Fjerritslev Formation
(From NPD Bulletin no. 3)
Name
After the village of Fjerritslev, Jutland, Denmark, (Larsen, 1966)
Well type section
Fjerritslev No 2 well, Jutland, (Larsen 1966).
Well reference sections
In Norwegian waters wells 17/9-1 (Esso) from 2835 m to 2992 m, coord N 58°28'27.26", E O3°5O' 16.18" (Fig. 25), and 7/9-1 (Conoco) from 2524 m to 2601 m, coord N 57°20' 37.10", E02°51'21.4" (Fig. 24).
Thickness
In the Norwegian reference wells the thickness of the Fjerritslev Formation is 157 m (17/9-1) and 77 m (7/9-1).
Lithology
The formation consists predominantly of grey to dark grey or greyish brown marine claystone. It is variably calcareous and pyritic. Silty intervals occur frequently, grading into grey or buff micaceous siltstone.
In the Danish area the Fjerritslev Formation is divided into four members according to degree of siltiness (Michelsen, 1978). However, such a subdivision is not merited in the Norwegian sector.
Boundaries
The formation is distinguished from the underlying sandy deposits of the Gassum and Skagerak Formation, and from the overlying sands of the Vestland Group, by its higher gamma ray and lower sonic log velocity readings.
In reference well 17/9-1 the Fjerritslev Formation is overlain by a sequence of interbedded lavas and sediments over 500 m thick. They are belived to be of Lower-Middle Jurassic age (Dixon et al., 1980). This sequence has only been identified in one well, and is not named in this report. The boundary between the Fjerritslev Formation and the volcanics is again made by an upward change to lower gamma ray readings and higher sonic log velocities, (Fig. 25).
Distribution
The Fjerritslev Formation is only patchily developed in the Norwegian sector. The developments which are present probably represent those remnants of a once more widely distributed deposit which survived the mid-Jurassic ero-sional episode. The formation has been penetrated in two distinct and separate areas; around the Southern Vestland Arch (e.g., blocks 7/9 and 7/12) and in the Egersund Sub-Basin (e.g. block 17/9).
Age
The formation ranges in age from Hettangian to Pliensbachian. It is approximately equivalent to the Lower Jurassic Dunlin Group of the Northern North Sea, although no direct connection between the two sequences is thought likely.
In reference well 17/9-1 a dyke immediately below the Fjerritslev Formation has been dated as Pliensbachian (Furnes et al., 1982).
Depositional environment
The claystones of the Fjerritslev Formation are shallow marine sediments deposited during a widespread marine transgression.
Remarks
In block 17/12 (Bream area) a sequence of continental elastics has been dated as Pliensbachian to Toarcian (Olsen and Strass, 1982). They are therefore partially age-equivalent to the Fjerritslev Formation, but cannot on lithological grounds be referred to the latter, (Table 4). These deposits have not been named by the present nomenclature group.
