transported southwards, exemplified in good coastal
sections in Yorkshire.
The lower part of the Brent Group consists of
upward-coarsening beds of mica-rich sandstones that
represent prograding delta deposits, especially distributary mouth bars. This is the Etive and Rannoch
formations. The middle part is the Ness Formation,
comprised mostly of fluvial delta facies with channels,
crevasse sand deposits, lagoonal deposits and coal
beds. These delta top facies were deposited during a
northward progradation of the Brent Delta. At the top
of the Brent Group, the Tabert Formation is a generally well-sorted sandstone formed by the reworking of
deltaic deposits during a transgression. The sediment
supply from the south was no longer able to keep up
with the basin subsidence, resulting in a gradual
drowning of the Brent Delta. Each of these delta facies
has its characteristic reservoir properties, which are a
function both of primary sorting and mineralogy and
subsequent diagenetic modification.
Nearly all samples from these deltaic reservoir
rocks show evidence of meteoric water flushing and
have well developed authigenic pore-filling kaolinite
and partly dissolved feldspar (see Chap. 4). At burial
depth >4 km (130
C) kaolinite have reacted with Kfeldspar and is replaced by illite.
There is normally little clay coating on quartz in the
Brent sandstones and quartz cementation may be
extensive so that the porosity is reduced to 10-15%
at 4 km depth.
In the Late Jurassic, volcanism was very much
reduced and the areas within and surrounding the rift
systems subsided in response to lower geothermal
gradients. At the same time, normal faulting along
the Viking Graben led to the rotation of basement
blocks and their overlying sediments. The shoulders
of the tilted fault blocks were exposed to erosion,
removing Lower-Middle Jurassic and locally even
Upper Triassic strata. The Late Jurassic (Oxfordian)
transgression covered the Viking Graben with a thick
drape of clayey sediments of the Heather Formation,
while the coarser clastics (sand) were deposited as
turbidites and in deltas along the basin margins.
Some of the deltas appear to have been controlled by
the same structures that have determined the location
of the fjords in western Norway.
The uppermost Jurassic Kimmeridge Clay Formation is transgressive and often forms a several hundred
metres thick rich source rock which on the Norwegian
side is called the Draupne Formation. The rift topography produced numerous, locally overdeepened,
basins with poor bottom water circulation. Only a
relatively small proportion of the organic production
was oxidised while the sedimentation rate was fairly
high. The organic-rich shales of the Upper Jurassic are
thus the prime source rock in the North Sea, and
provided the main petroleum source in both the
Statfjord and Ekofisk areas. The thickness of the
Upper Jurassic sediments along the rift axis may
reach 3,000 m. The deposition of organic-rich shales
continued into the Early Cretaceous in some of the
basins. The edges of the rotated blocks suffered erosion and a few were not buried until the Late Cretaceous. The majority of the faults die out before the
Cretaceous but a few continue up into younger beds.
The rifting resulted in rotated fault blocks containing
sandstone reservoirs of Lower and Middle Jurassic age
(sandstones of the Brent Group and Statfjord Formation). Small fan deltas developed along the rift and
also deepwater sandstones including debris flows.
Some of these are good reservoir rocks occuring
within the Upper Jurassic source rocks (see Chap. 12).
Upper Jurassic sandstones deposited on submarine
slopes are much less leached and have less kaolinite
than the Brent sandstones. They may however contain
grain-coating micro-quartz which retards quartz
cementation, thus preserving porosity. This is linked
to a siliceous sponge (Rhaxella), see Chap. 4.
In much of the North Sea basin Kimmeridge shale
equivalents are immature but in the Upper Jurassic rift
structures this source rock is buried to much greater
depth corresponding to the oil window, and even gas
window.
25.2.2.6 Cretaceous
The last phase of rifting in the North Sea in the Late
Jurassic was followed by a major transgression, but the
uplifted rift stuctures remained dry and were islands
for most of the Early Cretaceous. There is a major
unconformity between the Cretaceous and the Jurassic
except in the deep parts of the rifts where there may
have been continuous sedimentation (Figs. 25.6 and
25.7). The Base Cretaceous Unconformity is very
well marked on most seismic sections from the North
Sea. Fault activity diminished during the Cretaceous,
and the Cretaceous subsidence was due primarily to
crustal cooling after the Jurassic rifting. The transition
from syn- to post-rift configuration was strongly
614
J.I. Faleide et al.
sections in Yorkshire.
