Shetland Group (Shetlandsgruppen)
(Original definition in NPD Bulletin no. 5)
Name
Named from the Shetland Islands off the north coast of Scotland (Deegan & Scull 1977). The group has now been expanded to include the formations of the former Chalk Group.
Type area
The group is typically developed in the central and Northern North Sea.
A chalk facies is developed in the central North Sea and a siliclastic facies
in the Northern North Sea (Fig. 32 a). A typical section of the chalk
facies in the central area is represented by Norwegian well
1/3-1 (Fig. 24),
while Norwegian well 25/1-1
(Fig. 33) provides a typical section of the siliciclastic
facies in the northern area. UK well 22/1-2A illustrates a section
in the transition zone between the two facies (Figs. 5 and 25).
Thickness
In well 1/3-1 the group is 1183 m thick, and in well 25/1-1 it measures 1284 m. Seismic interpretation and well data indicate that the thickness of the group ranges between 1000 and 2000 m in the graben areas. The group shows considerable thinning towards and in the platform areas.
Depth to the top of the Shetland Group in the Norwegian Sea based on released well data.
Click for PDF.
Interactive Norlex map of same.
3D image of the depth to the top of the Shetland Group (revised definition) in the Norwegian Sea based on
released well data.
Note that the figure is viewed from a location in the SE (Møre Basin) looking towards the NW (Trøndelags
Platform). The green arrow is the North pointer.
The grid is based on the updated NorLex tops data for this group and the revised NorLex definition of the Shetland
Group.
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Regional isopach of the Shetland Group thickness in the Norwegian Sea based on released well data.
The isochore map is generated from Norlex data using thin plate splines (thickness constrained to original range).
Thicknesses in metres.
Circled well contain both top and base horizons. In the case of the Shetland Group this includes wells with a well TD within
the group.
The red wells have Norlex biostratigraphy. Click for large version.
Interactive Function:
Note that this map is only a regional interpretation and the user can generate more specific, local area isochore maps
interactively within Norlex using the link below.
Interactive Norlex isochore map for the Shetland Group
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Lithology
The group consists of the chalk facies of chalky limestones, limestones,
marls, and calcareous shales and mudstones. Chert (flint) occurs
throughout the facies. The siliciclastic facies consists of mudstones
and shales, partly interbedded with limestones. Minor amounts of
sandstones are present in the lower part in the Agat Field area
(block 35/3). The shales and sandstones are slightly to very calcareous.
In the Maastrichtian sequence the quantity of limestones are generally
higher on the Horda Platform than in the Viking Graben.
Logs in reference well 6506/12-1
Logs in reference well 6506/12-4
Basal stratotype
Typically the lower boundary is the contact to the calcareous mudstones
or marlstones of the Cromer Knoll Group.
On structural highs like the Horda Platform, Tampen Spur, Sørvestlandet
and Mandal Highs the lower part of the group is occasionally absent and
the remainder rests unconformably on the Cromer Knoll Group, Jurassic or
older rocks.
Characteristics of the upper boundary
The group is overlain by Paleocene mudstones, marls
or sandstones of the Rogaland Group.
Distribution
The group is present throughout the Norwegian North Sea, being absent
only locally on highs (e.g. 16/5-1,
31/2-9)
and a few salt diapirs (e.g.
2/7-12). A
transition between the chalk and siliciclastic facies
of the group occurs relatively abruptly in the Norwegian sector along
the Utsira High (Figs. 32a and b)and more gradually in the graben areas.
Occurrences of group tops in wells
Age
The group ranges in age from Cenomanian to Danian.
The siliciclastic facies is restricted in age to the Late
Cretaceous.
Depositional environment
The Upper Cretaceous sequence in the North Sea was deposited in an
open marine environment during a general rise in sea level
(Hancock & Kauffman 1979). The chalk facies formations were deposited
as coccolith debris and other carbonate grains and sequences often
show a cyclic pelagic sedimentation pattern termed periodite (d'Heur 1986).
In the Central Trough, extensive subsidence resulted in the chalk facies
being dominated by allochthonous, redeposited chalks which were transported
downslope as major slides, slumps, debris flows, and proximal and distal
turbidites. The siliciclastic facies is less well studied. The influx of
siliciclastic mud was higher and the carbonate production probably
lower than in the area with chalk facies.
Subdivision
The Shetland Group is represented by four chalk facies formations:
the Hidra, Hod,
Tor and Ekofisk
Formations (all erected by Deegan & Scull 1977) and six siliciclastic
facies formations: the Svarte (new),
Blodøks (new), Tryggvason
(new), Kyrre (new), Jorsalfare
(new) and Hardråde (new) Formations.
The Herring and Flounder Formations in the UK sector (Deegan & Scull 1977)
are regarded as equivalents of the lowermost part of the Hod Formation and
of the Kyrre Formation, respectively (Fig. 5 and 6).
