The oil began to migrate into the reservoir during
early to mid-Tertiary times, when the source rock
(Kimmeridge Clay) began to be mature. The oil/
water contact in the Statfjord Formation and the
Brent Group is at different levels which relate to two
different pressure cells. The reservoir lies at 2.5–3 km
depth and the sandstones have little quartz cement,
such that loose sand is often recovered instead of
solid core samples. There is also a danger of some
sand coming up during oil production.
During recent years, increasing focus has been set
on the prospectivity of sands in the post-rift sequences,
and some discoveries in different settings have been
made (e.g. the Agat and Grane fields). The greatest
exploration success during recent years is the Edvard
Grieg and Johan Sverdrup oil discoveries in southern
parts of the Utsira High (see Chap. 26 for more
details).
Some of the reservoirs in the northern North Sea
leak oil or gas, and the gas in particular is seen in
seismic reflection data. The leakages are probably
the result of pressure having built up to the point
where the cap rock has fractured, as at Gullfaks
(block 34/10). This means that some of the oil has
migrated up through the Cretaceous shale cap rock
and into the Cenozoic succession. Parts of this oil and
gas have accumulated in Palaeocene and Eocene
sandstones that were deposited as turbidites
emanating from the Shetland Platform to the west.
After deposition, the eastern side towards the
Norwegian coast was uplifted, imparting a gentle
westward dip to the top of these sandstone beds.
They now form traps, as in the Frigg Field where
the reservoir is Eocene turbidite sands. Heimdal,
Balder and Grane fields also contain Palaeocene and
Eocene sandstones, which are good reservoir rocks.
Eocene clays rich in smectite make good cap rocks.
These clays are lightly compacted, have low seismic
velocities and often form small diapirs. During the
Palaeocene a lot of sediment was supplied from the
west, but these beds also dip westwards and rarely
have closure towards the east.
In the northern North Sea a gas field (Peon) is found
in the Quaternary glacial sediments only about 160 m
below the seafloor. This shows that compacted glacial
sediments may serve as a cap rock and be capable of
trapping gas.
25.3 Mid-Norwegian Shelf/Margin
25.3.1 Structure
Along strike the mid-Norwegian margin comprises
three main segments (Møre, Vøring and LofotenVestera ˚len), each 400–500 km long, separated by the
East Jan Mayen Fracture Zone and Bivrost Lineament/
Transfer Zone (Fig. 25.10).
The Møre Margin is characterised by a narrow shelf
and a wide/gentle slope, underlain by the wide and
deep Møre Basin with its thick Cretaceous fill
(Figs. 25.11 and 25.12). The inner flank of the Møre
Basin is steeply dipping basinward and the crystalline
crust thins rapidly from >25 km to <10 km. The
sedimentary succession is thickest along the western
part of the basin, 15–16 km, decreasing landwards to
12–13 km. The Møre Basin comprises sub-basins
separated by intrabasinal highs formed during Late
Jurassic-Early Cretaceous rifting. Most of the structural relief was filled in by mid-Cretaceous time. Sill
intrusions are widespread within the Cretaceous succession in central and western parts of the Møre Basin,
and lava flows cover the western part. Seaward of the
Faeroe-Shetland Escarpment, at the Møre Marginal
High, thickening of the crystalline crust and
shallowing of the pre-Cretaceous sediments and top
crystalline basement occur near the continent-ocean
transition.
The 500 km wide Vøring Margin comprises, from
southeast to northwest, the Trøndelag Platform, the
Halten and Dønna terraces, the Vøring Basin and the
Vøring Marginal High (Figs. 25.10, 25.11, 25.12 and
25.13). The Trøndelag Platform has been largely stable since Jurassic time and includes deep basins filled
by Triassic and Upper Palaeozoic sediments. The
Vøring Basin can be divided into a series of sub-basins
and highs, mainly reflecting differential vertical
movements during the Late Jurassic–Early Cretaceous
basin evolution.
The Vøring Plateau is a distinct bathymetric feature
(Fig. 25.1), and includes the Vøring Marginal High
and the Vøring Escarpment. The Vøring Marginal
High consists of an outer part of anomalously thick
oceanic crust, and a landward part of stretched continental crust, covered by thick Early Eocene basalts
and underplated by mafic intrusions.
