A well-defined geological model of the Gassum Formation in the subsurface of the Skagerrak Strait is established in order to ensure safe storage of CO2. This is particularly important in this open aquifer where the inclined layers slope northwards towards the seafloor. Petrography and provenance analyses are integrated with seismic interpretation and well-log analyses to estimate the reactivity of the sandstones and thereby the CO2 immobilization potential. The results show that the zircon age distributions, the mineralogical compositions and the grain-size parameters are different in the upper and lower part of the Gassum Formation in the J-1 well in the southern part of the study area. The upper part is mineralogically immature and is derived locally from erosion of southernmost Norway, whereas the lower part with smaller grain size, better sorting and lower contents of feldspar and rock fragments has a higher mineralogical maturity and a larger provenance area. The northward onlapping internal seismic reflectors in the formation show that the lower part gradually disappears towards the basin margin, which indicate that injected CO2 will migrate into the most mineralogically immature sediments and these have the highest trapping potential since they are expected to be most reactive.


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