The Architecture and Geomorphology of the Shelf Tidal Ridges of the Cook Formation in the Northern North Sea

The Cook Formation in the North Sea has for several decades been an important HC-bearing formation but is now also an interesting site for CCS. It is the shelf tidal ridges within the Cook Formation that serves these two purposes.

The Cook Formation is a subaqueous delta built southward through the confinement of the Early Jurassic Seaway during regression, whereas several internal transgressive phases produced sands that were reworked into shelf tidal ridges.

The tidal ridges of the Cook Formation are elongated with stacked, well-sorted, cross-bedded sandstone sets with mudstone drapes. Both the elongate tidal sand-ridges and intervening mudstone-rich, inter-ridge zones are proven by numerous well observations and illustrated by seismic amplitudes. The ridges are built up of a stack of asymmetrical sub-ridge and can reach about 50 meters of thickness. The internal stacking and architecture of the ridges are reflected in cores and bore-hole image logs. The image logs also provide paleo-currents directions indicating direction of migration. The general shape of a ridge and the geomorphology of the top-surface of the ridge itself is reflected in an amplitude map.

The tidal ridges stand out with excellent reservoir properties at burial depth where other sandstones are cemented tight from quarts-cementation. This is caused by chlorite-coating on the grains, preventing quarts overgrowth, combined with the grain-size and sorting by the tidal processes forming the tidal ridge itself. This renders these shelf tidal ridges of high interest both for the purpose of HC exploration and CCS storage sites in the North Sea.