620
J.I. Faleide et al.
early to mid-Tertiary times, when the source rock
(Kimmeridge Clay) began to be mature. The oil/
water contact in the Statfjord Formation and the
Brent Group is at different levels which relate to two
different pressure cells. The reservoir lies at 2.5–3 km
depth and the sandstones have little quartz cement,
such that loose sand is often recovered instead of
solid core samples. There is also a danger of some
sand coming up during oil production.
During recent years, increasing focus has been set
on the prospectivity of sands in the post-rift sequences,
and some discoveries in different settings have been
made (e.g. the Agat and Grane fields). The greatest
exploration success during recent years is the Edvard
Grieg and Johan Sverdrup oil discoveries in southern
parts of the Utsira High (see Chap. 26 for more
details).
Some of the reservoirs in the northern North Sea
leak oil or gas, and the gas in particular is seen in
seismic reflection data. The leakages are probably
the result of pressure having built up to the point
where the cap rock has fractured, as at Gullfaks
(block 34/10). This means that some of the oil has
migrated up through the Cretaceous shale cap rock
and into the Cenozoic succession. Parts of this oil and
gas have accumulated in Palaeocene and Eocene
sandstones that were deposited as turbidites
emanating from the Shetland Platform to the west.
After deposition, the eastern side towards the
Norwegian coast was uplifted, imparting a gentle
westward dip to the top of these sandstone beds.
They now form traps, as in the Frigg Field where
the reservoir is Eocene turbidite sands. Heimdal,
Balder and Grane fields also contain Palaeocene and
Eocene sandstones, which are good reservoir rocks.
Eocene clays rich in smectite make good cap rocks.
These clays are lightly compacted, have low seismic
velocities and often form small diapirs. During the
Palaeocene a lot of sediment was supplied from the
west, but these beds also dip westwards and rarely
have closure towards the east.
In the northern North Sea a gas field (Peon) is found
in the Quaternary glacial sediments only about 160 m
below the seafloor. This shows that compacted glacial
sediments may serve as a cap rock and be capable of
trapping gas.
25.3 Mid-Norwegian Shelf/Margin
25.3.1 Structure
Along strike the mid-Norwegian margin comprises
three main segments (Møre, Vøring and LofotenVestera ˚len), each 400–500 km long, separated by the
East Jan Mayen Fracture Zone and Bivrost Lineament/
Transfer Zone (Fig. 25.10).
The Møre Margin is characterised by a narrow shelf
and a wide/gentle slope, underlain by the wide and
deep Møre Basin with its thick Cretaceous fill
(Figs. 25.11 and 25.12). The inner flank of the Møre
Basin is steeply dipping basinward and the crystalline
crust thins rapidly from >25 km to <10 km. The
sedimentary succession is thickest along the western
part of the basin, 15–16 km, decreasing landwards to
12–13 km. The Møre Basin comprises sub-basins
separated by intrabasinal highs formed during Late
Jurassic-Early Cretaceous rifting. Most of the structural relief was filled in by mid-Cretaceous time. Sill
intrusions are widespread within the Cretaceous succession in central and western parts of the Møre Basin,
and lava flows cover the western part. Seaward of the
Faeroe-Shetland Escarpment, at the Møre Marginal
High, thickening of the crystalline crust and
shallowing of the pre-Cretaceous sediments and top
crystalline basement occur near the continent-ocean
transition.
The 500 km wide Vøring Margin comprises, from
southeast to northwest, the Trøndelag Platform, the
Halten and Dønna terraces, the Vøring Basin and the
Vøring Marginal High (Figs. 25.10, 25.11, 25.12 and
25.13). The Trøndelag Platform has been largely stable since Jurassic time and includes deep basins filled
by Triassic and Upper Palaeozoic sediments. The
Vøring Basin can be divided into a series of sub-basins
and highs, mainly reflecting differential vertical
movements during the Late Jurassic–Early Cretaceous
basin evolution.
The Vøring Plateau is a distinct bathymetric feature
(Fig. 25.1), and includes the Vøring Marginal High
and the Vøring Escarpment. The Vøring Marginal
High consists of an outer part of anomalously thick
oceanic crust, and a landward part of stretched continental crust, covered by thick Early Eocene basalts
and underplated by mafic intrusions.
620
J.I. Faleide et al.