Brent Group (elevated)
(From NPD Bulletin no. 3)
Name
Named by Deegan and Scull (1977). According to Norwegian usage the unit earlier had formation status and the type well was UK well 211/29-3 (Shell). In this report the unit is given group status, also, in the Norwegian sector.
Type area
The type area is the East Shetland Basin. In this report the following wells are used to illustrate the Brent Group: UK well 211/29-3 (Shell), Norwegian wells 33/9-1 (Mobil), 30/6-7 (Norsk Hydro) and 31/4-4 (Norsk Hydro).
Thickness
The thickness of the group varies considerably. In UK well 211/29-3 (Brent Field) it is 226.5 m, while the Norwegian well 33/9-1 in the Statfjord Field has 204 m of Brent Group sediments. Wells used to illustrate the group on and around the Horda Platform have thicknesses between 159 m (30/6-7) and 78 m (31/4-4). Thicknesses of 200 m or more are present to the north in quadrant 35.
Lithology
The group consists of grey to brown sandstones, siltstones and shales with subordinate coal beds and conglomerates.
Boundaries
In the Brent-Statfjord area the group normally rests with a minor disconformity on the predominantly argillaceous Dunlin Group. To the west and in the southern Viking Graben it cuts down onto lower levels within the Dunlin Group. On the Horda Platform the lower boundary is either picked at the base of a "coarsening upward" log motif, with underlying marine shales, or at the base of a homogenous sandstone with a "blocky" log appearance. The upper boundary of the Brent Group may vary in nature due to post-middle Jurassic tectonism and erosion. Variable amounts of the group may be missing, particulary, towards the crests of tilted fault blocks, but the contact is nevertheless easy to pick where the sandy Brent Group is overlain by Heather Formation shales.
Distribution
The Brent Group is recognizable over most of the East Shetland Basin and the northern part of the Horda Platform. It passes southwards into the Vestland Group south of the Frigg Field area. The absence of the basal marine sandstones is considered to be the distinguishing feature of the Vestland Group. Northwards, the Brent Group shales out within the East Shetland Basin between 61o30' N and 62°N.
Age
Mainly Bajocian to Early Bathonian but including Late Toarcian to the east.
Subdivision
The group is divided into five formations. These are: the Broom (base), Rannoch, Etive, Ness and Tarbert (top) Formations. According to Norwegian usage, the units earlier had a member status (Deegan and Scull, 1977). In this report it is proposed to give the units formation status also in the Norwegian sector. All formations are recognizable in the Brent-Statfjord area. However, difficulties are met when moving away from the type area. On the Horda Platform the Broom Formation appears to be absent, and the presence of the Rannoch Formation is under debate.
