*3.1. Measurement of the scCO2 Storage Ratio for the Conglomerate and Sandstone Cores*

Results of the modal analyses for the rudaceous sandstone and conglomerate cores are shown in Table 1 and the photomicrographs of their thin sections are shown in Figure 4. The conglomerate was mostly composed of rhyrolitic and andesitic rock fragments (average 74.5%), followed by quartz, clay/accessory minerals, feldspar, micas, and calcite (in descending order). The sandstone mainly consisted of quartz, rock fragments, clay/accessory minerals, feldspar, micas, and calcite. Their average proportions were 32.5%, 23.8%, 18.3%, 19.4%, 2.5%, and 2.5%, respectively (Table 1). In previous studies it was observed that calcite, feldspars, chlorite, micas, and clay minerals bearing Ca and Mg might control the geochemical reactions with CO2 in the storage site, thereby regulating the physical properties of the reservoir rock [25–28]. The results from the XRD analyses (not shown in this paper) showed a mineral composition similar to that indicated by the modal analysis.

**Figure 4.** Photomicrographs of thin sections of the conglomerate (**a**,**b**) and the rudaceous sandstone (**c**,**d**); left column—closed mode; right column—open mode; Qtz—quartz; Flds—feldspars; Cal—calcite; Bit—biotite; and Rxs—rock fragments.

The porosity of the conglomerate and the sandstone cores were measured and are shown in Table 2. The average porosity of the conglomerate and the rudaceous sandstone was 17.8% and 14.5%, respectively, suggesting that they fall within the porosity range of typical CO2 storage formations in other places, where large amounts of scCO2 have been injected [22,29–34]. The scCO2 storage ratio

of each conglomerate and sandstone core was measured and their averages are shown in Table 2. The calculated average scCO2 storage ratio of the Janggi conglomerates was 30.7%, suggesting that 30.7% of the void space in the conglomerate was filled with scCO2, while the pressure difference between the scCO2 injection and the pore water was maintained at 10 bar. The average scCO2 storage ratio of rudaceous sandstones was 13.0%, which was about two-fifths that of the conglomerate. From these results, both kinds of rock had great capability for storing CO2 in their pore spaces, but the conglomerate was considered to be a better option for a CO2 storage site than the rudaceous sandstone.


**Table 1.** Petrographic detrital modal analysis of the rock cores.

\* JG1-C—conglomerate cores; JG1-S—rudaceous sandstone cores, JG1-M—mudstone cores and JG1-T—dacitic tuff cores.



The scCO2 storage ratio values were used for the estimation of the CO2 storage capacity for the Janggi Basin (Table 3). From previous studies [7–9] it was known that the Janggi Basin extends horizontally over about 0.25 km<sup>2</sup> below a depth of 800 m and that the thickness of both the conglomerate and sandstone layers available for the CO2 reservoir is 100 m (about 1:1 ratio), assuming that the maximum volume of the CO2 storage formation in the Janggi Basin under 800 m in depth is about 0.025 km3. The parameter values used to calculate the CO2 storage capacity and the calculated scCO2 capacity for the two formations are shown in Table 3. The scCO2 storage capacity of the reservoir rocks around the probable scCO2 injection site in Janggi Basin was calculated to be 368,742 metric tons, demonstrating that the conglomerate and sandstone formations in Janggi Basin have great potential for use as a pilot test site for CO2 storage in Korea (to receive more than 100,000 metric tons of CO2 injection at the test storage site). As mentioned earlier, the amount of dissolved CO2 in the pore water of each core was ignored while calculating the amount of scCO2 storage in Table 3. When the amount of dissolved CO2 in the pore water was considered, the already substantial CO2 storage capacity of the Janggi Basin was increased. Thus, if enough scCO2 was secured in the conglomerate and the sandstones, the Janggi Basin could be evaluated as a positive storage site, regardless of the amount of CO2 dissolved in the pore water.

**Table 3.** The scCO2 storage capacity of conglomerate and rudaceous formations at the Janggi Basin (unit—metric ton).

