The opening of the Greenland Sea was complex,
involving jumps in the location of the spreading axis
and the splitting off of microcontinents. Since earliest
Oligocene time (magnetic chron 13) Greenland moved
with North America in a more westerly direction relative to Eurasia (Fig. 25.3). This gave rise to extension,
break-up and onset of seafloor spreading also in the
northern Greenland Sea west of Svalbard, and a deepwater gateway between the North Atlantic and Arctic
was finally established sometime in the Miocene. This
had major implications for the palaeo-oceanography
and climate in the region.
The late Cenozoic evolution was characterised by
subsidence and burial of the margins by thick
sediments in a clastic wedge derived from the uplifted
Barents Sea area. Late Cenozoic uplift and erosion of
the Barents Sea has removed most of the Cenozoic
sediments, and even older strata. The subcrop pattern
is dominated by Mesozoic units. The erosion seems to
have been most extensive in the western Barents Sea,
especially in the northwestern part including Svalbard
where more than 3,000 m of sedimentary strata have
been removed. In the SW Barents Sea the erosion
estimates are in the range 1,000–1,500 m. In the eastern Barents Sea, little work has so far been done on
Cenozoic uplift and erosion, however, erosion has
been calculated to be between 250 and 1,000 m.
High seismic velocities at the top of the bedrock in
the NW Barents Sea, decreasing south- and eastwards,
reflect the pattern of differential uplift and erosion.
The Neogene and Quaternary, resting unconformably
on Palaeogene and Mesozoic rocks, thickens dramatically in the huge sedimentary wedge at the margin.
The glacial sediments are dated as Late Pliocene to
Pleistocene/Holocene in age.
25.4.3 Exploration History and Petroleum
Provinces/Systems
More than three decades of research and petroleum
exploration in the Barents Sea have revealed a deep
and complex sedimentary basin system affected by a
variety of geological processes. A large petroleum
potential has been proven including multiple source
and reservoir intervals. However, there are still many
uncertainties and problems linked to understanding
the prospectivity of the Barents Sea. Areas in the
southern Barents Sea were opened for exploration in
1980 and the first discovery was made in 1981.
A great variety of traps (fault and salt structures,
stratigraphic pinchout) and sealing mechanisms exist
in the Barents Sea area, and several different play
models have proven hydrocarbon accumulations. The
early discoveries were dominated by gas and gas condensate accumulations (e.g. Snøhvit Field), but several
oil discoveries have been made recently in the SW
Barents Sea (see below). Potential reservoirs are
distributed across several stratigraphic levels from
Devonian to Tertiary and comprise both sandstone
and carbonate lithologies. Jurassic sandstones are the
main proven reservoir but hydrocarbons have also
been found in sandstones of Triassic and Cretaceous
age. Several proven and potential source rock units are
present in the Barents Sea. The most important are the
Upper Jurassic and Triassic organic-rich (anoxic)
shales. Lower Cretaceous, Permian and Carboniferous
shales, as well as Lower Permian evaporites and
Lower Jurassic coals, also have source potential.
About 25 discoveries have been made in the
Barents Sea, most of them in the Hammerfest Basin
(Fig. 25.20) where their reservoirs are in sandstones,
mainly Jurassic, as in the Snøhvit Field. Deeper
discoveries have also been made, such as in Triassic
sandstones in 7122/7-1 (Goliath Field) and 7125/4-1
(Nucula Field). Oil and gas have also been found in
Triassic sandstones in the Nordkapp Basin, which is
dominated by salt tectonics (Fig. 25.18). Gas and oil
have been found in carbonates of Carboniferous to
Permian age on the Finnmark Platform. Recently,
several oil discoveries have been made in the SW
Barents Sea at or close to the Loppa High (Johan
Castberg and Gohta/Alta).
The most significant exploration problem in the
Western Barents Sea relates to the severe uplift and
erosion of the area that took place during the Cenozoic. The quantity of sediments removed, and the
timing of this removal, are still a matter of debate,
but it is agreed that the uplift and erosion have had
important implications for oil and gas exploration in
the Barents Sea. Residual oil columns found beneath
gas fields in the Hammerfest Basin indicate that the
structures were once filled, or partially filled, with oil.
