CARBON SEQUESTRATION POTENTIAL OF MAFIC VOLCANIC ROCKS IN SOUTHERN IDAHO: ROLE OF MINERAL TRAPPING

Geologic sequestration of CO₂ occurs via three interrelated mechanisms, namely hydrodynamic trapping, solubility trapping, and mineral trapping. Hydrodynamic and solubility trapping have received considerable attention because of the potential importance of depleted hydrocarbon reservoirs as sequestration site. However, trapping due to mineralization in which CO₂ is entombed by increased weathering of subsurface minerals containing geochemically reactive base cations (primarily Ca²⁺, Mg²⁺, and Fe²⁺) has the potential to permanently entombed anthropogenic CO₂ in the subsurface. In addition, the permanency of mineral trapping may not depend on the long term integrity of a cap rock. In the northwester United States thick sequences of mafic volcanic rock provide the potential for the permanent disposal of large quantities of CO₂. Carbon dioxide injected into the subsurface dissolves in pore-water to forms a weak acid that is neutralized by weathering minerals to produce carbonate and bicarbonate ions (alkalinity) and/or mineral carbonates. Mineralization potential will be highest in rocks with abundant Ca-, Mg-, and Fe-silicates such as basalt and lowest in rocks poor in these phases (e.g., sandstone). We have conducted a preliminary assessment of the CO₂ sequestration potential of mafic volcanic rocks in southern Idaho using The Geochemist's Workbench computer code and a generalized mineral dissolution rate law that we have developed. Model results suggest that more that 20 kg carbon per m³ of rock and be sequestered in typical basalt. Furthermore, mineral trapping becomes the dominate form of sequestration after approximately 300 years. Our assessment indicates that mafic rocks of southern Idaho have significant potential for the permanent sequestration of CO₂.