For the purpose of geological carbon storage, it is necessary to understand the long-term effects of introducing CO2 and sulfur-species into saline aquifers. CO2 stripped from the flue gas during the carbon capture process may contain trace SO2 and H2S and it may be economically beneficial to inject S-bearing CO2 rather than costly purified CO2. Further, reactions between the S-bearing CO2, formation brines and formation minerals will increase pH and promote further dissolution and precipitation reactions. To investigate this we model reactions in a natural analogue where CO2 and SO4-H2S bearing fluids have reacted with clay-rich siltstones. In the Mid-Jurassic Carmel formation in a cap rock to a natural CO2-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, 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 years to form before the fluid self-sealed, and that literature hematite dissolution rates between 10-8 and 10-7 mol/m2/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 CO2-bearing, SO4-rich and minor H2S-bearing fluids is shown to be much faster than with CO2-poor, SO4-rich with minor H2S-earing fluids. The substantial buffering capacity of mineral reactions demonstrated by the S and CO2–related alteration of hematite-bearing siltstones at the Green River CO2 accumulation implies that corrosion of such a cap rock are, at worst, comparable to the 10,000-year timescales needed for carbon storage.