Dissolved CO2 and Alkane Gas in Clay Formations

Abstract : Characterization of dissolved CO 2 and alkane gas in clayrocks may help assessing the confinement properties of geological barriers considered as potential host rocks for a deep geological disposal as well as for caprocks of gas storages. A monitoring of alkanes with CO 2 , combined with carbon isotopes was performed on core samples coming from Underground Research Laboratories (Bure, Mont Terri, Tournemire) and the Schlattingen borehole in France and Switzerland. Composition of hydrocarbon gas and δ 13 C of methane strongly suggest a dominant thermogenic origin of methane which is mixed with a bacterial origin for the Toarcian shales, Pliensbachien and Callovian-Oxfordian clayrocks. Results also evidence the contrasted behavior of CO 2 , which is controlled by chemical equilibrium between pore water and carbonate mineralogy, compared to the alkanes which are present in the porosity as a stock of dissolved gases which can be depleted during degassing experiments.