Gassum Formation
(From NPD Bulletin no. 3)
Name
After the village of Gassum, Jutland, Denmark,
(Larsen, 1966).
Well type section
Danish well Gassum No. 1 from 1613 m to 1643 m below ground (Ground evelation 53.3 m) (Bertelsen, 1978).
Well reference sections
In Norwegian waters wells 17/10-1 (Shell) from 2682 m to 2825 m, coord N 58°01' 54",
E 03°09'58", (Fig. 23), and 7/9-1 (Conoco) from 2601 m to 2609 m, coord N 57°20'37.1'', E 02°51'21.4", (Fig. 24). The section in well 17/10-1 was included by Deegan and Scull (1977) in the Skagerak Formation.
Thickness
In the Norwegian reference wells the thickness of the Gassum Formation is 143 m (17/10-1) and 8 m (7/9-1).
Lithology
In the type well the formation consists of predominantly light grey to whitish, in places rather coarse-grained sandstones, with subordinate dark-coloured clay bands and coal lenses (Larsen, 1966). In Norwegian waters the formation is predominantly a white to light grey, mainly fine to medium grained sandstone, but frequently contains coarse sand and gravel. It is often calcite cemented and in some instances contains glauconite.
Boundaries
In the Norwegian sector the lower boundary may be characterized by a general lowering of the velocity when entering the underlying Skagerak Formation. Often this boundary coincides with a distinct horizon with high gamma ray response (e.g. well 17/4-1 (Elf) and well 9/12-1 (Shell)).
The upper boundary of the Gassum Formation is easily picked where it is overlain by Lower Jurassic shales of the Fjerritslev Formation or Upper Jurassic shales of the Boknfjord or Tyne Groups. Where the Gassum Formation is overlain by the Middle Jurassic Bryne Formation (Norwegian sector), the boundary may only show a slight decrease in the gamma ray response and an overall decrease in velocity marking the appearance of the Bryne Formation.
In the reference well 17/10-1, where the Fjerritslev and Bryne Formations are missing, the Gassum Formation is overlain by the shales of the Boknfjord Group.
Distribution
In the Norwegian sector th eGassum Formation occurs throughout theNOrwegian Danish Basin, on the Southern Vestland Arch and aong the northeastern margin of the Central Graben. Its distribution towards the axial part of the Central Graben is unknown. It is often completely or partially eroded as a result of mid-Jurassic earth movements.
Occurrences of formation tops in wells
Age
Rhaetian in the type well, but seems to become younger northwards (Bertelsen, 1978). Sparse dating in the Norwegian sector gives Rhaetian to Sinemurian ages.
Depositional environment
The sedimentology of the Gassum Formation in the Norwegian sector has not been much studied, but is assumed to represent fluvial to marginal marine deposits laid down during a transgressive phase at the Triassic/Jurassic transition.
