Evaporite controls on basin-inversion structures in the southern Danish Central Graben, North Sea – a 3D seismic-data study

During Cretaceous and Paleogene times, tectonic shortening caused mild basin inversion of earlier rifting depocentres in the Danish Central Graben. This exerted an important control on the thickness variations and geometries of e.g. the Late-Cretaceous and Danian Chalk Group. Structural highs formed by inversion and especially Permian-salt movements, host important hydrocarbon reservoirs in the sector. Earlier researchers have linked basin inversion in the North-Sea area to Alpine deformational phases and the onset of seafloor spreading in the North Atlantic.

The objective of this 3D seismic-data study is an analysis of the relationships between basement (sub-salt) faults, salt movements, and salt-cover deformation, as well as fluid migration near and within inversion structures.

We find that the northeastern margin of the larger inverted area generally has a thick-skinned style. Here, reverse reactivation of the rift-bounding master fault is coupled between the strata above and below the salt. Oppositely, the southwestern margin has a thin-skinned style. Here, buttressed hangingwall folds sit above reverse faults detaching into even thin evaporite sequences. The strike of this cover-fault trend mimics that of the underlying basement faults, although they dip in opposite directions. A triangle-zone model explains how sub-salt shortening (reactivation of major basement faults) can be balanced to the shortening observed in the sedimentary cover. As the current thickness of Permian salt increases and mobile-salt structures become predominant towards the south, the effects of basin inversion grow difficult to distinguish from those of halokinesis.

Interestingly, the shallow crests of inversion folds, especially along the southwestern margin, host groups of smaller normal faults. These formed to some degree during inversion, indicating that local extensional tectonism (crestal collapse) took place during the overall shortening. We conclude that the shallow parts of the folds experienced forced bending rather than buckling during folding.

A significant number of hydrocarbon reservoirs sit within basin-inversion structures. Potentially, this work can increase our understanding of deformation within these and similar structures.

Acknowledgements: We thank the Centre for Oil and Gas – DTU (DHRTC) for funding and supporting this project and for providing data. We also thank Schlumberger and Eliis for providing seismic-interpretation software.