Norwegian and East Shetland hinterlands within a
generally evolving post-rift basin. There was still considerable sediment supply from the Variscan
mountains to the south and there is also evidence of
an uplift of Scandinavia. A broadly similar geometry
of the megasequences in both continental Triassic and
marine Jurassic successions was related to subsidencerate variations. Differential subsidence across faults
throughout Triassic time has also been reported. The
Øygarden Fault Zone, forming the eastern margin of
the Permo-Triassic basin, was active throughout most
of the time interval. The sedimentation rate was in
most cases high enough to keep up with the subsidence, resulting in a rather flat landscape with gently
flowing rivers. If the sediment supply had been less,
these rift basins would have been marine. Along the
rift structure along the middle of the North Sea the
thickness of the Triassic sediments may exceed 5 km.
The underlying Permian salt started to form diapirs
and Triassic sediment was eroded, or not deposited, at
the top of the structures.
The climate in northwest Europe during the Triassic was still arid, and continental red beds continued to
be predominant. In England these are referred to as the
New Red Sandstone because they are similar to the
Devonian continental sediments (Old Red Sandstone).
The equivalent facies in Germany is the Bunter
sandstone of Lower Triassic age. In south England
east of Exmouth there are also good exposures of
Triassic deposits with sandstones and conglomerates
(Sherwood Sandstone Group) and mudstones (Mercia
Mudstone Group). The Sherwood Sandstone is a very
important groundwater reservoir in large areas of
England. In Southern Norway north of Hamar, the
Brumundal sandstone is an outlier of a more extensive
cover of Permian red bed facies.
In the Upper Triassic we find carbonates
(Muschelkalk) and salt deposits (Keuper salt) in the
southern part of the North Sea. In the central and
northern areas, however, continental clastic sedimentation continued right up to the end of the Triassic
(Rhaetian). Sabkha environments fringed the evaporite basins and caliche, that is carbonate precipitation
in soil profiles, is typical. Towards the end of the
Triassic the climate became less arid with more normal fluvial sedimentation and gradually also marine
sedimentation when the Statfjord Formation was
deposited.
The Triassic fluvial sandstones may have variable
sorting and reservoir quality depending upon facies
and distance to basement sources. In the fluvial channel facies there may be well developed clay coatings
on quartz grains, retarding quartz cementation. The
clay coatings have probably formed in the vadose
zone where surface water has filtered down to the
groundwater table.
25.2.2.5 Jurassic
The transition from Triassic to Jurassic approximately
coincides with a change from continental to shallow
marine depositional environments. The climate also
gradually became more humid in the Jurassic as northwest Europe was pushed northward out of the arid belt
at about 30
N. A transgression in the Early Jurassic
(Lias) led to the accumulation of black shales over
large parts of NW Europe and they are exposed in
Yorkshire in northeast England and Dorset in southern
England. This is because a transgression over an
uneven land surface results in a shallow sea with
poor vertical circulation in the water. The Upper Lias
(Toarcian) in particular contains good source rocks for
oil and gas in southern parts of the North Sea. In
Britain and much of the North Sea the Lias deposits
consist of black shales with thin carbonate and sand
beds. Iron-rich layers with siderite (FeCO 3 ) and
chamosite (Fe-chlorite) are common. Beds rich in
these two minerals were the basis for iron mines
which were especially important during the industrial
revolution in England, Germany and France.
In the northern part of the North Sea fluvial and
partly marine sandstones of Lower Jurassic age
(Lunde and Statfjord formations) are important reservoir rocks in the Viking Graben. The Statfjord Formation is succeeded by the Dunlin Group, which is a dark
marine shale but normally without enough organic
content to become a significant source rock. Then
follows the Brent Group sandstone, a prograding
delta sequence which forms the main reservoir rock
in the northern North Sea. This sandstone was deposited in a delta that drained the central part of the North
Sea towards the marine embayment to the north,
between the Shetland and Horda platforms. It was
sourced from an uplifted area in the south associated
with Middle Jurassic (Bajocian-Bathonian) volcanic
activity. The volcanic centre was located south of the
Viking Graben and east of Scotland (Moray Firth). On
the other side of the uplifted area, the sediments were
25 Geology of the Norwegian Continental Shelf
613
generally evolving post-rift basin. There was still considerable sediment supply from the Variscan
mountains to the south and there is also evidence of
an uplift of Scandinavia. A broadly similar geometry
of the megasequences in both continental Triassic and
marine Jurassic successions was related to subsidencerate variations. Differential subsidence across faults
throughout Triassic time has also been reported. The
Øygarden Fault Zone, forming the eastern margin of
the Permo-Triassic basin, was active throughout most
of the time interval. The sedimentation rate was in
most cases high enough to keep up with the subsidence, resulting in a rather flat landscape with gently
flowing rivers. If the sediment supply had been less,
these rift basins would have been marine. Along the
rift structure along the middle of the North Sea the
thickness of the Triassic sediments may exceed 5 km.
