The Tryggvason Formation consists generally of mudstones with interbedded limestones. Interbedded sandstones are common in the Agat area in the northeastern Norwegian North Sea (Blocks 35/6-9). The content of limestones relative to mudstones is generally lower in the northern part of the Viking Graben (from Blocks 30/2 and 30/3 northwards) than in the southern part. At the transition between the Viking Graben and the Horda Platform the Tryggvason Formation consists of limestone. The mudstones are light to dark grey, often calcareous, occasionally micaceous, glauconitic and pyritic. The limestones are white to light grey or brownishgrey and argillaceous. The sandstones are clear to light grey, very fine to fine grained and cemented by calcite.
The lower boundary is defined by a decrease in gamma-ray intensity and an increase in velocity from the Blodøks Formation into the Tryggvason Formation. This is due to the difference in carbonate content.
The upper boundary shows an increase in gamma-ray intensity and a decrease in velocity from the Tryggvason Formation upwards into the Kyrre Formation. This log change is due to the lower carbonate content of the Kyrre Formation.
The Tryggvason Formation is time-equivalent with the Herring Formation and the lower part of the Hod Formation in the central North Sea, and also with the informal "formation C" of Deegan & Scull (1977).
Distribution and thickness
The formation is present in the Viking Graben and northern Tampen Spur area towards the Marulk Basin in the northern Norwegian North Sea. In the Viking Graben, the formation is 208 m thick in the type well (25/1-1), 326 m in well 35/3-2 and 145 m in well 24/9-1. It is 45 m thick in well 30/11-3 on the western margin of the Horda Platform.
After rifting in the Permian, Triassic and Late Jurassic, thermally induced subsidence became focused in the axis of the Søgn Graben, located in the northeastern Norwegian North Sea. The previously formed fault-block topography was filled during the early part of the post-rift period, such that by the Late Cretaceous, the Måløy slope had been transformed
into a gently westward-dipping slope (Bugge et al., 2001; Gabrielsen et al., 2001; Jackson et al., 2011). The Cretaceous succession reached a maximum thickness of 800 m (2625 ft) in the axis of the Søgn Graben and was dominated by hemipelagic mudstones and thin carbonates. However, during the Late Cretaceous (Late Turonian), sand supply from the Norwegian hinterland to the east was abundant and resulted in deposition of a series of slope-channel complexes and associated terminal fans that were fed through a series of shelf-edge canyons (Martinsen et al., 1999, 2005; Bugge et al., 2001; Jackson, 2007; Jackson et al., 2008). On seismic data, these
sandstone-rich depositional systems are expressed as high-amplitude anomalies, set within a background of predominantly low-amplitude, chaotic reflection events interpreted as hemipelagic mudstones (Jackson et al., 2011).
References in scientific journals and books:
Bugge, T., B. Tveiten, and S. Bäckström, 2001, The depositional
history of the Cretaceous in the northeastern North Sea: Norwegian Petroleum Society Special Publication 10, p. 279–291.
Gabrielsen, R. H., R. Kyrkjebø, J. I. Faleide, W. Fjeldskaar, and T. Kjennerud, 2001, The Cretaceous post-rift basin configuration of the northern North Sea: Petroleum Geoscience, v. 7, p. 137–154.
Christopher A.-L. Jackson, C. A.L., M. Huuse, and Gillian P. Barber, 2011: Geometry of winglike clastic intrusions adjacent to a deep-water channel complex: Implications for hydrocarbon exploration and production. AAPG Bulletin, v. 95, no. 4 (April 2011), pp. 559–584.
Martinsen, O. J., T. Lien, and C. A.-L. Jackson, 2005, Cretaceous
and Paleogene turbidite systems in the North Sea and Norwegian Sea basins: Source, staging area and basin physiographic controls on reservoir development, in A. G. Dore and B. Vining, eds., Petroleum geology of
northwest Europe and global perspectives: Proceedings of the 6th Petroleum Geology Conference, Geological Society (London), p. 1147–1164.