Dunlin Group (elevated)
(From NPD Bulletin no. 3)
Name
Named by Deegan and Scull (1977). According to earlier Norwegian usage the unit had formation status, and the type well was UK well 211/29-3. In this report the unit is given group status in Norwegian as well as UK usage.
Type area
The type area is the East Shetland Basin, in particular the region of Brent Field. The group is illustrated in the following wells: UK well 211/29-3 (Shell),
Norwegian wells 33/9-1
(Mobil), 30/6-7
(Norsk Hydro), 31/2-1
(Shell) and 25/2-4 (Elf).
Lithology
The group consists mainly of dark to black argillaceous marine sediments, but in the marginal areas of the basin marine sandstones are developed at several stratigraphic levels and can extend a considerable distance into the basin. The sandstones are white to light grey, very fine to medium grained and generally well sorted. The group tends to be more calcareous in the Norwegian sector, and in places limestone beds, some of which contain chamosite and siderite ooliths, are found.
Thickness
222 m in the UK well 211/29-3, 255 m, 365.5 m, 308 m and 204 m in Norwegian wells 33/9-1, 30/6-7, 31/2-1 and 25/2-4 respectively. The group is thickest in the northern Viking Graben area.
Boundaries
The lower boundary with the Statfjord Formation and the upper boundary with the Brent Group are clearly marked by gamma ray log breaks. The Dunlin Group generally has a more regular log ca-racter than the underlying and overlying sediments. In the northernmost area where the Brent Group is not recognised, the Dunlin Group is often unconformably overlain by the Viking Group.
Distribution
The group is more widespread than the underlying Statfjord Formation and is thickest in the Viking Graben area, east of the Statfjord and Brent Fields. It is recognizable over most of the East Shetland Basin and northern part of the Horda Platform. In places the Dunlin Group rests with an apparent unconformity on the Statfjord Formation. In the western part of the basin higher formations within the group are thought to be transgressive onto pre-Statfjord Formation sediments. Variation in thickness on tilted fault blocks probably reflects syndepositional movement.
Age
The group ranges from Hettangian to Bajocian in age.
Subdivisions
The Dunlin Group is divided into five formations. These are named the Amundsen (base), Johansen, Burton, Cook and Drake (top) Formations and can be clearly differentiated on sonic and gamma ray logs. The Amundsen, Cook and Drake Formations are found throughout the East Shetland Basin. The Burton Formation is found over most of the area but is not present on the Horda Platform. The Johansen Formation, on the other hand, has so far only been found on the Horda Platform. It should be stressed that the upper part of the calcareous sands in the Statfjord Formation passes laterally into the lower part of the calcareous silts and shales of the Amundsen Formation in the central parts of the basin.
