there is a secondary porosity provided by fissures.
Fissures are particularly important in connecting the
small pores, so that the permeability increases to circa
one to a few millidarcy. Without the fissuring the
permeability would be much lower and it would have
been impossible to produce the reservoir. The fissures
are probably related to the horizontal tension which
developed when the beds were bent upwards above the
salt structure, so that the cracks are mostly vertical.
The high overpressure also aids the fissuring process.
The source rock for the hydrocarbons at Ekofisk is the
underlying Kimmeridge Clay which attains its optimal
maturity at this depth.
Gas injection and water injection have been important for maintaining reservoir pressure and preventing
the fissures from closing. The reservoir rock has also
been deliberately fissured by hydrofracturing, to
increase the permeability. The platform at Ekofisk
has sunk several metres during production, and the
subsidence has continued despite the pressure being
maintained with water injection. According to the laws
of soil- and rock-mechanics the subsidence should
cease when the effective stresses are not increasing,
but here there has probably also been a chemical
compaction which is not simply a function of effective
stresses. This may be due to water substituting for the
oil during production. The injected sea water contains
high concentrations of sulphate and also Mg
++ which
were not present in the porewater of the reservoir. This
led to increased compaction (water-weakening) and
also changed the surface properties and wettability.
These factors helped to increase the recovery from
the Ekofisk Field. There are several satellite fields,
including Cod, Albuskjell, Tor, Eldfisk, West Ekofisk
and Edda.
25.2.3.2 Northern North Sea
The northern North Sea contains the majority of the
giant fields discovered so far in the Norwegian continental shelf. The area is, however, still under intensive
exploration, and the existing infrastructure makes
even relatively small and complex traps interesting.
The province is extensively drilled, particularly along
the margins of the basin, and a whole series of trap
types have been investigated. The most common trap
type is provided by rotated fault blocks along both
margins of the Viking Graben (Figs. 25.6 and 25.7),
generated during Bathonian-Ryazian extension. The
reservoirs are commonly found within sandstones of
the Rhaetian-Sinemurian Statfjord Formation, in the
Aalenian-Bathonian Brent Group, and locally in the
Pliensbachian-Toarcian Cook Formation of the Dunlin
Group. Sealing faults are frequently important for
this trap type. Also Upper Jurassic (BathonianKimmeridgian) sandstones provide good reservoirs,
perhaps particularly in the platform areas, where
fault block rotation is moderate (e.g. the Troll Field),
and in mixed structural-stratigraphic traps along faultblock crests. Post-rift marine sands (Cretaceous and
Cenozoic) add to the prospectivity of the northern
North Sea. Indeed, the Cenozoic marine sands of the
Frigg type were the major target of early exploration
activity.
In the northern North Sea there are no Permian salt
deposits, but there are large thicknesses of Permian
and Triassic sediments at depth (Fig. 25.6). The large
oil fields in the northern part of the North Sea have
Brent Group sandstones, and often also Statfjord Formation sandstones, as reservoir rocks. This applies to
Statfjord, Gullfaks, Vigdis, Visund, Snorre and
Oseberg. The traps consist of rotated fault blocks
formed during rifting in the Late Jurassic. They stood
as islands in the sea, and much of the Upper Jurassic
and Lower Cretaceous is absent through nondeposition or erosion. At the top of the Gullfaks structure we only find a thin shale from the Upper Cretaceous, and the structure probably remained above sea
level until then.
The source rock is the Kimmeridge Clay from the
Upper Jurassic (Draupne Formation), and the oil has
migrated up to the top of the fault blocks, but stratigraphically downwards from Upper to Middle or Lower
Jurassic. In the Snorre Field the erosion at the top of
the rotated fault blocks has reached right down to
sandstones in the Upper Triassic (Lunde Formation)
which is the reservoir rock together with the Statfjord
Formation. The Statfjord Field was discovered in 1974
and was the first giant oil field on the Norwegian shelf
after Ekofisk. The field lies in blocks 33/9 and 13/12,
and extends into the British Sector. The Statfjord Field
is large, estimated to contain about 575 mill. tons oil
and 85 · 10
9 m
3 gas and NLG. Since the sand has such
high porosity and permeability, the production rate
was very high (330,000 barrels a day in 1997). Most
of the oil and gas has now been produced. Statfjord is
located on part of the platform-like Tampen Spur
which is situated north of the Viking Graben and east
of the East Shetland Basin (Figs. 25.6 and 25.7). The
618
J.I. Faleide et al.
