Late Cretaceous stratigraphy and basin development in the Danish Central Graben

A regional study on the Late Cretaceous has been carried out and this paper presents the results of the evaluation on the structural development in the greater Danish Central Graben area during the Late Cretaceous. More than 100 wells have been examined and both 2D and 3D seismic data have been used for the comprehensive evaluation in the area. The structural development is illustrated by regional time-isochore maps, flattened seismic profiles and log correlation panels.

Based on the integration of the biostratigraphy, the petrophysical and the geophysical data a modified Late Cretaceous lithostratigraphy is introduced. The Chalk Group is divided into 10 informal lithostratigraphic units. The distribution of the various units is closely related to the structural development of the inversion ridges associated with the Late Campanian inversion tectonism.

For illustration of the structural development during Upper Cretaceous 10 flattened seismic profiles have been made (Enclosure 1). The study imply that the Chalk Group in the Central Graben can be divided into two successions (Lower Chalk and Upper Chalk) separated by the distinct and regional Top Hod Unconformity (THU).

The THU is a stratigraphic hiatus and a tectonic feature closely related to the Late Campanian inversion within the Central Graben. The Late Campanian inversion phase was accompanied with a relative sea level fall and in large areas the inverted Lower Chalk section was above the storm wave base level which gave rise to severe erosion of the succession. The THU is a major hiatus with variable time gap depending on the degree of truncation into the Lower Chalk section. The variation and range in time gap is illustrated in the stratigraphic correlation panels in Enclosure 2. The deposition of the Upper Chalk is highly controlled by the relief created during the Late Campanian inversion. Initially, the eroded material from the ridges was delivered to the basin centers outside the inversion zone. During Maastrichtian time relative thick deposits covers the inversion areas but the succession reveals a high amount of reworked chalk. The reworked chalk is closely related to the flanks of the inversion structures where it is associated with down dip gravity processes. Down-dip sliding and slumping are common along the inversion ridges in addition to turbiditic deposits in the basin centres.

The Upper Chalk encounters high porosity chalk due to the prevalence of reworked chalk and represents the most prolific interval in the Chalk Group. Because all structural closures on Top Chalk have been drilled new chalk prospects need to be related to stratigraphic traps. It is therefore necessary to outline areas and intervals with reworked chalk. As a result of this study, it is recommended that upcoming attempts to outline prospective intra-Chalk intervals should be linked to the structural development related to the inversion.