Impact of diagenesis on the suitability of reservoir rocks for CO2 storage - Examples from a Cambrian sandstone in the Baltic Sea

Two areas in Sweden are currently being investigated by the Swedish geological survey as possible sites for geological storage of CO₂. One of them is in the Baltic Basin south of Gotland, where Cambrian sandstone from the Faludden Member (Borgholm Formation) has been recognized as a suitable storage reservoir. The Faludden reservoir is a well-sorted, fine- and medium-grained quartz arenite with high porosity and permeability. Here, we report new results obtained from scanning electron microscope analyses of the secondary mineralogy of the Faludden sandstone and implications for its CO₂ storage potential. Our study shows that there are several phases of secondary mineralization in the sandstone, which to a varying degree affects the interaction with injection and storage of CO₂. The results provide information on the diagenetic history of the reservoir and assessment of depth-related P-T mineralizations. The most common secondary mineralization is a patchy carbonate cement. The patches, embedding several detrital grains, are round to irregular with individual diameters of up to 5 mm. The carbonate is mostly dolomite with microscale variations towards more Mn- and Fe-rich compositions. Calcite is also present, especially in connection to occasional microfractures. In general, patchy cementation is favorable for CO₂ storage since it can prevent compaction, while still allowing a relatively high porosity and permeability. Quartz cement in the Faludden sandstone is limited to scattered occurrences of fringed overgrowths at the edges of detrital quartz grains. However, we observe that the amount of secondary quartz is increasing with depth in the Baltic Basin. Subcropping lower Cambrian sandstone beds, such as the När and Viklau sandstone members are often more or less completely cemented with quartz. Thus, the process of secondary quartz mineralization is an important factor to consider when assessing the optimal depth range for CO₂ storage in the basin. Other secondary minerals in the Faludden sandstone include pyrite, apatite, iron(titanium)oxides and small amounts of clay minerals (illite and kaolinite). In some samples there is evidence of dissolution of both quartz cement and dolomite, introducing a secondary microporosity. This study confirms that the sandstone from the Faludden Member is a suitable reservoir rock for CO₂ storage. A thorough understanding of geological processes, including reservoir diagenesis, is essential for predicting the physiochemical interactions between the injected CO₂ and the reservoir rock.