Fissures are particularly important in connecting the
small pores, so that the permeability increases to circa
one to a few millidarcy. Without the fissuring the
permeability would be much lower and it would have
been impossible to produce the reservoir. The fissures
are probably related to the horizontal tension which
developed when the beds were bent upwards above the
salt structure, so that the cracks are mostly vertical.
The high overpressure also aids the fissuring process.
The source rock for the hydrocarbons at Ekofisk is the
underlying Kimmeridge Clay which attains its optimal
maturity at this depth.
Gas injection and water injection have been important for maintaining reservoir pressure and preventing
the fissures from closing. The reservoir rock has also
been deliberately fissured by hydrofracturing, to
increase the permeability. The platform at Ekofisk
has sunk several metres during production, and the
subsidence has continued despite the pressure being
maintained with water injection. According to the laws
of soil- and rock-mechanics the subsidence should
cease when the effective stresses are not increasing,
but here there has probably also been a chemical
compaction which is not simply a function of effective
stresses. This may be due to water substituting for the
oil during production. The injected sea water contains
high concentrations of sulphate and also Mg
++ which
were not present in the porewater of the reservoir. This
led to increased compaction (water-weakening) and
also changed the surface properties and wettability.
These factors helped to increase the recovery from
the Ekofisk Field. There are several satellite fields,
including Cod, Albuskjell, Tor, Eldfisk, West Ekofisk
and Edda.
25.2.3.2 Northern North Sea
The northern North Sea contains the majority of the
giant fields discovered so far in the Norwegian continental shelf. The area is, however, still under intensive
exploration, and the existing infrastructure makes
even relatively small and complex traps interesting.
The province is extensively drilled, particularly along
the margins of the basin, and a whole series of trap
types have been investigated. The most common trap
type is provided by rotated fault blocks along both
margins of the Viking Graben (Figs. 25.6 and 25.7),
generated during Bathonian-Ryazian extension. The
reservoirs are commonly found within sandstones of
the Rhaetian-Sinemurian Statfjord Formation, in the
Aalenian-Bathonian Brent Group, and locally in the
Pliensbachian-Toarcian Cook Formation of the Dunlin
Group. Sealing faults are frequently important for
this trap type. Also Upper Jurassic (BathonianKimmeridgian) sandstones provide good reservoirs,
perhaps particularly in the platform areas, where
fault block rotation is moderate (e.g. the Troll Field),
and in mixed structural-stratigraphic traps along faultblock crests. Post-rift marine sands (Cretaceous and
Cenozoic) add to the prospectivity of the northern
North Sea. Indeed, the Cenozoic marine sands of the
Frigg type were the major target of early exploration
activity.
In the northern North Sea there are no Permian salt
deposits, but there are large thicknesses of Permian
and Triassic sediments at depth (Fig. 25.6). The large
oil fields in the northern part of the North Sea have
Brent Group sandstones, and often also Statfjord Formation sandstones, as reservoir rocks. This applies to
Statfjord, Gullfaks, Vigdis, Visund, Snorre and
Oseberg. The traps consist of rotated fault blocks
formed during rifting in the Late Jurassic. They stood
as islands in the sea, and much of the Upper Jurassic
and Lower Cretaceous is absent through nondeposition or erosion. At the top of the Gullfaks structure we only find a thin shale from the Upper Cretaceous, and the structure probably remained above sea
level until then.
The source rock is the Kimmeridge Clay from the
Upper Jurassic (Draupne Formation), and the oil has
migrated up to the top of the fault blocks, but stratigraphically downwards from Upper to Middle or Lower
Jurassic. In the Snorre Field the erosion at the top of
the rotated fault blocks has reached right down to
sandstones in the Upper Triassic (Lunde Formation)
which is the reservoir rock together with the Statfjord
Formation. The Statfjord Field was discovered in 1974
and was the first giant oil field on the Norwegian shelf
after Ekofisk. The field lies in blocks 33/9 and 13/12,
and extends into the British Sector. The Statfjord Field
is large, estimated to contain about 575 mill. tons oil
and 85 · 10
9 m
3 gas and NLG. Since the sand has such
high porosity and permeability, the production rate
was very high (330,000 barrels a day in 1997). Most
of the oil and gas has now been produced. Statfjord is
located on part of the platform-like Tampen Spur
which is situated north of the Viking Graben and east
of the East Shetland Basin (Figs. 25.6 and 25.7). The
618
J.I. Faleide et al.