Tampen Spur is one of several fault blocks which
subsided and rotated along low-angle faults in Middle
to Upper Jurassic time. The reservoir rocks consist of
the Statfjord Formation (Lower Jurassic) and the Brent
Group (Middle Jurassic).
The Statfjord Formation is comprised of fluvial
sandstones which are replaced towards the top by
shallow marine deposits. The Dunlin Shale succeeds
the Statfjord Formation, and has enough organic material to be a source rock even if its contribution is very
modest compared to the Kimmeridge Clay (Draupne
Formation). During the Middle Jurassic there was
considerable volcanic activity in the central part of
the North Sea (Rattray Formation). This volcanism
and the high geothermal gradients caused this area to
be uplifted. The Triassic and Permian sandstones were
then eroded and the sediments were for the most part
transported northwards in a fluvial system which
debouched in the Brent delta. The delta built out
across almost the entire area between west Norway
and Shetland, and sediment was also supplied from
these areas to the delta. A several hundred metre thick
series called the Brent Group was deposited,
consisting of five formations that represent different
facies. First the Broom Formation was deposited,
mostly on the British side, and the Oseberg Formation
on the Norwegian side. These were deposited as local
fans related to faults along the sides of the initial rift.
The Rannoch Formation consists of a mica-bearing
sandstone which represents an upwards-coarsening
sequence – from “offshore” to “shoreface” – and
represents the actual progradation of the Brent delta.
The mica settled out of suspension outside the fluvial
channels. The Etive Formation consists of sandstones
deposited on the delta front above wave base and is
therefore better sorted. The Ness Formation represents
the fluvial delta-top facies. Here there are meandering
fluvial channels and crevasse splays. The clay and
mica contents are higher in the leve ´e and overbank
deposits. The Ness Formation represents the maximum northward extent of the delta. The Tarbert Formation is much better sorted, having been deposited as
marine delta-front sediments while the delta was being
transgressed by the sea. The sediment supply from the
Mid-North Sea High was reduced as this area was
gradually transgressed and sediment supply to the
delta could then no longer keep pace with the subsidence. Part of the Ness Formation was probably
eroded and redeposited as marine sediments as part
of the Tarbert Formation.
The reservoir properties are a function of these
depositional conditions, both on account of primary
sorting and because diagenetic changes are usually
determined by the primary mineralogical composition
and sorting. The sandstones of the Brent Group are
generally immature with a high content of feldspar
and mica indicating a basement source and a relatively
short transport into the prograding Brent delta. This
implies that all sandstone bodies where flushed intensively by freshwater shortly after deposition. Nearly
every sample of Brent Sandstone therefore contains
abundant evidence of feldspar leaching and porefilling authigenic kaolinite. Carbonate-cemented beds
are often associated with marine environments with
aragonitic fossils which have dissolved and formed
cement.
The Brent delta was transgressed by the mudstones
of the Heather Formation towards the end of the Late
Jurassic. The main source rock for the area is the
Upper Jurassic Kimmeridge Clay. It was deposited
over a large part of the North Sea basin and also
onshore present Britain. In the northern North Sea it
is referred to as the Draupne Formation and the
Mandal Formation in the Norwegian Central Graben.
It may be more than 500 m thick along the rotated fault
blocks which emerged as islands. The irregular bottom
topography in the basin gave poor circulation and
stagnant conditions with the deposition of a good
source rock. The crests of many of the blocks were
so heavily eroded during rotation that most of the
Jurassic sequence is missing from them.
Shale at less than 3.5 km depth on the shallower
parts of the structures is not sufficiently mature to have
generated oil or gas in significant quantity. Where the
Kimmeridge Clay is downfaulted deeply enough along
the listric faults, oil has migrated up into the tectonically overlying but stratigraphically underlying
Statfjord and Brent Groups. Most of the oil comes
from the area north of the Snorre Field where the
source rock (Draupne Formation) is buried to
4–5 km, but there is also some from the areas south
of Gullfaks. Such areas that have generated a lot of oil
are called kitchen areas.
In the Troll Field the reservoir rock is of Upper
Jurassic age and contains mostly gas formed in the
deepest parts of the basin (>4.5 km).
