SOLUBILITY OF SILICA POLYMORPHS IN ELECTROLYTE-SOLUTIONS .2. ACTIVITYOF AQUEOUS SILICA AND SOLID SILICA POLYMORPHS IN DEEP SOLUTIONS FROM THE SEDIMENTARY PARIS BASIN

Activity coefficient for aqueous silica in saline waters and brines fr om the Paris Basin was calculated using Fitter's specific interaction model. Quartz and chalcedony are the only reported authigenic silica m inerals in the Dogger aquifer of the Paris Basin (France). However, th e measured silica concentrations fall between those of these two phase s. The silica concentrations measured in Dogger fluids seem to be cont rolled by a microcrystalline quartz phase with a grain size computed t o be about 20 nm. Studies have shown that pressure can preserve small grain size for a long time at the geological scale. The effective mech anism of pressure action is probably linked to the fact that pressure simultaneously favours dissolution at the grain-contact inducing a qua rtz supersaturation and prohibits the increase in size of reprecipitat ed microcrystalline quartz grains. This hypothesis is supported by oth er studies reported in the literature. The proposed model, which incor porates silica mineralogy and a precise calculation of aqueous silica activity, allows us to explain measured silica concentrations in the d eep sedimentary solutions of the Dogger aquifers. In the Keuper brines , silica solubility can in most cases be explained by an equilibrium w ith either chalcedony or quartz. Another application of the present wo rk is shown by an example, where we examined the importance of precise ly evaluating the activity coefficient in basin characterisation, as t he goal of reservoir characterisation is to describe the spatial distr ibution of petrophysical parameters such as porosity, permeability, an d saturations. (C) 1997 Elsevier Science B.V.