eastwards through the Jurassic sandstones up to the
coast so that pressure has not built up much above
hydrostatic pressure; this has been an important factor
in hindering leakage. The main problem in the area is
that many of the reservoirs are very deep and oil can
have escaped due to overpressure. Typical trap types
drilled so far on the mid-Norwegian continental shelf
are rotated fault blocks of Jurassic age in the horstand-graben terrane of the Halten Terrace, and complex
fault blocks within the major fault zones. Here too the
source rock formed during rifting in the Late Jurassic.
It is called the Spekk Formation, which approximately
equates with the Draupne Formation in the North Sea
and the Kimmeridge Formation in England. In the
Early Jurassic A ˚ re Formation there are coal beds,
which may be the source for the gas and possibly
some of the oil.
The most important reservoir rocks are of Early and
Middle Jurassic age, with alternating sandstones and
shales from the A ˚ re, Tilje, Tofte, Ile and Garn
formations (Fig. 25.4). The fields in the east, for
example Heidrun Field, are not so very deep, but in
the Smørbukk Field the reservoir rocks lie at more
than 4 km. Also here it was block rotation during
rifting which created the majority of the structures.
West of the Smørbukk Field there is a large fault,
and to the west of that there is high overpressure in the
Jurassic sandstones. Several wells have been drilled
here that show signs of there having once been oil
present, and that it has leaked out. This is presumably
due to the high pressure having reached fracture pressure. In the Kristin Field, however, some of the oil is
still in place. The Lavrans and Kristin fields lie at
more than 5 km, where the temperature can reach
170–180
C. Here the porosity and permeability in
the reservoir rocks are critical, and it is the degree of
quartz cementation which essentially determines
whether there is sufficient porosity to produce petroleum. The sand grains are coated with chlorite in much
of the Tilje Formation, and in places also in the Garn
Formation. This hinders quartz precipitation and thus
helps maintain the relatively high porosities (20–25%)
despite the great burial depth. Where the chlorite is
absent and the sand is pure quartz, as in other parts
of the Garn Formation, the porosity is quite low
(<10–12%). Precipitation of illite from kaolinite
and K-feldspar has in many cases reduced the
permeabillity significantly at a depth of >3.8-4.0 km.
Some of the sandstones contain however dominantly
plagioclase, and illitisation of kaolinite is then
prevented (See Chap. 4). These compositional
differences may reflect source areas both from the
east and from the west (Greenland).
The Draugen Field has Upper Jurassic sandstone
reservoirs and is fairly shallow, just 1.5 km. The
source rock in this area is not mature and the oil
found at Draugen has actually had a relatively long
migration path from deeper-lying areas to the west.
The subsidence and sedimentation rates were high
during the Quaternary, providing a sequence about
1 km thick (Fig. 25.13). This is considerably greater
than in most of the North Sea. Thus neither the reservoir nor source rocks have been deeply buried for a
geologically long period. This has left the source rocks
somewhat immature for their depth, but also means
that the reservoir rocks have less quartz cement.
25.3.3.2 Vøring and Møre Basins
This area was first opened for exploration at the end of
the 1990s, with great expectation because of the large
structures that could be seen on the regional seismic.
Inversion structures (mainly huge, gentle anticlines
with wavelengths in the order of tens of kilometres)
are common in the deep Cretaceous basins, and provide traps of considerable size. These were generated
during Tertiary inversion.
The geology proved to be very different from the
North Sea and Haltenbanken. There had been considerable magmatic activity associated with the break-up
between Norway and Greenland and the initiation of
seafloor spreading. Tertiary lavas had flowed out
across great tracts of land and intrusive dykes and
sills penetrated the Cretaceous sediments. There was
some concern that this could have raised the temperature so much that all the oil had become gas. However,
it was found that this heating had been quite local and
had had little overall effect.
More significant was the extreme thickness of the
Cretaceous sequence, up to 6–7 km. This meant that
the Upper Jurassic source rocks had already matured
by the mid-Cretaceous. This was before the reservoir
rocks, of Upper Cretaceous and Lower Tertiary age,
had even been deposited, and before the structures we
now see (e.g. Ormen Lange Dome, Helland Hansen
Arch, Gjallar Ridge) had formed. It thus appears that
the structures have only trapped some of the gas
formed at depth from the oil. Gas has been found in
several prospect in the western Vøring Basin, in Upper
25 Geology of the Norwegian Continental Shelf
625
coast so that pressure has not built up much above
hydrostatic pressure; this has been an important factor
in hindering leakage. The main problem in the area is
that many of the reservoirs are very deep and oil can
have escaped due to overpressure. Typical trap types
drilled so far on the mid-Norwegian continental shelf
are rotated fault blocks of Jurassic age in the horstand-graben terrane of the Halten Terrace, and complex
fault blocks within the major fault zones. Here too the
source rock formed during rifting in the Late Jurassic.
