Alteration of Wall Rocks by CO₂-Rich Water Ascending in Fault Zones: Natural Analogues for Reactions Induced by CO₂Migrating Along Faults in Siliciclastic Reservoir and Cap Rocks

The recent tectonic activity of the Rhenish Massif is associated with the discharge of CO2-rich fluids of mantle origin. Ascending CO2 dissolves in groundwater of the fractured sedimentary rocks of the Variscan basement. Alteration reactions have been derived from petrological analyses of wall rocks, fluid chemistry, and thermodynamic modelling. The principle reaction is the alteration of chlorite and feldspar to kaolinite, Fe-carbonate, quartz, and bicarbonate water. Fluid flow rates, temperatures, depths, groundwater residence times, rock types, and reaction products vary among the natural systems studied. Reaction conditions can be constrained by comparison of natural parameter variations with thermodynamic reaction progress simulations. Depending on the degree of reaction progress, either porosity enhancement due to dissolution of primary minerals or porosity reduction, and eventually even self-sealing of fractures can occur within the same rock.