particularly after burial to more than 2 km depth and
quartz cementation, giving rise to strong seismic
marker horizons (top Balder Formation).
In Eocene times progradation from the East Shetland Platform was dominant and major depocentres
developed in the Viking Graben area, with deep water
along the basin axis. The Palaeocene and Eocene
submarine fans were built up with turbidite sands
carried out into the central part of the North Sea with
the Utsira High limiting their eastward extent. Parts of
Fennoscandia were probably covered by sea during
Middle-Late Eocene times.
At the Eocene-Oligocene transition, southern
Norway became uplifted. This uplift, in combination
with prograding units from both the east and west,
gave rise to a shallow threshold in the northern North
Sea, separating deeper waters to the south and north.
The uplift and shallowing continued into Miocene
time when a widespread hiatus formed in the northern
North Sea, as revealed by biostratigraphic data. Miocene outbuilding from the north into the southeastern
North Sea was massive (Fig. 25.8). Coastal
progradation of the Utsira Formation in the northern
North Sea reflects Late Miocene-Early Pliocene uplift
and erosion of mainland Norway. This relatively thick
sandstone is a good aquifer and is used for injection of
CO 2 in the Sleipner Field. It was still relatively warm
in the Early Pliocene, compared to the Late Pliocene
when mountain glaciation started to develop.
The Late Pliocene-Early Pleistocene basin configuration was dominated by the progradation of thick
clastic wedges in response to uplift and glacial erosion
of eastern source areas (Figs. 25.7 and 25.8). Considerable uplift of the eastern basin flank is documented by
the strong angular relationship and tilting of the complete Cenozoic succession below the mid-Pleistocene
angular unconformity. The Plio-Pleistocene sediments
are partly glacial and partly marine representing
reworked glacial sediments and are typically poorly
sorted. Such sediments compact readily because of the
poor sorting of glacial and glaciomarine sediments and
periods of glacial advances may have contributed to the
compaction.
The ice sheets advanced repeatedly out onto the
shelf, but often deposited much of their debris load
relatively near to land. Only during relatively short
periods did the glaciers cover most of the North Sea
basin. When major ice sheets developed on the Baltic
Shield much of the ice flowed southwards along the
valleys to the Oslo region and deepened the Oslofjord.
The ice flow continued along the south coast of
Norway and eroded a trough in the Tertiary and Mesozoic sediments. This is up to 700 m deep and continues
up along western Norway. Thick Pleistocene sedimentary fans were deposited at the slope in front of the
bathymetric trough.
Periods of ice loading resulted in a tectonic tilting
of much of the shelf, particularly in the inner part. This
has in some cases influenced oil migration and resulted
in tilted OWC (e.g. in the Troll Field). Ice loading
has also caused compaction so that Quaternary
moraines and glaciomarine sediments have become
overconsolidated.
At most places on the shelf the Holocene (the last
10,000 years) is represented by only a thin layer of
silty sediment, chiefly reworked from Quaternary or
older sediments exposed on topographic highs or
along the margins of the North Sea. Very little “new”
sediment has been supplied from land during the
Holocene. This is because the fjords act as very efficient sediment traps, collecting the sediment from
the rivers. Because fjords are deep but have shallow
thresholds, not much clastic sediment reaches the
shelf. On the seafloor there are in many areas frequent depressions which are typically a few tens of
metres across and several metres deep. These are
called pockmarks and have formed by fluid, generally
gas, seeping from deeper layers to emerge at the
seafloor.
The Cenozoic sedimentation was relatively rapid
and the clayey sediments had little time to compact
sufficiently to reduce the water content. Some beds
therefore display plastic folding and diapir structures
due to the under-compacted clays, especially in the
Eocene. Polygonal faults are also common in these
mudstones. They form a network which are from several hundred metres to 1 km across.
25.2.3 Exploration History and Petroleum
Provinces/Systems
Exploration has been taking place in the North Sea for
over 40 years and most of the large Norwegian fields
are situated here. This is a mature part of the continental shelf and most of the plays are confirmed. The
Norwegian part of the North Sea can be divided into
two petroleum provinces characterised by different
616
J.I. Faleide et al.
quartz cementation, giving rise to strong seismic
marker horizons (top Balder Formation).
