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For the purpose of geological carbon storage, it is necessary to understand the long-term effects of introducing CO 2  and sulfur-species into saline aquifers. CO 2  stripped from the flue gas during the carbon capture process may contain trace SO 2  and H 2 S and it may be economically beneficial to inject S-bearing CO 2  rather than costly purified CO 2 . Furthermore, reactions between the S-CO 2 -bearing formation brines and formation minerals will increase pH and promote further dissolution and precipitation reactions. To investigate this we model reactions in a natural analog where CO 2 - and SO 4 -H 2 S bearing fluids have reacted with clay-rich siltstones. In the Mid-Jurassic Carmel formation in a cap rock to a natural CO 2 -bearing reservoir at Green River, Utah, a 3.1 mm wide bleached alteration zone is observed at the uppermost contact between a primary gypsum bed and red siltstone. Gypsum at the contact is ~1 mm thick and shows elongate fibers perpendicular to the siltstone surface, suggesting fluid flow along the contact. Mineralogical concentrations, analyzed by Quantitative Evaluation of Minerals by SCANning electron microscopy (QEMSCAN), show the altered siltstone region comprises two main zones: a 0.8 mm wide, hematite-poor, dolomite-poor, and illite-rich region adjacent to the gypsum bed; and a 2.3 mm wide, hematite-poor, dolomite-poor, and illite-poor region adjacent to the hematite alteration front. A one-component analytical solution to reactive-diffusive transport for the bleached zone implies it took less than 20 yr to form before the fluid self-sealed, and that literature hematite dissolution rates between 10 –8  and 10 –7  mol/m 2 /s are valid for likely diffusivities. Multi-component reactive-diffusive transport equilibrium modeling for the full phase assemblage, conducted with PHREEQC, suggests dissolution of hematite and dolomite and precipitation of illite over similar short timescales. Reaction progress with CO 2 -bearing, SO 4 -rich, and minor H 2 S-bearing fluids is shown to be much faster than with CO 2 -poor, SO 4 -rich with minor H 2 S-bearing fluids. The substantial buffering capacity of mineral reactions demonstrated by the S- and CO 2 -related alteration of hematite-bearing siltstones at the Green River CO 2  accumulation implies that corrosion of such a cap rock are, at worst, comparable to the 10 000 yr timescales needed for carbon storage.

A siltstone reaction front related to CO 2 - and sulfur-bearing fluids: Integrating quantitative elemental mapping with reactive transport modeling