Seismic Geomorphology of Remobilized Cretaceous Chalks within the Danish Central Graben

The Cretaceous chalks in the Danish North Sea are biogenic limestones, composed mainly of nano-sized tests of algae with minor additions of microfossils. The consistency of freshly deposited carbonate ooze is like a soup ground, making it fairly unstable at dips beyond several degrees. Remobilized chalks are therefore common, and their recognition within cores and on seismic data in the 80s was the first clue to a more dynamic depositional environment besides pelagic fall-out. At present we are able to visualize these remobilized bodies in much greater detail, due to the availability of high-quality 3D seismic data, and by using an integrated seismic geomorphological workflow. In this study, we reveal some spectacular examples of km-scale remobilization within the Lower and Upper Cretaceous chalks of the Danish Central Graben with horizon slicing, spectral decomposition data, and color processing of seismic data. Wells penetrating the features show that the body has higher porosities than the background pelagic counterparts, which results in an acoustic anomaly that makes the bodies stand out in 2D section and on 3D horizon slices. Rapid sedimentation rates combined with poor dewatering is thought to have led to local overpressure establishment, causing lower effective stresses within the remobilized body and therefore porosity could be preserved. Therefore, in general, remobilized chalks are an interesting subsurface target for extraction of natural resources (hydrocarbons, geothermal energy, and drink water). Seismic geomorphology thus provides a convenient framework to investigate the stratigraphic and spatial extent of these reservoirs, as well as revealing these catastrophic events within the seismic sedimentary record.