**2. Estimating CO2 Mineralization Potential (I): Model Parameterization**

In CO2 storage, the chemical characteristics of the sediment are of particular importance with respect to estimating mineralization potential. Thorough, descriptive petrographic studies using optical- and scanning electron-microscopy (SEM) methods in addition to quantitative bulk mineralogy analysis such as X-ray diffraction data (XRD) are necessary to characterize the reservoir rock with respect to reactivity.

#### *2.1. Qualitative and Quantitative Reservoir Mineralogy*

It is useful to define reactivity and make separate geochemical categories within the sedimentological framework. Changes in grain size and mineralogy (phase and occurrence) are particularly important. Bed stacks, or para-sequences (sensu Van Wagoner et al. [36,37]), may serve as a scale of reservoir subdivision, depicting depositional trends; e.g., upwards coarsening or fining trends in grain sizes, indicating changes in depositional regime with time. In the case of the Johansen Formation, reservoir grade sandstones recognised in wells are subdivided in lower shoreface (very fine-grained) and upper shoreface (medium-grained) deposits, interbedded with mudstones and/or carbonate cemented layers [28,29] (Figure 2).

**Figure 2.** (**a**) Simplified sedimentary facies distributions for the Johansen Formation. Mineralization potential for the reservoir intervals are given for upper and lower shoreface sandstones; (**b**) an interpreted lithological log (vertical section through the sandy Johansen Formation with the muddy over- and underlying Amundsen Formation) from well 31/2-3: based on wire line log data (available at npd.factpages.no), cuttings, and a short cored section (2116–2134 m) from which rock samples were collected. The succession consists of prograding and aggrading parasequences of upper and lower shoreface deposits, with mudstones representing flooding events. Carbonate cemented sandstone layers may form within or on the top of beds due to dissolution and re-precipitation of calcareous material (e.g., shells). These layers are close to impermeable, and provide barriers to fluid flow in otherwise permeable reservoir sandstone. Generally, micro-scale observations from the different facies settings show that total grain surface area in contact with pore water (white void) increases with decreasing grain size.

Though desirable, geological cores through the entire reservoir zone from saline aquifers are rarely available. Usually data from shorter core sections or sidewall cores must be interpolated with respect to vertical and lateral facies changes. In the case of the Johansen Formation, available core data are collected from a well tens of kilometres away from potential injection areas, and at shallower burial depths (Figure 1). Thin sections provide means for 2D porosity estimations, grain size, and mineral content (vol. %). Porosity and permeability plug test data are available from side wall cores [28].

One of the most common means for mineralogical quantification is X-ray diffraction (XRD) (e.g., the Rietveld method), as it is inexpensive, fast, and requires little sample material. The method may be crude or specific with respect to mineral phases, depending on the effort and knowledge put into interpretation of the results and treatment of sample material [38]. Analyses of grain size specific fractions are more suited for reactivity estimates—e.g., clay separation in fluid suspension.

Identification of main cation donors for mineralization in a given reservoir can be performed using the bulk mineralogy. Chlorite is a major constituent in the clay fraction of the Johansen Formation [29], and geochemical studies find chlorite to be a significant cation donor (i.e., Fe2+ and Mg2+ supply through rapid dissolution) in CO2 carbonatization [9,10,18,39]. Feldspars also provide a significant reactant, as plagioclase (albite and oligoclase) dissolve within relatively short time-scales (100 s of years), contributing Na<sup>+</sup> and Ca2<sup>+</sup> to solution [10,18,19]. This study will focus on the characterization of chlorites and feldspars, while the same kind of analyses should be undertaken in case of other or more reactive constituents (e.g., mafic minerals).

