8b). Where fault-bend folds are associated with a large
flat with a significant inclination to the regional dip of
the master fault, and where sediment supply is good,
the trapped volumes might be considerable. In fact, the
total volume may further be increased since the
depocentre will move as the hangingwall is
transported across the flat, causing stacking of the
units. The major disadvantage foreseen for this type
of stratigraphic trap – especially in view of its distal
position – would be the difficulty for sediments to
bypass the hangingwall anticline. On the other hand,
the model allows the possibility of eroding the top of
the anticline, thus utilising a local source area. Largescale fault-bend folds are not uncommon (e.g. Njord
Field), but we are not aware that stratigraphic traps
related to such features have been reported in the
literature from the North Sea.
By continued subsidence along the master fault,
antithetic faults are likely to form on the distal flank
of the hangingwall fold, and a graben develops parallel
to the master fault. If the sedimentation rate does not
keep up with subsidence the graben may influence the
drainage pattern, and generate a sediment trap as well.
This eventually heralds the shift from an open to a
closed system. At starved basin margins the accommodation graben may further restrict development of
the primary fan, and in systems where the graben floor
is close to the marine level, the top of the accommodation structure may be eroded, and there may be local
sediment transport towards the master fault
(Fig. 12.11, trap types 7a, 7b).
Examples of this type of trap are found in the “Ula
trend”, which locally defines the margin of the Central
Graben. In block 2/2 a system of antithetic faults,
partly triggered by halokinesis, has developed a local
high, contributing to the initiation of a graben where
the sands of the Ula Formation have been trapped.
Gravity slides across fault escarpments may occur
along the crestal areas of the rotated fault blocks
(Fig. 12.11, trap type 7c). These are the most unstable
areas of the graben system and have their instability
enhanced by the flexural cantilever effect, which
predicts accentuation of uplift in this area. Eventually,
slight inversion may further contribute to the uplift of
fault-block crests along basin margins.
In reflection seismic data, erosion of uplifted and
rotated fault block crests may be the most easily
detectable effect, but deformation of the escarpment
by gravity sliding should not be neglected. The basic
transport mechanisms would be block sliding, rock
fall, or mass flow. In all these cases large volumes of
reservoir rocks may become resident in the
hangingwall in proximal or distal positions relative
to the master fault.
Different types of gravity slides have been reported
in this position in front of rotated fault-blocks from the
North Sea, but have not so far been deliberately
drilled.
12.3.1.3 Unconformity Traps Related to
Transition from Active Stretching to
Thermal Cooling
The transition from active stretching to thermal
cooling is manifested by a change in style of subsidence pattern. In the active stretching stage, fault
blocks rotate away from the graben axis, causing similar tilting in the sedimentary cover. The thermal
cooling stage, however, causes tilting of strata towards
the graben axis, mainly because the most rapid subsidence takes place along the graben centre (Fig. 12.9).
In strongly asymmetric grabens, this picture will of
course be modified accordingly.
Overall transgression and onlap of the crestal areas
may be expected towards the end of the active
stretching stage because of the overall subsidence of
the rifted area, and because sediment starvation is not
uncommon when subsidence outpaces sediment yield.
On a regional scale relief may be enhanced by
upheaval of graben shoulders due to isostacy and
elastic response to faulting. The spatial distribution
and the magnitude of elevated (possibly eroded) and
subsiding areas is influenced by the geometry and
depth of the detachments, and by whether the crustal
thinning happens during simple or pure shear. Accordingly, these factors will be of importance to the development, distribution and preservation of stratigraphic
traps.
The accelerated axial subsidence, which is likely to
occur during the early stages of cooling because of the
exponential nature of the thermal decay, will normally
be associated with marine transgression, and at the
stage where earlier graben walls are onlapped and
drowned, a break-up unconformity will develop.
