Towards an improved lithostratigraphic subdivision of the Chalk Group in the Netherlands North Sea area – A seismic stratigraphic approach

In the Netherlands North Sea area, the Chalk Group has thus far been subdivided into the Cenomanian Texel Formation, the Turonian to MaastrichtianOmmelanden Formation and the Danian Ekofisk Formation. This paper describes the attempt to arrive at a more detailed lithostratigraphicsubdivision for this area, particularly of the Ommelanden Formation. To this end, a seismic stratigraphic analysis was carried out on a regional2D and 3D seismic dataset. The Chalk Group was subdivided into eleven seismic stratigraphic sequences, named CK1 through CK11, based on themapping and correlation of unconformities. The identified seismic sequence boundaries were used as the main chronostratigraphic markers inthe Chalk Group interval. The seismic dataset was subsequently expanded with well log data of 45 boreholes. These were tied to the seismicdataset by constructing a synthetic seismogram for each borehole, after which the seismic sequence boundaries were noted and correlated onthe logs. Finally, micropaleonthological data available in 15 boreholes were used to date the seismic sequences. The sequences were interpretedto be of Cenomanian (CK1), Turonian (CK2), Coniacian (CK3), Santonian (CK4), Early Campanian (CK5; CK6), Middle to Late Campanian (CK7),Early Maastrichtian (CK8; CK9), Late Maastrichtian (CK10) and Danian (CK11) age. The seismic units recognised in this study were compared withformal lithostratigraphic units defined in the Chalk Group in the surrounding North Sea sectors. Based on this comparison, a revision of theformal lithostratigraphic scheme, recognising Tor Formation and Herring Formation equivalents, is suggested for the Netherlands North Sea area