resulted in expulsion of most of the oil from the traps.
Seal breaching and spillage probably also occurred as
a result of the uplift and tilting. A further consequence
of these late movements was the cooling of the source
rocks in the area, which effectively caused most
hydrocarbon generation to cease.
The Neogene uplift and erosion has had important
consequences for Barents Sea petroleum systems –
different from other basins at the Norwegian Continental Shelf.
Much of the North Sea basin and Haltenbanken
offshore mid-Norway have been dominated by
continued subsidence and sedimentation. Permian
and Jurassic rifting resulted in uplift and erosion but
most of the sedimentary sequences making up the
basin fill are now at their maximum burial depth.
This simplifies the reconstruction of the burial history
also with respect to temperature and time for
modelling of source rock maturation and migration.
Similar time-temperature integrals may be used to
model diagenetic processes like quartz cementation
in reservoir rocks to predict reservoir quality. Particularly at Haltenbanken up to 1.5 km of Plio-Pleistocene
basin subsidence and sedimentation imply that
reservoirs now at 4 km burial depth were at 2.5 km
depth 1.5–3.0 million years ago. The limited time at
elevated temperature (>100˚C) resulted in limited
quartz cement compared to sandstones with less recent
subsidence. In the Northern North Sea Quaternary
uplift has affected the eastern parts including the
Troll Field and has also caused late tilting of some of
the sequences thus influencing petroleum migration
and the fluid contacts.
In the western part of the Barents Sea late Neogene
uplift and erosion may total 1.5–2 km. The timing of
maximum burial depth and temperature is then more
difficult to estimate with respect to maturation, migration and sometime remigration of oil and gas into
traps. Maturation of source rocks is not only a function
of the maximum burial temperature but of the time
exposed to temperatures above 90–100˚C.
Sandstones undergo diagenesis and quartz cementation both during subsidence and uplift as an exponential function of temperature when it is above about
80˚C. Chemical compaction due to quartz cementation
and porosity loss will therefore continue also during
uplift. If the rock volume is reduced the shrinkage
results in a reduction in differential stresses during
tectonic deformation. In the parts of the Barents Sea
where the uplift and erosion has been more than 1.5–
2.0 km the maximum temperature may have exceeded
70–80˚C and Jurassic or Cretaceous rocks are quartz
cemented to some degree. This also means that the
temperature has pasteurised the reservoir rocks with
respect to bacteria. Oil migrating into such reservoir
rocks will not easily be biodegraded. An example to
illustrate this may be the Wisting discovery where
normal unbiodegraded oil is found at very shallow
depth, 2–300 m below the seafloor.
During glaciations maximum ice loadings of up
to 2 km or more could add up to 10–20 MPa to the
total overburden stress depending on the sea level.
Overpresssured water below the ice would reduce the
effective stress transmitted to the seafloor, but increase
the pore pressure in the rocks below the seafloor. This
means that more gas will be dissolved in the porewater
of the underlying sedimentary rocks, and in reservoirs
more gas could also dissolve in the oil phase.
During interglacial periods with no ice load,
reduced pressure will cause gas to bubble out of the
porewater and appear as shallow gas. It is important to
distinguish between shallow biogenic gas produced by
bacterial processes and thermogenic gas formed from
maturation of source rocks at greater depth. This may
be shallow gas due to upwards migration and also due
to erosion of the overburden.
The Quaternary cover of glacial sediments may be
hard and well compacted by the ice load and could in
some cases also serve as a cap for oil and gas. The
timing for trapping of petroleum would however have
been very short (from the last glaciation). Because of
their poor sorting of clay, silt and sand glacial
sediments compact very effectively at moderate effective stresses (see Chap. 11). In the northern North Sea
the Peon field is an example where the reservoir is only
at about 200–300 m depth in Pleistocene glacial
sediments which have been compacted by glacial
loading.
The late uplift of the Barents Sea basin and proximal parts of the Norwegian offshore basins may have
caused leakage of petroleum reservoir. The many prolific onshore basins in the world show however that not
all traps for oil and gas have leaked during uplift and
erosion.
