Figure 4: Diffusion-reaction modelling in PHREEQC.

(a) PHREEQC⁴² model run for 125,000 years for constant CO₂ at saturation on basal boundary. A 15 cm-long one-dimensional reactive–diffusive model comprising 30 cells of 5 mm length was used with initial mineralogy (mol l⁻¹) calculated from XRD analyses, pore volumes from SANS of the unaltered portion of the caprock, the invading pore fluid chemistry based on the reservoir fluids (Supplementary Table 4) and the initial caprock pore fluid chemistry was taken to be a fluid in equilibrium with the caprock mineralogy, with pCO₂, pO₂ and salinity estimates for typical Jurassic marine shales. The initial redox state of the invading fluid was defined using the SO₄ ²⁻/H₂S redox couple. Models were run assuming local fluid–mineral equilibrium and a constant D_e value of 5 × 10⁻¹² m² s⁻¹ was used for all aqueous species. The model timescale was 125,000 years with a time step of 7 days. (b) One-dimensional model with the same starting conditions as a but with two 25,000 year phases of CO₂-saturated brine in the basal cell with an intervening 75,000 year period where the basal cell contained a CO₂-poor brine. The results of the two episodes of CO₂ saturation are seen as double peaks in pyrite and dolomite modes as the CO₂ from each pulse diffuses into and reacts with the caprock.