Polygonal faulting in chalk - An example at the Tyra field, Danish north sea

Polygonal faults have been mapped in several basins worldwide within fine-grained sedimentary sections. In a few cases, such features have been noticed also for chalk rocks at both outcrop and seismic scale. In this paper we present a case history of polygonal faults in the Late Cretaceous chalk units at the Tyra Field, Danish North Sea. The use of 3D seismic data has allowed producing images of the polygonal faults of excellent quality. Seismic data show polygonal faults within the Tor Formation (Maastrichtian Age) that do not extend into the overlying Ekofisk Formation reservoir of Danian age. In the case of the Tyra Field, the possible changes in porosity/lithology, hence permeability, associated with the polygonal faults, do not reach the Ekofisk Formation reservoir, but may have acted as preferential pathways for vertically migrating hydrocarbons. The present study illustrates the morphology of the polygonal faults, addressing also their spatial relationship with major Cretaceous faults and gravitational mass movements in the area.