634
J.I. Faleide et al.
Seal breaching and spillage probably also occurred as
a result of the uplift and tilting. A further consequence
of these late movements was the cooling of the source
rocks in the area, which effectively caused most
hydrocarbon generation to cease.
The Neogene uplift and erosion has had important
consequences for Barents Sea petroleum systems –
different from other basins at the Norwegian Continental Shelf.
Much of the North Sea basin and Haltenbanken
offshore mid-Norway have been dominated by
continued subsidence and sedimentation. Permian
and Jurassic rifting resulted in uplift and erosion but
most of the sedimentary sequences making up the
basin fill are now at their maximum burial depth.
This simplifies the reconstruction of the burial history
also with respect to temperature and time for
modelling of source rock maturation and migration.
Similar time-temperature integrals may be used to
model diagenetic processes like quartz cementation
in reservoir rocks to predict reservoir quality. Particularly at Haltenbanken up to 1.5 km of Plio-Pleistocene
basin subsidence and sedimentation imply that
reservoirs now at 4 km burial depth were at 2.5 km
depth 1.5–3.0 million years ago. The limited time at
elevated temperature (>100˚C) resulted in limited
quartz cement compared to sandstones with less recent
subsidence. In the Northern North Sea Quaternary
uplift has affected the eastern parts including the
Troll Field and has also caused late tilting of some of
the sequences thus influencing petroleum migration
and the fluid contacts.
In the western part of the Barents Sea late Neogene
uplift and erosion may total 1.5–2 km. The timing of
maximum burial depth and temperature is then more
difficult to estimate with respect to maturation, migration and sometime remigration of oil and gas into
traps. Maturation of source rocks is not only a function
of the maximum burial temperature but of the time
exposed to temperatures above 90–100˚C.
Sandstones undergo diagenesis and quartz cementation both during subsidence and uplift as an exponential function of temperature when it is above about
80˚C. Chemical compaction due to quartz cementation
and porosity loss will therefore continue also during
uplift. If the rock volume is reduced the shrinkage
results in a reduction in differential stresses during
tectonic deformation. In the parts of the Barents Sea
where the uplift and erosion has been more than 1.5–
2.0 km the maximum temperature may have exceeded
70–80˚C and Jurassic or Cretaceous rocks are quartz
cemented to some degree. This also means that the
temperature has pasteurised the reservoir rocks with
respect to bacteria. Oil migrating into such reservoir
rocks will not easily be biodegraded. An example to
illustrate this may be the Wisting discovery where
normal unbiodegraded oil is found at very shallow
depth, 2–300 m below the seafloor.
During glaciations maximum ice loadings of up
to 2 km or more could add up to 10–20 MPa to the
total overburden stress depending on the sea level.
Overpresssured water below the ice would reduce the
effective stress transmitted to the seafloor, but increase
the pore pressure in the rocks below the seafloor. This
means that more gas will be dissolved in the porewater
of the underlying sedimentary rocks, and in reservoirs
more gas could also dissolve in the oil phase.
During interglacial periods with no ice load,
reduced pressure will cause gas to bubble out of the
porewater and appear as shallow gas. It is important to
distinguish between shallow biogenic gas produced by
bacterial processes and thermogenic gas formed from
maturation of source rocks at greater depth. This may
be shallow gas due to upwards migration and also due
to erosion of the overburden.
The Quaternary cover of glacial sediments may be
hard and well compacted by the ice load and could in
some cases also serve as a cap for oil and gas. The
timing for trapping of petroleum would however have
been very short (from the last glaciation). Because of
their poor sorting of clay, silt and sand glacial
sediments compact very effectively at moderate effective stresses (see Chap. 11). In the northern North Sea
the Peon field is an example where the reservoir is only
at about 200–300 m depth in Pleistocene glacial
sediments which have been compacted by glacial
loading.
The late uplift of the Barents Sea basin and proximal parts of the Norwegian offshore basins may have
caused leakage of petroleum reservoir. The many prolific onshore basins in the world show however that not
all traps for oil and gas have leaked during uplift and
erosion.
634
J.I. Faleide et al.