25 Geology of the Norwegian Continental Shelf
619
subsided and rotated along low-angle faults in Middle
to Upper Jurassic time. The reservoir rocks consist of
the Statfjord Formation (Lower Jurassic) and the Brent
Group (Middle Jurassic).
The Statfjord Formation is comprised of fluvial
sandstones which are replaced towards the top by
shallow marine deposits. The Dunlin Shale succeeds
the Statfjord Formation, and has enough organic material to be a source rock even if its contribution is very
modest compared to the Kimmeridge Clay (Draupne
Formation). During the Middle Jurassic there was
considerable volcanic activity in the central part of
the North Sea (Rattray Formation). This volcanism
and the high geothermal gradients caused this area to
be uplifted. The Triassic and Permian sandstones were
then eroded and the sediments were for the most part
transported northwards in a fluvial system which
debouched in the Brent delta. The delta built out
across almost the entire area between west Norway
and Shetland, and sediment was also supplied from
these areas to the delta. A several hundred metre thick
series called the Brent Group was deposited,
consisting of five formations that represent different
facies. First the Broom Formation was deposited,
mostly on the British side, and the Oseberg Formation
on the Norwegian side. These were deposited as local
fans related to faults along the sides of the initial rift.
The Rannoch Formation consists of a mica-bearing
sandstone which represents an upwards-coarsening
sequence – from “offshore” to “shoreface” – and
represents the actual progradation of the Brent delta.
The mica settled out of suspension outside the fluvial
channels. The Etive Formation consists of sandstones
deposited on the delta front above wave base and is
therefore better sorted. The Ness Formation represents
the fluvial delta-top facies. Here there are meandering
fluvial channels and crevasse splays. The clay and
mica contents are higher in the leve ´e and overbank
deposits. The Ness Formation represents the maximum northward extent of the delta. The Tarbert Formation is much better sorted, having been deposited as
marine delta-front sediments while the delta was being
transgressed by the sea. The sediment supply from the
Mid-North Sea High was reduced as this area was
gradually transgressed and sediment supply to the
delta could then no longer keep pace with the subsidence. Part of the Ness Formation was probably
eroded and redeposited as marine sediments as part
of the Tarbert Formation.
The reservoir properties are a function of these
depositional conditions, both on account of primary
sorting and because diagenetic changes are usually
determined by the primary mineralogical composition
and sorting. The sandstones of the Brent Group are
generally immature with a high content of feldspar
and mica indicating a basement source and a relatively
short transport into the prograding Brent delta. This
implies that all sandstone bodies where flushed intensively by freshwater shortly after deposition. Nearly
every sample of Brent Sandstone therefore contains
abundant evidence of feldspar leaching and porefilling authigenic kaolinite. Carbonate-cemented beds
are often associated with marine environments with
aragonitic fossils which have dissolved and formed
cement.
The Brent delta was transgressed by the mudstones
of the Heather Formation towards the end of the Late
Jurassic. The main source rock for the area is the
Upper Jurassic Kimmeridge Clay. It was deposited
over a large part of the North Sea basin and also
onshore present Britain. In the northern North Sea it
is referred to as the Draupne Formation and the
Mandal Formation in the Norwegian Central Graben.
It may be more than 500 m thick along the rotated fault
blocks which emerged as islands. The irregular bottom
topography in the basin gave poor circulation and
stagnant conditions with the deposition of a good
source rock. The crests of many of the blocks were
so heavily eroded during rotation that most of the
Jurassic sequence is missing from them.
Shale at less than 3.5 km depth on the shallower
parts of the structures is not sufficiently mature to have
generated oil or gas in significant quantity. Where the
Kimmeridge Clay is downfaulted deeply enough along
the listric faults, oil has migrated up into the tectonically overlying but stratigraphically underlying
Statfjord and Brent Groups. Most of the oil comes
from the area north of the Snorre Field where the
source rock (Draupne Formation) is buried to
4–5 km, but there is also some from the areas south
of Gullfaks. Such areas that have generated a lot of oil
are called kitchen areas.
In the Troll Field the reservoir rock is of Upper
Jurassic age and contains mostly gas formed in the
deepest parts of the basin (>4.5 km).
25 Geology of the Norwegian Continental Shelf
619