Accessibilities of reactive minerals in consolidated sedimentary rock: An imaging study of three sandstones

Widespread implementation of geological storage of CO₂ requires an understanding of dissolution reactions with formation minerals. This will be aided by reactive transport modeling, which relies on accurate estimates of the accessible surface areas of reactive minerals in consolidated sedimentary rocks. For three Viking sandstones (Alberta sedimentary basin, Canada), we have employed backscattered electron microscopy and energy dispersive X-ray spectroscopy to examine mineral content and to statistically characterize mineral contact with pore space. Porosities range from 20% in a lightly-cemented sandstone with grains on the order of 100 μm, to 8% in a highly-cemented shaly sandstone with a mix of primary pore space and fractures, to 7% in a lightly-cemented conglomerate sandstone with grain sizes between 500 μm and 1 mm. In all three specimens, kaolinite is the primary authigenic clay mineral cementing quartz grains. It accounts for only 5% to 31% of mineral content, but 65% to 86% of pore-mineral contact boundaries. The sandstone specimen has 6% minerals in the "reactive" category, which in this study includes minerals other than kaolinite and quartz, such as K-feldspar, apatite and pyrite. For this specimen, only one third of the reactive minerals are accessible to pore fluids due to clay-mineral grain coatings. For the shaly sandstone, only one fifth of its 5% reactive minerals are accessible to pore fluids due to regions of cementation of fine detrital matrix. Thus, if a mineral volume fraction is used in reactive transport modeling as a proportional measure of accessible surface area in consolidated sandstones, the reaction rates are likely to be overestimated by three to five times. The conglomerate sandstone has only 1% of its mineral matter in this category, and these are often found as inclusions rather than grains.