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The Johansen and Cook Formations are mainly separated by shales and silt-stones, but due to fault juxtaposition, they will be treated as one aquifer. The Johansen Formation sandstones have good reservoir properties in several wells in the Troll Field, and seismic data imply that the sand distribution is similar to the overlying Sognefjord Delta. The Cook Formation and the underlying Statfjord Formation extend to the Tampen Spur. The upper part of the Dunlin Group in the Troll area consists of the thick Drake Formation shale which is the main seal (figure).
The Johansen Formation south of the Troll Field was suggested by the NPD in 2007 as a potential storage site for CO2 from Mongstad, and several studies have been carried out in order to qualify the aquifer for CO2 storage. The NPD and Gassnova have acquired 3D seismic data in the most promising area. The studies indicate that the for-mation has sufficient capacity to store the volumes from Mongstad, but a well is important to clarify the reservoir and seal properties in the area south of Troll. Migration of CO2 to the surface is unlikely due to the large capacity of the Sognefjord Delta aquifer.
The capacity of the Johansen and Cook aquifer depends on the commu-nication within the aquifer, and if it is in communication with the Statfjord and/or the Sognefjord Delta aquifers across major faults.
The pore volume and the storage capacity in prospects given in the table are based on calculations by Gassnova. These calculations do not include the northernmost part of the aquifer in the area north of Troll, see figure.