Hydrogeochemical processes in clastic sedimentary rocks, South Korea: A natural analogue study of the role of dedolomitization in geologic carbon storage

For long-term mineral trapping of sequestered CO ₂ in deep saline aquifers, base cations such as Ca ²⁺ and Mg ²⁺ are essential. As a natural analogue study of geologic carbon storage in deep aquifers hosted in sedimentary formations, we examined the hydrochemistry of sulfate-rich (up to 1140mg/l SO ₄ ^2-) and moderately high P _CO2 (10 ^-1.1 to 10 ^-2.4atm) groundwater in a Cretaceous non-marine sedimentary basin in South Korea with the objective to elucidate water-rock interactions controlling the concentrations and behavior of base cations. Principal component analysis of the acquired hydrochemical data indicated that dissolution of carbonates (calcite and dolomite) and evaporite minerals (halite and gypsum) controls the chemical composition of groundwater, resulting in substantial increases of the concentration s of Ca ²⁺, Mg ²⁺, Na ⁺, Cl ^-, SO ₄ ^2- and total dissolved solids (TDS) in deep groundwater. Na ⁺ versus Cl ^- and Ca ²⁺+Mg ²⁺ versus HCO ₃ ^-+SO ₄ ^2- plots provided evidence for dissolution of halite and gypsum. Progressively increasing δ ³⁴S values of dissolved SO ₄ ^2- (from 15.1 to 19.2‰) with increasing sulfate concentrations indicated gypsum dissolution. Ion-ion plots (esp., Mg ²⁺/Ca ²⁺, Ca ²⁺/SO ₄ ^2- and Mg ²⁺/SO ₄ ^2-) and saturation indices of calcite, dolomite and gypsum suggest that the groundwater chemistry (esp., the concentrations of Ca ²⁺ and Mg ²⁺) is controlled by dedolomitization driven by gypsum dissolution. Groundwater in the study area does not reach complete equilibrium with respect to calcite and dolomite because of gypsum dissolution, which controls the Mg ²⁺/Ca ²⁺ ratios of groundwater. Continued calcite precipitation triggered by an excess Ca supply from gypsum dissolution reduces the concentrations of dissolved inorganic carbon (DIC) in groundwater. The increase of δ ¹³C _DIC values from -11.1 to -6.5‰ concomitantly with increasing sulfate concentration was explained via geochemical modeling by dedolomitization under the rate constant ratio of about 0.038 between dolomite and calcite. The model results agree well with the observed Mg ²⁺/Ca ²⁺ ratios and further suggest a potential increase of the void volume in the aquifer through dedolomitization by about 0.72cm ³/l. Based on this analogue, we suggest that dedolomitization in concert with dissolution of gypsum may constitute an important process releasing base cations for mineral trapping of injected CO ₂ in non-marine clastic sedimentary strata containing carbonates and gypsum in South Korea and elsewhere.