CARBONATE PRECIPITATION IN AN EXPERIMENTAL SUPERCRITICAL CARBON DIOXIDE-BRINE-ROCK SYSTEM
The brines present a range of reaction potential among rock, brine, and supercritical CO2. On one hand, carbonate may readily precipitate from Mg-rich brine containing aqueous carbonate ion buffered by coexisting supercritical CO2 (Kaszuba et al., 2003). On the other, Na-rich brine, initially devoid of divalent cations, can only derive the cations needed for carbonate precipitation by reaction with silicates. Magnesite precipitated in the Mg-NaCl brine-supercritical CO2-rock experiment whereas magnesite and siderite precipitated in the NaCl brine-supercritical CO2-rock system. Magnesite crystals in both experiments are large, up to a few mm in length, and occur as individual grains and splays. Euhedral magnesite precipitated in Mg-NaCl brine, indicating equilibrium growth. Pitted crystal faces of magnesite in the NaCl brine experiment indicate disequilibrium and are interpreted as dissolution of early-formed magnesite. Siderite occurs as individual crystals, 200-250 mm in diameter, on shale. Euhedral texture, indicative of equilibrium, is interpreted as precipitation of siderite after early-formed magnesite. Occurrence of pitted magnesite and euhedral siderite may represent a paragenetic sequence for carbonates. This sequence is not predicted by geochemical models, indicating complexity of geochemical reactions among supercritical CO2, brine, arkose, and shale.