petroleum systems/plays: (1) the Central Graben area,
and (2) the northern North Sea (Fig. 25.5).
The most important source rocks in the North Sea
are shales in the Upper Jurassic. Thick beds of shale
rich in organic matter were deposited over most of
the North Sea area in Late Jurassic time. In the Norwegian sector, these belong to the Draupne Formation
in the Viking Group or the Mandal Formation in the
Tyne Group. Middle Jurassic coal is another important
source rock, mainly for gas. In the southern part of the
Norwegian sector, this coal is found in the Bryne
Formation in the Vestland Group. In the northern
part of the North Sea, these coal seams are found in
the Brent Group. Source rocks may possibly be found
in Carboniferous or older strata in the southern part
of the North Sea, but these have so far not been
confirmed.
25.2.3.1 Central Graben
In the Central Graben area rifting took place during
Permian-Early Triassic and Middle-Late Jurassic
times, and Zechstein salt was deposited north of the
Mid North Sea High (Figs. 25.8 and 25.9). When this
area was first drilled, the target was the Permian
sandstones beneath the salt. It was by accident that
oil was discovered in the Upper Cretaceous rocks
(Chalk). Ekofisk was the first field to be discovered,
but now there is a string of fields with Upper Cretaceous reservoirs, both on the Norwegian side and in
the Danish Sector.
There are other sandstone reservoirs too, such as
those of Upper Jurassic age in the Ula and Gyda fields.
This lead is found south of the Brent delta, which
excluded the Brent Group from forming reservoir
rocks there. The area was uplifted and Middle Jurassic
sediments are mostly absent. Later, oil has been found
in Upper Palaeozoic reservoir rocks which had been
the original prospecting target in the area. In the
Embla Field the reservoir rock is a sandstone believed
to be of Devonian age.
The Ekofisk Field, found in 1969, was the first large
oilfield in Europe. The Chalk in the Ekofisk reservoir
is of the same type as we have onshore in Denmark
and eastern England in stratigraphic levels
approaching the Cretaceous/Cenozoic boundary
(Maastrichtian and Danian). Chalk lithologies had
previously been assumed to be far too fine-grained to
be reservoir rocks, and Ekofisk was the first large
oilfield of this type. The Austin Chalk in Texas is
one of the few other occurrences where Chalk forms
a reservoir rock, but the fields there are fairly small by
comparison.
Chalk
is
comprised
of
microscopic
(0.001–0.005 mm) skeletons of planktonic algae
which floated in the surface waters. After they died
they sank to the seafloor to accumulate as a calcareous
ooze. First when the scanning electron microscope
(SEM) was developed was it possible to study these
algae. The coccolithoporid skeleton consists of calcite,
which makes the sediment more stable during diagenesis. We do not find as much solution and
reprecipitation as would have been the case if the
fossils had been aragonitic. There was uplift and
some erosion of the Chalk in the Danian (earliest
Cenozoic) with a degree of resedimentation of Chalk
beds along the slopes of the shallower areas, i.e.
slumping. These redeposited Chalk sediments have
proved to be the ones with the best reservoir
characteristics.
The Chalk beds were then buried by Palaeocene
and Eocene clays which sealed the Chalk and
prevented the freshwater circulation which we otherwise often find in carbonate rocks along continental
shelves. These clays have high contents of expanding
clay minerals (smectite) and form a layer with very
low permeability, so that porewater could not be
forced out quickly enough with respect to the subsidence rate. The effective stresses and hence compaction have therefore been strongly reduced, and this is
an important factor explaining why the porosity can
still be as high as 30–35%. Both mechanical compaction and chemical pressure solution are reduced by the
overpressure. The Ekofisk reservoir lies at over 3 km
depth and without overpressure there would not have
been such good porosity. In the shallower Valhall
reservoir the effective stresses are even lower.
The Ekofisk structure is a dome-shaped anticlinal
which has formed in response to an underlying salt
diapir (Upper Permian). The structure covers c.50 km
2
and has a closure height of 244 m. The oil column is
306 m in thickness, which means there is oil below the
structure’s lowest point (spill point). This situation
requires an additional diagenetic trap, i.e. low permeability coupled with the capillary forces in the Chalk
hinder the oil from seeping out laterally from below
the structural trap. The porosity within the reservoir
rock varies from 30 to 35% to tighter layers with
0–20%. However, in addition to this primary porosity
25 Geology of the Norwegian Continental Shelf
617
and (2) the northern North Sea (Fig. 25.5).
