques are used to gather point and continuous records. Leakage occurs along discrete sections relying on the nature of surface formations and the permeability of discontinuities. Main gas vents are perennial, relative proportions of different gas phases (CO2, 222Rn and 4He) being dependent from the interaction with surface deposits and the distance to main tectonic pathways. Mineral waters were also monitored as they represent integrative bodies that exist above nearly all CCS sites. Constraints on chemical processes occurring at depth are brought by characterisation of the dissolved constituents and by related isotopic systematics. Such determination of mixing processes and their influence is important, as leakage from CCS site may be evidenced through the mixing of the water from the storage complex and overlying aquifers. Longer term monitoring was conducted for physico-chemical parameters highlighting noticeable variations for pH, dissolved oxygen, redox potential and dissolved CO2.