Depending on the nature of the subsidence, the unconformity will be diachronously onlapped both along the
axis and transversely in the graben system. Depending
upon the relation between subsidence and sedimentation rate, isostatic stability of the graben margins and
336
R.H. Gabrielsen
flat with a significant inclination to the regional dip of
the master fault, and where sediment supply is good,
the trapped volumes might be considerable. In fact, the
total volume may further be increased since the
depocentre will move as the hangingwall is
transported across the flat, causing stacking of the
units. The major disadvantage foreseen for this type
of stratigraphic trap – especially in view of its distal
position – would be the difficulty for sediments to
bypass the hangingwall anticline. On the other hand,
the model allows the possibility of eroding the top of
the anticline, thus utilising a local source area. Largescale fault-bend folds are not uncommon (e.g. Njord
Field), but we are not aware that stratigraphic traps
related to such features have been reported in the
literature from the North Sea.
By continued subsidence along the master fault,
antithetic faults are likely to form on the distal flank
of the hangingwall fold, and a graben develops parallel
to the master fault. If the sedimentation rate does not
keep up with subsidence the graben may influence the
drainage pattern, and generate a sediment trap as well.
This eventually heralds the shift from an open to a
closed system. At starved basin margins the accommodation graben may further restrict development of
the primary fan, and in systems where the graben floor
is close to the marine level, the top of the accommodation structure may be eroded, and there may be local
sediment transport towards the master fault
(Fig. 12.11, trap types 7a, 7b).
Examples of this type of trap are found in the “Ula
trend”, which locally defines the margin of the Central
Graben. In block 2/2 a system of antithetic faults,
partly triggered by halokinesis, has developed a local
high, contributing to the initiation of a graben where
the sands of the Ula Formation have been trapped.
Gravity slides across fault escarpments may occur
along the crestal areas of the rotated fault blocks
(Fig. 12.11, trap type 7c). These are the most unstable
areas of the graben system and have their instability
enhanced by the flexural cantilever effect, which
predicts accentuation of uplift in this area. Eventually,
slight inversion may further contribute to the uplift of
fault-block crests along basin margins.
In reflection seismic data, erosion of uplifted and
rotated fault block crests may be the most easily
detectable effect, but deformation of the escarpment
by gravity sliding should not be neglected. The basic
transport mechanisms would be block sliding, rock
fall, or mass flow. In all these cases large volumes of
reservoir rocks may become resident in the
hangingwall in proximal or distal positions relative
to the master fault.
Different types of gravity slides have been reported
in this position in front of rotated fault-blocks from the
North Sea, but have not so far been deliberately
drilled.
12.3.1.3 Unconformity Traps Related to
Transition from Active Stretching to
Thermal Cooling
The transition from active stretching to thermal
cooling is manifested by a change in style of subsidence pattern. In the active stretching stage, fault
blocks rotate away from the graben axis, causing similar tilting in the sedimentary cover. The thermal
cooling stage, however, causes tilting of strata towards
the graben axis, mainly because the most rapid subsidence takes place along the graben centre (Fig. 12.9).
In strongly asymmetric grabens, this picture will of
course be modified accordingly.
Overall transgression and onlap of the crestal areas
may be expected towards the end of the active
stretching stage because of the overall subsidence of
the rifted area, and because sediment starvation is not
uncommon when subsidence outpaces sediment yield.
On a regional scale relief may be enhanced by
upheaval of graben shoulders due to isostacy and
elastic response to faulting. The spatial distribution
and the magnitude of elevated (possibly eroded) and
subsiding areas is influenced by the geometry and
depth of the detachments, and by whether the crustal
thinning happens during simple or pure shear. Accordingly, these factors will be of importance to the development, distribution and preservation of stratigraphic
traps.
The accelerated axial subsidence, which is likely to
occur during the early stages of cooling because of the
exponential nature of the thermal decay, will normally
be associated with marine transgression, and at the
stage where earlier graben walls are onlapped and
drowned, a break-up unconformity will develop.
Depending on the nature of the subsidence, the unconformity will be diachronously onlapped both along the
axis and transversely in the graben system. Depending
upon the relation between subsidence and sedimentation rate, isostatic stability of the graben margins and
336
R.H. Gabrielsen