Rannoch Formation (elevated)
(From NPD Bulletin no. 3)
Brent Group
Name
Named by Deegan and Scull (1977) who gave it "sub-unit" status.
Well type section
UK well 211/29-3 (Shell), from 2783 m to 2818 m, coord N 61°08'06", E 01 °43'36.5", (Fig. 11).
Well reference section
Norwegian well 33/9-1 (Mobil), from 2602 m to 2664 m, coord N 61°15'07.5", E 01°50'25.8", (Fig. 11).
Thickness
35 m in the type well and 62 m in 33/9-1.
Lithology
In the type well the formation is a light brown, fine grained, well sorted, friable, very micaceous sandstone. In the Brent Field area the lower part is more argillaceous with siltstones and thin shales. Towards the top the formation tends to be coarser grained and less micaceous, often resulting in a distinctive gamma ray pattern suggesting a "coarsening upward" sequence. Some workers consider that the Rannoch Formation is occasionally represented on the Horda Platform by one or more such coarsening upward cycles. However, in the majority of cases these cannot be observed and the presence of the Rannoch Formation must be considered debatable.
Boundaries
The high mica content of the sandstone produces an anomalously high gamma ray log response and generally distinguishes the Rannoch Formation from the overlying and underlying sandstones. Where the Broom Formation is missing the lower boundary is gradational into the dark, silty shales of the Dunlin Group. The upper boundary is normally defined by the "blocky" gamma ray log of the overlying Etive Formation.
Distribution
The distribution of the Rannoch Formation is essentially the same as that of the Brent Group, except on the Horda Platform where the presence of the formation is under debate (see "lithology").
Age
Late Toarcian to Bajocian.
Depositional environment
The formation is generally interpreted as delta front sheet sands and/or prograding shoreface
sands.