#### 2.1.1. Characterization of Chlorites

Chlorite is a phyllosilicate mineral, with Fe-rich chamosite (Fe,Mg)5Al(Si3Al)O10(OH)8 and Mg-rich clinochlore (Mg,Fe)5Al(Si3Al)10(OH)8 as common varieties. Detrital chlorites derived from mafic volcanic or metamorphic terrains are commonly Mg-rich clinochlores, whereas chlorites sourced from peralkaline granites tend to generate Fe-rich chamosites [40]. Diagenetic chlorite is a significant constituent in many siliciclastic reservoirs in the Norwegian North Sea [33]. Autigenic chlorite may form by recrystallization of precursor clay minerals during early burial; e.g., from smectite (<70 ◦C) [41,42] or from berthierine (90 ◦C) [33,34,43]. Chlorite may also form as an alteration product from degradation of mafic minerals (e.g., biotite, pyroxene, amphibole). In addition to provenance and detrital mineralogy, depositional environment also exerts a control on chlorite occurrence. As summarized in a literature review by Maast [44] (and references therein), Fe-rich chlorite coating is associated with sediments deposited in marine environments, near river mouths and under tropical conditions, whereas Mg-rich chlorite coating is commonly found in continental sediments, deposited under arid- to semi-arid conditions. Chemical speciation with respect to Fe/Mg ratios is important in the selection of suitable kinetic parameters for geochemical simulations, as chamosite and clinochlore display different reaction potentials. Chamosites with Fe/(Fe+Mg) values between 0.57 and 0.91 are the most common in studied North Sea reservoirs [45]. Most geochemical studies implement kinetic data from [46], which only provide kinetic constants for Mg-rich clinochlore. It seems that also recent investigations into chlorite kinetics focus mainly on clinochlore (e.g., [47–49]). The effect of varying chemical composition of chlorite on dissolution rates is uncertain, as no thorough studies have been performed in this realm. It has been claimed to have little effect [50]. However, some experimental studies indicate significantly higher rate constants for Fe-chlorite [33]. However, the precipitation rates for siderite (FeCO3), magnesite (MgCO3), and Fe-Mg-Ca solid solutions are not the same, which provides another argument for differentiation. It is likely that more kinetic data will become available and include more detailed solid solution speciation in the future. As part of the geological characterization, XRD-spectra may be modelled for estimation of element ratios, as shown for a typical Fe-rich chlorite (chamosite) from the Johansen Formation (Figure 3a).

**Figure 3.** Chlorite solid solutions and occurrence: (**a**) Modelled X-ray diffraction pattern by use of Newmod II, showing a fit with typical chlorites found in a potential CO2 reservoir in the North Sea; the Johansen Formation. The best fit was found for a Fe2.34 chamosite, with a Fe/(Fe+Mg) ratio of 0.93; (**b**) Scanning electron microscope image of grain coating chlorite from laboratory experiments of daphnite growth (yellow colour applied for reference). Individual crystals are half disk-shaped, growing perpendicular to the host grain surface, ranging in size from 2–15 μm; (**c**) Scanning electron microscope image of ooidal chlorite. Crystal growth occurs in dense, concentric layers around a nucleus grain, which has been dissolved in this case. Grain coating chlorite covers the surface of framework quartz grains. Ghost rims of chlorite coats remain where the framework grain has been dissolved.

In a disaggregated sample, e.g., separated in clay (<2 μm), fine (2–250 μm), and medium (>250 μm) grain size classes, chlorite may be present in all fractions as quantified by XRD. The clay fraction would comprise pore-filling chlorites from diagenetic degradation of detrital, percolated clay and/or diagenetic chlorite from disassembled pseudomorphs of altered grains (e.g., degraded biotite) or mud-clasts. The sand fraction classes could comprise grain coating chlorite, from precursor clay coats. These appear as platy clay-fraction crystals growing tangential or perpendicular on the host grain surface (Figure 3b), and may be more or less resistant to mechanical sample treatment. Another common chlorite occurrence, ooidal, may also be included in the sand fraction. Ooids are spherical grains, with concentric layers of a coating mineral (e.g., clays, carbonates, phosphates) adsorbing on and accumulating around a nucleus-grain. Ooidal chlorite (Figure 3c) forms by recrystallization of precursor clay. These grains have a dense structure with low permeability. Other examples are chloritic pellets and dense diagenetically altered pseudomorphs. Thus, reaction potentials calculated as surface area per wt% mineral from XRD analysis, assuming a uniform clay fraction, would be overestimated in ooid-rich sediments. Additionally, pore-filling clays may not be accessible for intruding reactive fluid [22,23,51], which may cause overestimation of the clay fraction reactivity.

