**Anja Sundal \* and Helge Hellevang**

Department of Geosciences, University of Oslo, 0371 Oslo, Norway; helge.hellevang@geo.uio.no **\*** Correspondence: anja.sundal@geo.uio.no; Tel.: +47-22-85-66-52

Received: 9 September 2019; Accepted: 24 October 2019; Published: 31 October 2019

**Abstract:** Reservoir characterization specific to CO2 storage is challenging due to the dynamic interplay of physical and chemical trapping mechanisms. The mineralization potential for CO2 in a given siliciclastic sandstone aquifer is controlled by the mineralogy, the total reactive surface areas, and the prevailing reservoir conditions. Grain size, morphologies and mineral assemblages vary according to sedimentary facies and diagenetic imprint. The proposed workflow highlights how the input values for reactive mineral surface areas used in geochemical modelling may be parameterized as part of geological reservoir characterization. The key issue is to separate minerals both with respect to phase chemistry and morphology (i.e., grain size, shape, and occurrence), and focus on main reactants for sensitivity studies and total storage potentials. The Johansen Formation is the main reservoir unit in the new full-value chain CO2 capture and storage (CCS) prospect in Norway, which was licenced for the storage of CO2 as of 2019. The simulations show how reaction potentials vary in different sedimentary facies and for different mineral occurrences. Mineralization potentials are higher in fine-grained facies, where plagioclase and chlorite are the main cation donors for carbonatization. Reactivity decreases with higher relative fractions of ooidal clay and lithic fragments.

**Keywords:** CCS; CO2 storage; mineralization; carbonatization; mineral trapping; mineral sequestration; Johansen Formation; North Sea; sedimentary facies
