Solubility of silica polymorphs in electrolyte solutions, II. Activity of aqueous silica and solid silica polymorphs in deep solutions from the sedimentary Paris Basin

Activity coefficient for aqueous silica in saline waters and brines from the Paris Basin was calculated using Pitzer's specific interaction model. Quartz and chalcedony are the only reported authigenic silica minerals in the Dogger aquifer of the Paris Basin (France). However, the measured silica concentrations fall between those of these two phases. The silica concentrations measured in Dogger fluids seem to be controlled by a microcrystalline quartz phase with a grain size computed to be about 20 nm. Studies have shown that pressure can preserve small grain size for a long time at the geological scale. The effective mechanism of pressure action is probably linked to the fact that pressure simultaneously favours dissolution at the grain-contact inducing a quartz supersaturation and prohibits the increase in size of reprecipitated microcrystalline quartz grains. This hypothesis is supported by other studies reported in the literature. The proposed model, which incorporates silica mineralogy and a precise calculation of aqueous silica activity, allows us to explain measured silica concentrations in the deep sedimentary solutions of the Dogger aquifers. In the Keuper brines, silica solubility can in most cases be explained by an equilibrium with either chalcedony or quartz. Another application of the present work is shown by an example, where we examined the importance of precisely evaluating the activity coefficient in basin characterisation, as the goal of reservoir characterisation is to describe the spatial distribution of petrophysical parameters such as porosity, permeability, and saturations.