Petrographic studies (e.g., modal mineralogy or point counting) of thin sections in optical microscopes provide a volumetric estimate (vol %) of the mineral assemblage and porosity, which in combination with a description of grain shapes, sizes, micro-porosity from SEM, pore connectivity, and extent of coating translates directly to 2D specific surface areas. As with sieving before XRD analysis, point-counting methods may be used to separate mineral occurrences in grain size classes, in combination with descriptions of grain shape. In addition to chlorite, several reactants may appear in different grain size classes, representative of different reaction potentials.

#### 2.1.2. Characterization of Feldspars

It is relevant to quantify the relative contributions and occurrences of feldspars (i.e., microcline/orthoclase/sanidine, albite, anorthite, and their solid solutions), as kinetics and dissolution potentials in the presence of CO2 vary significantly [46]. Anorthite is rarely preserved in clastic rocks, as it is chemically unstable and weathers easily [52]. Generally the feldspar assembly varies according to provenance and hinterland geology (i.e., felsic or mafic, igneous or methamorpic rock), and relative feldspar/quartz contents are higher in finer grained facies.

In the Johansen Formation (and aforementioned siliciclastic reservoirs of the North Sea) K-feldspar and Na-plagioclase are abundant (Figure 4). Plagioclase occurs as monocrystalline, diagenetically etched grains, partly dissolved and/or severely altered to sericite. The plagioclase fraction is less than the original detrital composition, but the overall reactive surface area is probably higher than the direct relation to average grain size, due to the diagenetic, secondary porosity within individual grains. The chemical composition in single grains is closer to the albite endmember, with Na >> Ca (determined with electron microprobe). Albitization of K-feldspar grains is common in siliciclastic reservoirs at temperatures >65 ◦C [53], which would add to the more reactive fraction of feldspars compared to assemblages at shallower depths. If the reservoir conditions in the injection area differ from the sample site it is necessary to extrapolate such diagenetic alterations, or perform sensitivity studies. In the available data set from the Johansen Formation microcline is the most abundant phase, and occurs as monocrystalline grains, some with authigenic overgrowths. K-feldspar is less corroded than plagioclase, and the reactivity is thus likely more directly proportional to average grain size. In perthitic grains, one constituent may be more corroded than the other (Figure 4b), increasing the proportion of reactive surface areas. The feldspar component in lithic fragments (e.g., gneiss and granite) is less corroded and exposes smaller mineral surface areas relative to the absolute volume fraction.

**Figure 4.** Feldspar occurrences and elemental mapping: (**a**) Scanning electron micrograph of K-feldspar grain with spiky, euhedral overgrowths. Note the compositional change across the outline of the original, detrital grain. The autigenic component is pure microcline, compared to the detrital K-feldspar grain with more heavy elements; (**b**) feldspar perthite grain (scanning electron micrograph). The albite component (dark colour) is partly dissolved, while microcline (light colour) is preserved. This grain is likely to display large mineral surface area compared with average grain sizes; (**c**) mineral maps from Scanning Electron Microscopy (QemScan analysis, Equinor—by C. Kruber), showing the relative volume fractions of the main mineral constituents. Corresponding wt% from XRD (Rietveld) are: 10 wt% K-felspar, 6 wt% Albite, and 2 wt% chlorite in this f-m grained sandstone (2125.4 m), and 12 wt% K-felspar, 5 wt% Albite and 9 wt% chlorite in this very fine grained sandstone (2129.3 m).
