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NORLEX


Boknfjord Group (new)

(From NPD Bulletin no. 3)

Name

From the main fjord in Rogaland, Norway.

Type area

The Fiskebank and Egersund Sub-Basins comprise the type area. The group is illustrated in the Norwegian well 9/4-3 (Conoco), (Fig. 36).

Thickness

In the type area wells show thicknesses of up to 500 m. In well 9/4-3 the thickness of the group is 290 m. Towards the basin margins the section thins considerably.

Lithology

The sediments of the group are dominated by shales. Varying amounts of siltstone, sandstone, limestone stringers and differences in organic content make it possible, however, to subdivide the group into formations (Olsen and Strass, 1982).

Boundaries

The lower boundary is characterized by a distinct log break with the underlying sandstones of the Vestland Group. The upper boundary is usually characterized by abrupt changes in log response to lower gamma ray and interval transit times in the overlaying Valhall Formation. In the Egersund Sub-Basin this boundary may be difficult to identify due to a large supply of clastic material.

Distribution

The group is confined to the Fiskebank and Egersund Sub-Basins although the upper two formations extend further westwards than those lying below.

Occurrences of group tops in wells
Isopach map VESTLAND-BOKNFJORD

Age

The group ranges in age from Callovian to Ryazanian.

Subdivisions

The group can be subdivided into four formations, the Egersund (base), Tau, Sauda and Flekkefjord Formation (top).

Remarks

The term "Bream Formation" was first used by Deegan and Scull (1977) to describe a Callovian-Portlandian (Volgian) sequence, mainly claystones and siltstones, distributed throughout the Norwegian-Danish Basin. The formation comprised the Egersund Member, BÝrglum Member and Fredrikshavn Member. The Bream Formation was adopted with some modification by Michelsen (1978) for the Danish Sub-Basin where it comprises the Borglum and Fredrikshavn Members. Recent correlation work in the Egersund and Fiskebank Sub-Basin (e.g. Olsen and Strass, 1982) shows the existence of four argillaceous units, ranging in age from Callovian-Ryazanian, which are considered stratigraphically useful. They are widespread enough to deserve formation status and different enough from the Danish deposits to merit separate nomenclature. These units are the Egersund, Tau, Sauda and Flekkefjord Formations. The formal definitions of the unit in this volume outline their relationship to the Egersund/Barlum/ Fredrikshavn Members of Deegan and Scull (1977). Note that the Flekkefjord Formation was formerly part of the early Cretaceous Valhall Formation, also defined by Deegan and Scull.

The four formations fall naturally into a single "claystone" group. It is not however considered proper to elevate the former Bream Formation to a "Bream Group" which would encompass these units, since it may still desirable to retain the formation status of the Bream Formation and the member status of the Borglum/Fredrikshavn Members in the Danish sector.

It is therefore proposed that the term "Bream Formation" should be abandoned for the Fiskebank and Egersund Sub-Basins, and replaced by the Boknfjord Group, which is defined above.

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