Farsund Formation
(From NPD Bulletin no. 3)
Tyne Group
Name
After the town of Farsund on the south-west coast of Norway.
Well type section
Norwegian well 2/7-3
(Phillips) from 3414 m to 3626 m, coord B 56°23'02.9", E 03°14'45.9" Fig. 40).
Well reference sections
Norwegian well 7/12-2
(BP) from 3306 m to 3378.5 m, coord N 57°06'41.34",
E 02°50'50.73", (Fig. 32) and
2/8-3 (Amoco)
from 3594 m to 3761 m, coord N 56°18'31", E 03°26'54.1", (Fig. 41).
Thickness
200 m in the type well and 72.5 m (7/12-2) and 167 m (2/8-3) in the reference wells. The formation attains its maximum thickness in the axial part of the Central Graben and thins towards the flanking highs.
Lithology
The Farsund Formation consists predominantly of medium to dark grey shale. The shale is often well laminated and contains frequent calcareous streaks. Sandstone stringers are common in the type well 2/7-3, particularly in the lowermost part of the sequence. In the reference well 7/12-2, closer to the flank of the Southern Vestland Arch, a thinner Farsund Formation is present as a clear "coarsening upward cycle", becoming consistently less radioactive towards the top of the unit.
Boundaries
In the type well the base of the Farsund Formation occurs at the top of the
sandy Eldfisk Formation and as a consequence is a
pronounced gamma ray marker. Similarly, in the reference well the Farsund
Formation overlies the sandy Ula Formation and is easily
distinguished on logs. In several wells within the Central Graben, the
Farsund Formation overlies the shaly Haugesund
Formation with no intervening sand (e.g. reference well 2/8-3). Here the base
of the Farsund Formation is picked directly above the gamma ray minimum
which forms the top of the Haugesund Formation.
The top of the Farsund Formation is marked by a further gamma ray minimum.
Above this occurs the distinct log motif of the Mandal
Formation, with its high gamma ray and interval transit time readings.
Distribution
The formation is present throughout the Central Graben but thin or absent over the Southern Vestland Arch and intra-basinal highs.
Age
Kimmeridgian to Volgian.
Depositional environment
The Farsund Formation shales were mainly deposited in a low energy marine
environment. The gamma ray log profile suggests that the formation
represents an initial period of deepening followed by gradual shallowing.
In parts of the Central Graben the occurence of thin sand stringers in
the lower part of the formation probably represent minor turbidite influxes
from the adjacent shelf, where time equivalent sands of the
Ula Formation were being deposited.
