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

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doi: 10.1016/S0009-2541(97)00047-8
Authors:Azaroual, M.; Fouillac, C.; Matray, J. M.
Author Affiliations:Primary:
BRGM, Direction de la Recherche, Orleans, France
Volume Title:Chemical Geology
Source:Chemical Geology, 140(3-4), p.167-179. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0009-2541
Publication Date:1997
Note:In English. BRGM Contrib. No. 97002. 53 refs.; illus., incl. sect., 4 tables, sketch maps
Summary: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 favors 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 characterization, as the goal of reservoir characterization is to describe the spatial distribution of petrophysical parameters such as porosity, permeability, and saturations.
Sections:Experimental mineralogy
Subsections:Silicates
Subjects:Activity; Aqueous solutions; Aquifers; Basins; Brines; Chemical reactions; Dogger; Electrolytes; Fluid phase; Framework silicates; Geochemistry; Grain size; Ground water; Jurassic; Keuper; Mesozoic; Middle Jurassic; Mineral-water interface; Moganite; Permeability; Polymorphism; Porosity; Quartz; Sedimentary basins; Sedimentary rocks; Silica; Silica minerals; Silicates; Solubility; Thermodynamic properties; Triassic; Upper Triassic; Europe; France; Paris Basin; Western Europe
Abstract Numbers:98M/3751
Record ID:1997072322
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands
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