Geochemical characterisation of the Danish subsurface ash series for carbon mineralisation potential

The Danish subsurface ash series within the Eocene-aged Fur and Ølst Formations has a large theoretical CO2 storage capacity through carbon mineralisation, a potential that is the main motivation behind the C·ASH project initiated by researchers from the Department of Geoscience, Aarhus University. The potential carbon storage is achieved by injection of CO2 into the closely spaced volcanic ash beds in the Danish subsurface to accelerate silicate dissolution, leading to carbon fixation by mineralisation of CO2 as carbonate minerals. This provides a valuable alternative to conventional CO2 storage technologies that typically aim to store CO2 as a supercritical phase in deep porous aquifers. One of the essential requirements for the mineralisation technology is the identification of ash beds that contain enough reactive divalent cations (Ca2+, Fe2+, Mg2+) to sustain the silicate dissolution – carbonate precipitation reactions. This study aims to provide a geochemical proxy to identify and locate suitable ash beds with high CO2 sequestration potential. For this purpose, the shallow Harre borehole drilled in 1980 in northwestern Denmark is analysed using high-resolution XRF and SEM-EDS. In the Harre well the ash beds are most abundant within a five meters thick interval in the uppermost part of the Ølst Formation between 191 and 196 meters below surface. The XRF data show that titanium (Ti) is the most reliable elemental proxy for identifying the volcanic ash beds and that calcium (Ca), which is an important divalent cation for carbon mineralisation, is mostly enriched in the volcanic ash beds within the five meters interval. However, Ca may also be associated with gypsum (CaSO4·2H2O) and carbonates (CaCO3), suggesting that part of the Ca have already leached from silicates within the ash beds. This fraction is believed to be low and that the ash beds’ suitability for carbon mineralisation remain high.