The underlying Permian salt started to form diapirs
and Triassic sediment was eroded, or not deposited, at
the top of the structures.
The climate in northwest Europe during the Triassic was still arid, and continental red beds continued to
be predominant. In England these are referred to as the
New Red Sandstone because they are similar to the
Devonian continental sediments (Old Red Sandstone).
The equivalent facies in Germany is the Bunter
sandstone of Lower Triassic age. In south England
east of Exmouth there are also good exposures of
Triassic deposits with sandstones and conglomerates
(Sherwood Sandstone Group) and mudstones (Mercia
Mudstone Group). The Sherwood Sandstone is a very
important groundwater reservoir in large areas of
England. In Southern Norway north of Hamar, the
Brumundal sandstone is an outlier of a more extensive
cover of Permian red bed facies.
In the Upper Triassic we find carbonates
(Muschelkalk) and salt deposits (Keuper salt) in the
southern part of the North Sea. In the central and
northern areas, however, continental clastic sedimentation continued right up to the end of the Triassic
(Rhaetian). Sabkha environments fringed the evaporite basins and caliche, that is carbonate precipitation
in soil profiles, is typical. Towards the end of the
Triassic the climate became less arid with more normal fluvial sedimentation and gradually also marine
sedimentation when the Statfjord Formation was
deposited.
The Triassic fluvial sandstones may have variable
sorting and reservoir quality depending upon facies
and distance to basement sources. In the fluvial channel facies there may be well developed clay coatings
on quartz grains, retarding quartz cementation. The
clay coatings have probably formed in the vadose
zone where surface water has filtered down to the
groundwater table.
25.2.2.5 Jurassic
The transition from Triassic to Jurassic approximately
coincides with a change from continental to shallow
marine depositional environments. The climate also
gradually became more humid in the Jurassic as northwest Europe was pushed northward out of the arid belt
at about 30
N. A transgression in the Early Jurassic
(Lias) led to the accumulation of black shales over
large parts of NW Europe and they are exposed in
Yorkshire in northeast England and Dorset in southern
England. This is because a transgression over an
uneven land surface results in a shallow sea with
poor vertical circulation in the water. The Upper Lias
(Toarcian) in particular contains good source rocks for
oil and gas in southern parts of the North Sea. In
Britain and much of the North Sea the Lias deposits
consist of black shales with thin carbonate and sand
beds. Iron-rich layers with siderite (FeCO 3 ) and
chamosite (Fe-chlorite) are common. Beds rich in
these two minerals were the basis for iron mines
which were especially important during the industrial
revolution in England, Germany and France.
In the northern part of the North Sea fluvial and
partly marine sandstones of Lower Jurassic age
(Lunde and Statfjord formations) are important reservoir rocks in the Viking Graben. The Statfjord Formation is succeeded by the Dunlin Group, which is a dark
marine shale but normally without enough organic
content to become a significant source rock. Then
follows the Brent Group sandstone, a prograding
delta sequence which forms the main reservoir rock
in the northern North Sea. This sandstone was deposited in a delta that drained the central part of the North
Sea towards the marine embayment to the north,
between the Shetland and Horda platforms. It was
sourced from an uplifted area in the south associated
with Middle Jurassic (Bajocian-Bathonian) volcanic
activity. The volcanic centre was located south of the
Viking Graben and east of Scotland (Moray Firth). On
the other side of the uplifted area, the sediments were
25 Geology of the Norwegian Continental Shelf
613