Haugesund Formation
(From NPD Bulletin no. 3)
Tyne Group
Name
After the town of Haugesund on the west coast of Norway.
Well type section
Norwegian well 2/7-3
(Phillips) from 3695 to 4191 m, coord N 56°23'02.9", E 03°15'45.9'7 (Fig. 40).
See "remarks" for qualification of this well type section.
Well reference sections
Norwegian well 3/5-2
(Gulf) from 3182.5 to 3345 m, coord N 56°32'34.46", E 04°23'11.1", (Fig. 42)
and 2/8-3 (Amoco) from
3761 m to 4115 m (T.D.), coord N56°18'31", E03°26'54.1" (Fig. 41).
Thickness
496 m in the type well, 162.5 m in 3/5-2, and 354 m in 2/8-3. The formation
is thickest in the axis of the Central Graben and thins towards the flanking
highs, where it passes partially
or entirely into the sandy lithology of
the Ula Formation.
Lithology
The Haugesund Formation consists predominantly of shale
ranging in colour from light grey to brownish black. The shale is often
carbonaceous and calcareous, and contains frequent
thin sandstone interbeds. In general the upper part of the formation
represents an overall "coarsening-upward cycle", becoming sandier and
siltier upwards.
Boundaries
In the type well, 2/7-3, the Haugesund Formation overlies Zechstein salt and
the base of the formation is therefore obvious from both logs and cuttings.
However, the Zechstein salt is almost certainly penetrative at this
location and does not therefore provide a true stratigraphic base for
Haugesund Formation (see "remarks". In the reference well 2/8-3 and elsewhere
in the Central Graben the base of the Haugesund Formation is often found
overlying the Vestland Group. This is shown in the
reference well 3/5-2 where the upward change from the sandy
Bryne Formation to the shales of the Haugesund
Formation produces the expected gamma ray/sonic log break (see Fig. 42).
The top of the Haugesund Formation in the type well is the contact with
the sandy Eldfisk Formation. In areas of the Central
Graben where the Eldfisk formation is absent, the top of the Haugesund
Formation is picked at a clearly correctable gamma ray minimum, above
which the gamma ray increases to the higher values of the basal
Farsund Formation (e.g. well 2/8-3).
Distribution
The formation is ubiquitous in the Central Graben
and widely distributed around the flanks of the basin and intra-basinal
highs. It is absent in the Ula Field where it is entirely replaced by
time-equivalent sands of the Ula Formation, and is
also absent on the crest of the Southern Vestland Arch and intra-basinal highs.
Age
Callovian to Early Kimmeridgian.
In neither the type nor the reference wells have pre-late Oxfordian ages
been proven but Callovian mudstones assignable to the Haugesund Formation
occur in the vicinity of the reference well, 3/5-2.
Depositional environment
The bulk of the shales of the Haugesund
Formation were deposited in a marine, low energy, basinal environment.
The common thin sand interbeds may represent sporadic turbidite influxes
emanating from the adjacent shelf where coarser elastics (i.e. the
Ula
Formation) were being deposited. The "coarsening-upward" nature of the
sequence represents an overall' regression which was terminated by a further
transgression and the deposition of the Farsund
Formation shales.
Remarks
The type well 2/7-3 penetrated a thick development of the Haugesund
Formation, considered to be typical of the formation as it is commonly
encountered in the Central Graben. However, the inadequately defined base
of the formation makes 2/7-3 ultimately unsatisfactory as a type well.
None of the other Central Graben wells available to this study establish
a base for this formation, and penetrations on the flanks of the basin
(such as the reference well 3/5-2) are fewer, atypical and potentially
controversial stratigraphically. Penetration of a well-defined base for
the Haugesund Formation by a future well in the Central Graben would
perhaps provide a rare instance in which replacement of a type well might
be justified.
