Cook Formation (elevated)
(From NPD Bulletin no. 3)
Dunlin Group
Name
Named by Deegan and Scull (1977) who gave it "sub-unit" status.
Well type section
UK well 211/29-3 (Shell), from 2887 m to 2950.5 m, coordN 61°08'06", E 01°43'36.5", (Fig. 10).
Well reference sections
Norwegian wells 33/9-1 (Mobil), from 2715 m to 2801 m, coord N 61 ° 15' 07.5 ", E01°50'25.8" (Fig. 11), 30/6-7 (Norsk Hydro), from 2975 m to 3023 m, coord N 60°38'39.49'' E 02°45'21.74" (Fig. 12), 31/2-1 (Shell) from 2093 m to 2134 m, coord N 60°46' 19.16" E 03°33'15.87" (Fig. 13), and 30/7-7 (Norsk Hydro) from 4735 m to 4801 m, coord N 60° 16'19.30", E 02° 16'07.30", (Fig. 14).
Thickness
63.5 m in the type well,
86 m in 33/9-1,
66 m in
30/7-7,
48 m in 30/6-7,
41 m in 31/2-1.
Lithology
In the type well section the formation is dominantly a marine siltstone with minor grey, silty claystone intercalations. The siltstones and clay-stones may contain sandy streaks, becoming more prominent away from the type well, especially in Norwegian waters. On the Horda Platform and along its western margin, sandstones are the dominant lithology in the formation. The sands are white to greyish brown, very fine to fine grained, subangular to subrounded and well sorted. Occasionally thin layers of medium to coarse grained sandstones are found. The sandstones are hard to friable. Silica is the most common cement. Mica, glauconite, carbonaceous material and calcareous cement may be present.
Boundaries
The formation can be distinguished from the more uniform and more argillaceous sediments above and below by a decrease in gamma ray response and an increase in velocity.
Distribution
The formation appears to be present throughout the East Shetland Basin and on the northern part of the Horda Platform.
Age
Pliensbachian - Toarcian.
Depositional environment
The sandstones can be divided into three types, related to depositional environment and basin geometry. In the Statfjord Field area the sandstones are believed to represent marine shoal sands (e.g. ref. well 33/9-1). On the Horda Platform and along its western margin the sandstones represent prograding shelf sands and several cycles can be identified within the formation (ref. 31/2-1 and 30/6-1). In the gra-ben area the sands are thinner bedded, and the shale intercalations show no gradations into the sands (ref 30/7-7). These sandstones are believed to represent redeposited sands from the edge of the shelf (the Horda Platform and East Shetland Basin west of the graben area).