In Eocene times progradation from the East Shetland Platform was dominant and major depocentres
developed in the Viking Graben area, with deep water
along the basin axis. The Palaeocene and Eocene
submarine fans were built up with turbidite sands
carried out into the central part of the North Sea with
the Utsira High limiting their eastward extent. Parts of
Fennoscandia were probably covered by sea during
Middle-Late Eocene times.
At the Eocene-Oligocene transition, southern
Norway became uplifted. This uplift, in combination
with prograding units from both the east and west,
gave rise to a shallow threshold in the northern North
Sea, separating deeper waters to the south and north.
The uplift and shallowing continued into Miocene
time when a widespread hiatus formed in the northern
North Sea, as revealed by biostratigraphic data. Miocene outbuilding from the north into the southeastern
North Sea was massive (Fig. 25.8). Coastal
progradation of the Utsira Formation in the northern
North Sea reflects Late Miocene-Early Pliocene uplift
and erosion of mainland Norway. This relatively thick
sandstone is a good aquifer and is used for injection of
CO 2 in the Sleipner Field. It was still relatively warm
in the Early Pliocene, compared to the Late Pliocene
when mountain glaciation started to develop.
The Late Pliocene-Early Pleistocene basin configuration was dominated by the progradation of thick
clastic wedges in response to uplift and glacial erosion
of eastern source areas (Figs. 25.7 and 25.8). Considerable uplift of the eastern basin flank is documented by
the strong angular relationship and tilting of the complete Cenozoic succession below the mid-Pleistocene
angular unconformity. The Plio-Pleistocene sediments
are partly glacial and partly marine representing
reworked glacial sediments and are typically poorly
sorted. Such sediments compact readily because of the
poor sorting of glacial and glaciomarine sediments and
periods of glacial advances may have contributed to the
compaction.
The ice sheets advanced repeatedly out onto the
shelf, but often deposited much of their debris load
relatively near to land. Only during relatively short
periods did the glaciers cover most of the North Sea
basin. When major ice sheets developed on the Baltic
Shield much of the ice flowed southwards along the
valleys to the Oslo region and deepened the Oslofjord.
The ice flow continued along the south coast of
Norway and eroded a trough in the Tertiary and Mesozoic sediments. This is up to 700 m deep and continues
up along western Norway. Thick Pleistocene sedimentary fans were deposited at the slope in front of the
bathymetric trough.
Periods of ice loading resulted in a tectonic tilting
of much of the shelf, particularly in the inner part. This
has in some cases influenced oil migration and resulted
in tilted OWC (e.g. in the Troll Field). Ice loading
has also caused compaction so that Quaternary
moraines and glaciomarine sediments have become
overconsolidated.
At most places on the shelf the Holocene (the last
10,000 years) is represented by only a thin layer of
silty sediment, chiefly reworked from Quaternary or
older sediments exposed on topographic highs or
along the margins of the North Sea. Very little “new”
sediment has been supplied from land during the
Holocene. This is because the fjords act as very efficient sediment traps, collecting the sediment from
the rivers. Because fjords are deep but have shallow
thresholds, not much clastic sediment reaches the
shelf. On the seafloor there are in many areas frequent depressions which are typically a few tens of
metres across and several metres deep. These are
called pockmarks and have formed by fluid, generally
gas, seeping from deeper layers to emerge at the
seafloor.
The Cenozoic sedimentation was relatively rapid
and the clayey sediments had little time to compact
sufficiently to reduce the water content. Some beds
therefore display plastic folding and diapir structures
due to the under-compacted clays, especially in the
Eocene. Polygonal faults are also common in these
mudstones. They form a network which are from several hundred metres to 1 km across.
25.2.3 Exploration History and Petroleum
Provinces/Systems
Exploration has been taking place in the North Sea for
over 40 years and most of the large Norwegian fields
are situated here. This is a mature part of the continental shelf and most of the plays are confirmed. The
Norwegian part of the North Sea can be divided into
two petroleum provinces characterised by different
616
J.I. Faleide et al.