Hegre Group

(See also NPD Bulletin no. 3 for older definition)


Named after the bird Heron (Norwegian: hegre).

Type area

The type area is the northern part of the northern North Sea. The following Norwegian wells are used to illustrate the Group: 31/6-1, 34/4-4 and 33/12-5.

Locations of wells (click image for full size).


The Hegre Group is thickest in the eastern part of the basin where major subsidence occurred along the present Norwegian coastline. More than 2 km of Triassic sediments has been penetrated in east (well NO 31/6-1). The Group is thinning towards the west in the northernmost part of the northern North Sea while in the southern part (Central Trough and Moray Firth) substantial thicknesses are recognized. There might have been depositional systems along the East Shetland Platform during deposition of the Hegre Group as great thicknesses of unspecified Triassic strata have been drilled here (see Comments below).


The Hegre Group consists of intervals of interbedded sandstone, claystones, mudstones and shales associated with sequences of dominantly sandstone or shale/claystone. Shales and claystones/mudstones usually have reddish colours whereas the sandstones show a range in colour from white, light grey, orange to brick red. The grain size varies from very fine to very coarse and the sediments are in parts of pebbly nature. The Hegre Group also has intervals of white limestone, anhydrite and brownish-red marl.


The base of the Hegre Group represents in the southern and central part of the northern North Sea a transition from Zechstein carbonates and evaporites to Triassic silica-clastics. It represents a transition from high gamma and sonic readings of the carbonate dominated Zechstein facies to low gamma and sonic of the Smith Bank Formation. It is uncertain which wells show a continuous succession of Permian to Triassic silica-clastics, as the lower boundary is generally not detectable as it represents a continuous succession of Rotliegendes red beds to Triassic sediments of similar characteristics. The Rotliegendes as well as the Triassic succession are generally barren of age diagnostic taxa. In the northernmost part of the Basin where Zechstein facies is absent, the base of the Hegre Group represents a disconformity to Caledonian basement (well NO 31/6-1).


Wells of Triassic strata which are the Hegre Group (click image for full size).


The area close to the margin of the East Shetland Platform and margin of the present Norwegian mainland shows generally a succession of sandstones and mudstones. A succession (of a well) which does not show a complete Triassic succession making it possible to subdivide it based on the overall lithostratigraphic pattern and totally absence of age diagnostic material makes it very difficult or impossible to decide which Triassic unit the sediments belong to. The actual succession should therefore be assigned unspecified to the Hegre Group.

While the Cormorant Formation by Vollset & DorŠ¹ (1984) was applied only to certain areas where a subdivision of Triassic is impossible, it is suggested here that unspecified Triassic strata should generally be assigned to undifferentiated Hegre Group rather than to a formation of unspecified strata. The Cormorant Formation is therefore here suggested to be abandoned.

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