It is called the Spekk Formation, which approximately
equates with the Draupne Formation in the North Sea
and the Kimmeridge Formation in England. In the
Early Jurassic A ˚ re Formation there are coal beds,
which may be the source for the gas and possibly
some of the oil.
The most important reservoir rocks are of Early and
Middle Jurassic age, with alternating sandstones and
shales from the A ˚ re, Tilje, Tofte, Ile and Garn
formations (Fig. 25.4). The fields in the east, for
example Heidrun Field, are not so very deep, but in
the Smørbukk Field the reservoir rocks lie at more
than 4 km. Also here it was block rotation during
rifting which created the majority of the structures.
West of the Smørbukk Field there is a large fault,
and to the west of that there is high overpressure in the
Jurassic sandstones. Several wells have been drilled
here that show signs of there having once been oil
present, and that it has leaked out. This is presumably
due to the high pressure having reached fracture pressure. In the Kristin Field, however, some of the oil is
still in place. The Lavrans and Kristin fields lie at
more than 5 km, where the temperature can reach
170–180
C. Here the porosity and permeability in
the reservoir rocks are critical, and it is the degree of
quartz cementation which essentially determines
whether there is sufficient porosity to produce petroleum. The sand grains are coated with chlorite in much
of the Tilje Formation, and in places also in the Garn
Formation. This hinders quartz precipitation and thus
helps maintain the relatively high porosities (20–25%)
despite the great burial depth. Where the chlorite is
absent and the sand is pure quartz, as in other parts
of the Garn Formation, the porosity is quite low
(<10–12%). Precipitation of illite from kaolinite
and K-feldspar has in many cases reduced the
permeabillity significantly at a depth of >3.8-4.0 km.
Some of the sandstones contain however dominantly
plagioclase, and illitisation of kaolinite is then
prevented (See Chap. 4). These compositional
differences may reflect source areas both from the
east and from the west (Greenland).
The Draugen Field has Upper Jurassic sandstone
reservoirs and is fairly shallow, just 1.5 km. The
source rock in this area is not mature and the oil
found at Draugen has actually had a relatively long
migration path from deeper-lying areas to the west.
The subsidence and sedimentation rates were high
during the Quaternary, providing a sequence about
1 km thick (Fig. 25.13). This is considerably greater
than in most of the North Sea. Thus neither the reservoir nor source rocks have been deeply buried for a
geologically long period. This has left the source rocks
somewhat immature for their depth, but also means
that the reservoir rocks have less quartz cement.
25.3.3.2 Vøring and Møre Basins
This area was first opened for exploration at the end of
the 1990s, with great expectation because of the large
structures that could be seen on the regional seismic.
Inversion structures (mainly huge, gentle anticlines
with wavelengths in the order of tens of kilometres)
are common in the deep Cretaceous basins, and provide traps of considerable size. These were generated
during Tertiary inversion.
The geology proved to be very different from the
North Sea and Haltenbanken. There had been considerable magmatic activity associated with the break-up
between Norway and Greenland and the initiation of
seafloor spreading. Tertiary lavas had flowed out
across great tracts of land and intrusive dykes and
sills penetrated the Cretaceous sediments. There was
some concern that this could have raised the temperature so much that all the oil had become gas. However,
it was found that this heating had been quite local and
had had little overall effect.
More significant was the extreme thickness of the
Cretaceous sequence, up to 6–7 km. This meant that
the Upper Jurassic source rocks had already matured
by the mid-Cretaceous. This was before the reservoir
rocks, of Upper Cretaceous and Lower Tertiary age,
had even been deposited, and before the structures we
now see (e.g. Ormen Lange Dome, Helland Hansen
Arch, Gjallar Ridge) had formed. It thus appears that
the structures have only trapped some of the gas
formed at depth from the oil. Gas has been found in
several prospect in the western Vøring Basin, in Upper
25 Geology of the Norwegian Continental Shelf
625