Drake Formation (elevated)
(From NPD Bulletin no. 3)
Dunlin Group
Name
Named by Deegan and Scull (1977) who gaveJt "sub-unit" status.
Well type section
UK well 211/29-3 (Shell), from 2829 m to 2887 m, coord N 61°08'06", E 01°43'36.5", (Fig. 10).
Well reference sections
Norwegian wells 33/9-1 (Mobil) from 2668 m to
2715 m, coord N 61° 15'07.5" E 01°50'25.8" (Fig. 11), 30/6-7 (Norsk Hydro) from 2786 m to 2975 m, coord N 60°38'39.49" E02°45'21.74"
15
(fig. 12), 31/2-1 (Shell) from 1985 m to 2093 m, coord N 60°46' 19.16" E O3°33'15.87" (Fig. 13), 25/2-4 (Elf) from 3876 m to 3895 m N 59°58'44.09", E 02°22'58.88" (Fig. 15).
Thickness
UK well 211/29-3: 58 m
Norwegian well 33/9-1: 47m
Norwegian well 30/6-7: 189m
Norwegian well
31/2-1: 108m
Norwegian well
25/2-4: 19m
Lithology
In the type well the lower part of the formation consists of medium grey, slightly sandy, calcareous claystone. The upper part is dark grey to black, fissile, micaceous shale containing calcareous nodules. On the Horda Platform and
along its western margin white to grey, fine to coarse sandstones are found within the formation. The sandstones are often hard and calcite cemented. They also show and "upward coarsening" on the gamma ray log. The claystone is often silty.
Boundaries
In the basinal areas, where the sandstones are not present, the formation has more regular gamma ray and sonic log responses than that of the underlying formation, the gamma ray response being uniformly higher and the velocity lower. The lower velocity is also apparent where sandstone beds are present. The upper boundary is marked by the presence of arenaceous sediments of the overlying Brent Group, and the upper boundary is placed at the base of the upward coarsening sequence of the Rannoch Formation of the Brent Group. The Brent Group shows a more erratic sonic log pattern than the Drake Formation. In the south the top of the formation is marked by an unconformity (e.g. well 25/2-4, Fig. 15, NPD Bulletin no. 3). In the northernmost area where the Brent Group is not recognised, the Dunlin Group is often unconformably overlain by the Viking Group.
Distribution
The formation is widely distributed throughout the East Shetland Basin and northern Horda Platform. It thins towards the west and south, where it may be absent due to erosion. The sand developments within the formation seem to be a function of marginal position within of the basin.
Age
Toarcian - Bajocian
Depositional environment
The Drake Formation is generally considered to
have been deposited in prodelta and delta front
environments
