Heather Formation
(From NPD Bulletin no. 3)
Viking Group
Name
Named by Deegan and Scull (1977).
Well type section
UK well 211/21-1A (Shell), from 2810 m to 2840 m, coord N 61°11'9.6", E 01°06'5.7", (Fig. 18).
Well reference sections
Norwegian wells 33/9-1
(Mobil), from 2450 m to 2464m, coord N61°15°07.5" E01°50'25.8"
(Fig. 11), 31/2-1
(Shell) from 1531.5 m to 1594.5 m, coord N 60°46'19.16'°,
E 03°33' 15.87" (Fig. 21), and
15/3-1 (Elf) from 4754 m to 4986 m, coord N 58°50'57.0",
E01°43' 13.25", (Fig. 19).
Thickness
30 m in the type well, but attains thicknesses in the order of a thousand metres in graben areas. In the reference wells the thicknesses are 14 m (33/9-1), 63 m (31/2-1) and 232 m (15/3-1).
Lithology
The formation consists of mainly of grey silty claystone with thin streaks of limestone. A further subdivision of the formation is possible (Fig. 18), although no formal status is proposed for this subdivision. Two divisions are commonly recognized. The lower division is light to dark grey, hard, silty claystone, often micaceous and calcareous. The upper division is separated into two further units by an unconformity detected by dipmeter data or biostratigra-phical gaps, but no distinct lithological difference is noted. The lithology is dark grey silty claystone, carbonaceous in part with limestone streaks. (For further discussion see Deegan and Scull (1977) p. 18). On the Horda Platform where the Heather Formation interdigitates with sandstones of the Krossfjord, Fensfjord and Sognefjord Formations, it becomes in places highly micaceous and may grade into a sandy siltstone.
Boundaries
The lower boundary is the contact with the arenaceous
Brent Group. The upper boundary is the contact with the
Draupne Formation, which has an anomalously high
gamma ray response and low velocity. Both boundaries are therefore marked
by log breaks.
Distribution
The formation can be recognized over most of the northern North Sea north of 58°N and east of the East Shetland Platform boundary faults.
Occurrences of formation tops in wells
Age
Bathonian to Kimmeridgian
Depositional environment
The silty claystones of the Heather Formation were deposited in an open marine environment, brought about by the marine transgression which initially deposited the youngest formation of the Brent group.
Remarks
Use of the Heather Formation is here restricted to the part of the North Sea north of approximately 58°N. Deegan and Scull (1977) indicated the presence of the formation in the Central Graben, but did not describe the area in any detail. Recent work (e.g. Ofstad 1983, and this report) suggests a subdivision of the Middle Jurassic-earliest Cretaceous claystones of this region into three new formations. None of these can alone be referred to the northern Heather Formation, and the name has therefore not been used in the southern area.