The most important source rocks in the North Sea
are shales in the Upper Jurassic. Thick beds of shale
rich in organic matter were deposited over most of
the North Sea area in Late Jurassic time. In the Norwegian sector, these belong to the Draupne Formation
in the Viking Group or the Mandal Formation in the
Tyne Group. Middle Jurassic coal is another important
source rock, mainly for gas. In the southern part of the
Norwegian sector, this coal is found in the Bryne
Formation in the Vestland Group. In the northern
part of the North Sea, these coal seams are found in
the Brent Group. Source rocks may possibly be found
in Carboniferous or older strata in the southern part
of the North Sea, but these have so far not been
confirmed.
25.2.3.1 Central Graben
In the Central Graben area rifting took place during
Permian-Early Triassic and Middle-Late Jurassic
times, and Zechstein salt was deposited north of the
Mid North Sea High (Figs. 25.8 and 25.9). When this
area was first drilled, the target was the Permian
sandstones beneath the salt. It was by accident that
oil was discovered in the Upper Cretaceous rocks
(Chalk). Ekofisk was the first field to be discovered,
but now there is a string of fields with Upper Cretaceous reservoirs, both on the Norwegian side and in
the Danish Sector.
There are other sandstone reservoirs too, such as
those of Upper Jurassic age in the Ula and Gyda fields.
This lead is found south of the Brent delta, which
excluded the Brent Group from forming reservoir
rocks there. The area was uplifted and Middle Jurassic
sediments are mostly absent. Later, oil has been found
in Upper Palaeozoic reservoir rocks which had been
the original prospecting target in the area. In the
Embla Field the reservoir rock is a sandstone believed
to be of Devonian age.
The Ekofisk Field, found in 1969, was the first large
oilfield in Europe. The Chalk in the Ekofisk reservoir
is of the same type as we have onshore in Denmark
and eastern England in stratigraphic levels
approaching the Cretaceous/Cenozoic boundary
(Maastrichtian and Danian). Chalk lithologies had
previously been assumed to be far too fine-grained to
be reservoir rocks, and Ekofisk was the first large
oilfield of this type. The Austin Chalk in Texas is
one of the few other occurrences where Chalk forms
a reservoir rock, but the fields there are fairly small by
comparison.
Chalk
is
comprised
of
microscopic
(0.001–0.005 mm) skeletons of planktonic algae
which floated in the surface waters. After they died
they sank to the seafloor to accumulate as a calcareous
ooze. First when the scanning electron microscope
(SEM) was developed was it possible to study these
algae. The coccolithoporid skeleton consists of calcite,
which makes the sediment more stable during diagenesis. We do not find as much solution and
reprecipitation as would have been the case if the
fossils had been aragonitic. There was uplift and
some erosion of the Chalk in the Danian (earliest
Cenozoic) with a degree of resedimentation of Chalk
beds along the slopes of the shallower areas, i.e.
slumping. These redeposited Chalk sediments have
proved to be the ones with the best reservoir
characteristics.
The Chalk beds were then buried by Palaeocene
and Eocene clays which sealed the Chalk and
prevented the freshwater circulation which we otherwise often find in carbonate rocks along continental
shelves. These clays have high contents of expanding
clay minerals (smectite) and form a layer with very
low permeability, so that porewater could not be
forced out quickly enough with respect to the subsidence rate. The effective stresses and hence compaction have therefore been strongly reduced, and this is
an important factor explaining why the porosity can
still be as high as 30–35%. Both mechanical compaction and chemical pressure solution are reduced by the
overpressure. The Ekofisk reservoir lies at over 3 km
depth and without overpressure there would not have
been such good porosity. In the shallower Valhall
reservoir the effective stresses are even lower.
The Ekofisk structure is a dome-shaped anticlinal
which has formed in response to an underlying salt
diapir (Upper Permian). The structure covers c.50 km
2
and has a closure height of 244 m. The oil column is
306 m in thickness, which means there is oil below the
structure’s lowest point (spill point). This situation
requires an additional diagenetic trap, i.e. low permeability coupled with the capillary forces in the Chalk
hinder the oil from seeping out laterally from below
the structural trap. The porosity within the reservoir
rock varies from 30 to 35% to tighter layers with
0–20%. However, in addition to this primary porosity
25 Geology of the Norwegian Continental Shelf
617