The lower part of the Brent Group consists of
upward-coarsening beds of mica-rich sandstones that
represent prograding delta deposits, especially distributary mouth bars. This is the Etive and Rannoch
formations. The middle part is the Ness Formation,
comprised mostly of fluvial delta facies with channels,
crevasse sand deposits, lagoonal deposits and coal
beds. These delta top facies were deposited during a
northward progradation of the Brent Delta. At the top
of the Brent Group, the Tabert Formation is a generally well-sorted sandstone formed by the reworking of
deltaic deposits during a transgression. The sediment
supply from the south was no longer able to keep up
with the basin subsidence, resulting in a gradual
drowning of the Brent Delta. Each of these delta facies
has its characteristic reservoir properties, which are a
function both of primary sorting and mineralogy and
subsequent diagenetic modification.
Nearly all samples from these deltaic reservoir
rocks show evidence of meteoric water flushing and
have well developed authigenic pore-filling kaolinite
and partly dissolved feldspar (see Chap. 4). At burial
depth >4 km (130
C) kaolinite have reacted with Kfeldspar and is replaced by illite.
There is normally little clay coating on quartz in the
Brent sandstones and quartz cementation may be
extensive so that the porosity is reduced to 10-15%
at 4 km depth.
In the Late Jurassic, volcanism was very much
reduced and the areas within and surrounding the rift
systems subsided in response to lower geothermal
gradients. At the same time, normal faulting along
the Viking Graben led to the rotation of basement
blocks and their overlying sediments. The shoulders
of the tilted fault blocks were exposed to erosion,
removing Lower-Middle Jurassic and locally even
Upper Triassic strata. The Late Jurassic (Oxfordian)
transgression covered the Viking Graben with a thick
drape of clayey sediments of the Heather Formation,
while the coarser clastics (sand) were deposited as
turbidites and in deltas along the basin margins.
Some of the deltas appear to have been controlled by
the same structures that have determined the location
of the fjords in western Norway.
The uppermost Jurassic Kimmeridge Clay Formation is transgressive and often forms a several hundred
metres thick rich source rock which on the Norwegian
side is called the Draupne Formation. The rift topography produced numerous, locally overdeepened,
basins with poor bottom water circulation. Only a
relatively small proportion of the organic production
was oxidised while the sedimentation rate was fairly
high. The organic-rich shales of the Upper Jurassic are
thus the prime source rock in the North Sea, and
provided the main petroleum source in both the
Statfjord and Ekofisk areas. The thickness of the
Upper Jurassic sediments along the rift axis may
reach 3,000 m. The deposition of organic-rich shales
continued into the Early Cretaceous in some of the
basins. The edges of the rotated blocks suffered erosion and a few were not buried until the Late Cretaceous. The majority of the faults die out before the
Cretaceous but a few continue up into younger beds.
The rifting resulted in rotated fault blocks containing
sandstone reservoirs of Lower and Middle Jurassic age
(sandstones of the Brent Group and Statfjord Formation). Small fan deltas developed along the rift and
also deepwater sandstones including debris flows.
Some of these are good reservoir rocks occuring
within the Upper Jurassic source rocks (see Chap. 12).
Upper Jurassic sandstones deposited on submarine
slopes are much less leached and have less kaolinite
than the Brent sandstones. They may however contain
grain-coating micro-quartz which retards quartz
cementation, thus preserving porosity. This is linked
to a siliceous sponge (Rhaxella), see Chap. 4.
In much of the North Sea basin Kimmeridge shale
equivalents are immature but in the Upper Jurassic rift
structures this source rock is buried to much greater
depth corresponding to the oil window, and even gas
window.
25.2.2.6 Cretaceous
The last phase of rifting in the North Sea in the Late
Jurassic was followed by a major transgression, but the
uplifted rift stuctures remained dry and were islands
for most of the Early Cretaceous. There is a major
unconformity between the Cretaceous and the Jurassic
except in the deep parts of the rifts where there may
have been continuous sedimentation (Figs. 25.6 and
25.7). The Base Cretaceous Unconformity is very
well marked on most seismic sections from the North
Sea. Fault activity diminished during the Cretaceous,
and the Cretaceous subsidence was due primarily to
crustal cooling after the Jurassic rifting. The transition
from syn- to post-rift configuration was strongly
614
J.I. Faleide et al.