The removal of up to 2 km of sedimentary overburden
from the area has had critical consequences for these
accumulations: exsolution of gas from the oil, and
expansion of the gas due to the decrease in pressure,
25 Geology of the Norwegian Continental Shelf
633
involving jumps in the location of the spreading axis
and the splitting off of microcontinents. Since earliest
Oligocene time (magnetic chron 13) Greenland moved
with North America in a more westerly direction relative to Eurasia (Fig. 25.3). This gave rise to extension,
break-up and onset of seafloor spreading also in the
northern Greenland Sea west of Svalbard, and a deepwater gateway between the North Atlantic and Arctic
was finally established sometime in the Miocene. This
had major implications for the palaeo-oceanography
and climate in the region.
The late Cenozoic evolution was characterised by
subsidence and burial of the margins by thick
sediments in a clastic wedge derived from the uplifted
Barents Sea area. Late Cenozoic uplift and erosion of
the Barents Sea has removed most of the Cenozoic
sediments, and even older strata. The subcrop pattern
is dominated by Mesozoic units. The erosion seems to
have been most extensive in the western Barents Sea,
especially in the northwestern part including Svalbard
where more than 3,000 m of sedimentary strata have
been removed. In the SW Barents Sea the erosion
estimates are in the range 1,000–1,500 m. In the eastern Barents Sea, little work has so far been done on
Cenozoic uplift and erosion, however, erosion has
been calculated to be between 250 and 1,000 m.
High seismic velocities at the top of the bedrock in
the NW Barents Sea, decreasing south- and eastwards,
reflect the pattern of differential uplift and erosion.
The Neogene and Quaternary, resting unconformably
on Palaeogene and Mesozoic rocks, thickens dramatically in the huge sedimentary wedge at the margin.
The glacial sediments are dated as Late Pliocene to
Pleistocene/Holocene in age.
25.4.3 Exploration History and Petroleum
Provinces/Systems
More than three decades of research and petroleum
exploration in the Barents Sea have revealed a deep
and complex sedimentary basin system affected by a
variety of geological processes. A large petroleum
potential has been proven including multiple source
and reservoir intervals. However, there are still many
uncertainties and problems linked to understanding
the prospectivity of the Barents Sea. Areas in the
southern Barents Sea were opened for exploration in
1980 and the first discovery was made in 1981.
A great variety of traps (fault and salt structures,
stratigraphic pinchout) and sealing mechanisms exist
in the Barents Sea area, and several different play
models have proven hydrocarbon accumulations. The
early discoveries were dominated by gas and gas condensate accumulations (e.g. Snøhvit Field), but several
oil discoveries have been made recently in the SW
Barents Sea (see below). Potential reservoirs are
distributed across several stratigraphic levels from
Devonian to Tertiary and comprise both sandstone
and carbonate lithologies. Jurassic sandstones are the
main proven reservoir but hydrocarbons have also
been found in sandstones of Triassic and Cretaceous
age. Several proven and potential source rock units are
present in the Barents Sea. The most important are the
Upper Jurassic and Triassic organic-rich (anoxic)
shales. Lower Cretaceous, Permian and Carboniferous
shales, as well as Lower Permian evaporites and
Lower Jurassic coals, also have source potential.
About 25 discoveries have been made in the
Barents Sea, most of them in the Hammerfest Basin
(Fig. 25.20) where their reservoirs are in sandstones,
mainly Jurassic, as in the Snøhvit Field. Deeper
discoveries have also been made, such as in Triassic
sandstones in 7122/7-1 (Goliath Field) and 7125/4-1
(Nucula Field). Oil and gas have also been found in
Triassic sandstones in the Nordkapp Basin, which is
dominated by salt tectonics (Fig. 25.18). Gas and oil
have been found in carbonates of Carboniferous to
Permian age on the Finnmark Platform. Recently,
several oil discoveries have been made in the SW
Barents Sea at or close to the Loppa High (Johan
Castberg and Gohta/Alta).
The most significant exploration problem in the
Western Barents Sea relates to the severe uplift and
erosion of the area that took place during the Cenozoic. The quantity of sediments removed, and the
timing of this removal, are still a matter of debate,
but it is agreed that the uplift and erosion have had
important implications for oil and gas exploration in
the Barents Sea. Residual oil columns found beneath
gas fields in the Hammerfest Basin indicate that the
structures were once filled, or partially filled, with oil.
The removal of up to 2 km of sedimentary overburden
from the area has had critical consequences for these
accumulations: exsolution of gas from the oil, and
expansion of the gas due to the decrease in pressure,
25 Geology of the Norwegian Continental Shelf
633