MINERALOGICAL AND GEOCHEMICAL ATTRIBUTES OF MIDCONTINENT RIFT: APPLICATION AS A TARGET FOR CO2 SEQUESTRATION

The Midcontinent Rift (MCR) contains several features that make it an attractive prospect for carbon sequestration. Sedimentary and volcanic rocks of the MCR are enriched in Ca-, Mg-, and Fe-silicates, and Fe-oxides; which when dissolved can induce the precipitation of carbonate minerals. The MCR also contains numerous stratigraphic and structural features that may hinder fluid and gas migration. Outcrop samples collected from the Keweenaw Peninsula (MI), L’Anse area (MI), Potato River Falls (WI), Copper Falls State Park (WI), and the Bayfield Peninsula (WI) were investigated by petrographic study and by XRD for mineralogy; and XRF for major chemical elements. The oldest sedimentary unit (Copper Harbor Conglomerate) showed abundant volcanic lithic fragments and traces of quartz, plagioclase feldspar, potassium feldspar, muscovite, iron oxides, biotite, and opaques. Intergranular pores and fractures were cemented by calcite and subordinate hematite. Cementation has significantly reduced the porosity and permeability of this unit. Matrix and clay sized minerals are dominated by chlorite and varied traces of illite, smectite, and laumontite. The bulk chemical composition of Copper Harbor Conglomerate showed enrichment of CaO (5 – 7.9 wt %), MgO (1.5 – 3.3 wt %), and Fe₂O₃ (5.2 – 12.4 wt %). With these mineralogical and geochemical attributes, the Copper Harbor was considered as the most attractive target to sequester CO₂ by carbonate mineralization. In contrast, sediments from the overlying Bayfield Group are feldspathic quartz arenites with some kaolinitic matrix and minor hematitic cement. The XRF results showed a significant amount of SiO₂ (more than 90 wt %), with minor Fe₂O₃ (0.9 – 1.4 wt %), and only traces of CaO and MgO. Thin section examination and vacuum permeability testing revealed porosity values up to 17%, which would provide effective storage space for injected CO₂. Rock samples from several startigraphic units were cut into wafers and polished, then reacted in deionized water + dry ice CO₂ at 90^oC in Teflon lined pressure vessels to mimic potential reactions that can be caused by injecting CO₂ into the subsurface. The maximum cation concentration released into the solution from all corrosion tests was Ca from Copper Harbor Conglomerate with concentration of 915 ppm after 100 days of testing.