*3.2. Carbonatization of Feldspars*

The feldspars in the Johansen Formation have been divided into five different types based on their chemistry and morphology. The lithic feldspars (plagioclase and K-feldspar) were assumed to be in a mineral mixture inside spherical fragments, and have reactive surface areas corresponding to the mineral fraction in the fragment. K-feldspar was also found along albite in perthitic fragments, and some plagioclase occurred as larger, preserved detrital grains. In these simulations we assumed that all grains (minerals and lithic) where in the same size, and we simulated two different settings: very fine grained sand (0.1 mm) corresponding to lower shoreface deposits, and medium grained sand (0.4 mm) being representative for upper shoreface deposits [29]. The difference in grain size leads to a four times larger reactive surface area for the very fine sandstone, and correspondingly faster dissolution of the feldspar grains and faster formation of the secondary dawsonite (Figure 8). In the very fine sand, detrital plagioclase dissolved completely within 50 years, leading to a corresponding dawsonite growth. In the medium-grained sandstone the same reaction takes four times longer, indicating that the

dissolution occurs at far-from-equilibrium under-saturation and that the rate therefore is proportional to the reactive surface area. The difference in carbonatization potential is mainly seen on the short term, with the very fine sandstone having a potential of about 50% more CO2 bound and immobilized in dawsonite after 100 years (Figure 8). The difference is smaller in the long term (1000–10,000 years) as all Na-feldspars are eventually replaced by dawsonite. Including minor calcite contents (e.g., 1 wt%) causes some minor recrystallization of calcite to ankerite, but the overall storage potential does not change much. The K-feldspars were not completely dissolved after 10,000 years, but K-feldspars have earlier not been regarded as a source for dawsonite ([59] and references therein). Some recent data may, however, indicate that K-dawsonite may also form, but this is still highly uncertain [60]. The feldspars in intact lithic fragments are much less reactive compared to individual plagioclase grains, and it takes thousands of years to dissolve the lithic plagioclase even in the very fine sandstone.

**Figure 8.** Simulated feldspar dissolution of equal wt% feldspars for (**a**) very fine grained sand, and (**b**) medium-grained sand. Feldspars were divided into perthitic K- and Na-feldspars, K-feldspar and plagioclase in lithic fragments, and detrital plagioclase grains; (**c**) micrographs from corresponding facies in the Johansen Formation: very fine grained and medium grained sandstones. Pore space is filled with blue epoxy; (**d**) amount of dawsonite that forms from feldspar dissolution for very fine grained and medium grained sand lithologies. The two cases correspond to (**a**) and (**b**) (this figure).

If available mineral data are XRD analyses only and no petrographic information is available, we cannot distinguish the lithic plagioclase component (as long as the chemical compositions of the two are similar). The rates of plagioclase dissolution and dawsonite formation may in such cases be highly uncertain. Although we know quite well the local composition (well 31/2-3) of the Johansen Formation rocks, there may be spatial variations and we illustrated this by simulating three

cases with different fractions of plagioclase bound in lithic fragments (Figure 9). If all plagioclase is monocrystalline and detrital (0% in lithic fragments), nearly 70% of the plagioclase has dissolved after only 100 years and some significant amounts of dawsonite forms (Figure 9b). On the other hand, if most (90%) of plagioclase is within lithic fragments, less than 20% has dissolved after 100 years and much less dawsonite forms. The difference is, however, smaller at longer time scales and quite small when approaching 10,000 years (Figure 9). We have so far simulated chlorites and feldspars and their carbonatization potentials separately. Because these mineral groups share elements such as Al and Si, dissolution of one may affect the other. We therefore simulated the combined chlorite-feldspar assemblage and compared the carbonatization potential with the individual mineral-group simulations (Figure 10).

**Figure 9.** The amount of dawsonite formed with varying amount of plagioclases lithic rock fragments (low-reactive) or as separate detrital crystals (high-reactive) over (**a**) 100 years, and (**b**) 10,000 years.

**Figure 10.** Amount of CO2 stored in secondary carbonates (mol/L fmw) in simulations only taking into account either feldspars or chlorites, and compared to the results in coupled simulations taking into account both feldspars and chlorites. No/little difference suggests that pH is the same in both cases. The motivation to run separate simulations is to save time in more complex large-scale reactive transport simulations.

#### **4. Discussion**

The reactivity depends on the mineral assemblage, grain size and morphology, which all vary according to sedimentary facies within one sandstone reservoir unit, as well as on the diagenetic imprint and in-situ reservoir conditions. Characterizing and describing a potential storage candidate with respect to the spatial distribution of sedimentary facies is an efficient means for separating zones with different mineralization potentials for CO2 in siliciclastic reservoirs.
