individual fault blocks, the unconformity may be very
complex. In this situation a number of unconformity
traps may develop.
Truncational unconformity traps are formed by
erosion of units deposited during the active stretching
stage. Fault block rotation and incipient compaction of
the sediment package are likely to cause slightly tilted
units which may be eroded and sealed by the thermal
subsidence shales (Fig. 12.11, trap type 10a). In principle, this trap type may form in all parts of the graben
system, but it is most likely to develop along the
graben shoulders.
On-lapping unconformity traps are found above the
unconformity, and are dependent upon whether erosion on nearby highs has taken place or not
(Fig. 12.11, trap type 10b). The relief across master
faults may be considerable at this stage, and it is likely
that there is significant erosion and reworking of sands
from the marginal highs at this stage. Since the fault
activity is retarded, the graben relief will diminish
during this process, and the traps may be of considerable lateral extent and contain large volumes of sand.
12.3.1.4 Stratigraphic Traps Related to
Thermal Subsidence and Sediment
Loading
At this stage sediment transport may be both axial and
transverse within the graben system, but it is likely that
the transverse systems will dominate in the basin margin areas close to major hinterland relief. Minor
continued fault activity should still be expected along
the master faults of the graben margins due to isostatic
adjustments. These areas will also act as pivots during
the shift in subsidence pattern. Altogether this implies
relatively smooth graben slopes, with an increased
possibilty to develop thick and extensive sandsheets
with axes oriented transversely to the graben axis.
Because of the high rate of subsidence of the graben
floor at this stage, deposition is likely to take place in a
marine environment and the axial basin may be
starved of sediment. As the thermal gradients of the
system approach equilibrium, the basin will fill in and
finally level out the relief completely.
Basin-margin fans (Fig. 12.11, trap type 11a) represent a well-described trap type. These are true stratigraphic or palaeotopographic in type, and will
normally have great lateral extent. Their thickness
will depend upon the degree to which the graben relief
was levelled out when deposition took place.
As the graben is expected to be filled by water,
transport agents may be gravity mass flows and turbidity currents (Fig. 12.11, trap type 11b). Large transport
distances are therefore possible and the submarine
fans may be completely separated from the delta
systems along basin margins, particularly in periods
with low-stand of sea level (Fig. 12.11, trap type 12).
Examples here include some of the main reservoirs in
the North Sea, like the Frigg, Forties and Bruce fields.
The platform-vergent fans are closely related to the
graben-vergent fans, but occur between crests of
rotated fault-blocks (Fig. 12.11, trap type 13). During
infilling of the graben, sedimentary packages in areas
with the thicker sedimentary fill will tend to suffer a
stronger compaction than areas with thin packages.
This results in development of a hangingwall compaction syncline, which, if it has surface expression, may
act as a local sediment trap.
The platform-vergent fans will, however, be more
restricted than the basin-vergent ones, and are more
dependent on a local sediment source. These
circumstances make this trap type less attractive due
to small potential sediment volumes.
12.4 Strike-Slip Systems
The strike-slip structural regime is characterised by
horizontal orientations of σ 1 and σ 3 , whereas σ 2 is
vertical (Fig. 12.2). Hence, the orientation of the
plane of τ max is vertical, which is also the orientation
of the master faults. The displacement along the master fault will be in the horizontal plane (parallel to
strike) and the faulting includes initiation of a complex
system of secondary fractures.
The general development of shear systems can
conveniently be analysed by the use of analogue
mechanical experiments. Strike-slip systems are
highly dynamic, and the geometry of the initial stages
is very different from that of the mature stages of
development. Figure 12.13a shows the relation
between the structural elements at the initial stage of
strain for a right-lateral (dextral) shear system. To
analyse the shear system, one decomposes the shearforces into compressional and tensional vectors by
constructing a vector parallelogram. The dominant
features define a system of conjugate fractures
(Riedel- and Riedel’-shears) that are related to the
compressive component of the shear. Of these, the
12 The Structure and Hydrocarbon Traps of Sedimentary Basins
337
complex. In this situation a number of unconformity
traps may develop.
Truncational unconformity traps are formed by
erosion of units deposited during the active stretching
stage. Fault block rotation and incipient compaction of
the sediment package are likely to cause slightly tilted
units which may be eroded and sealed by the thermal
subsidence shales (Fig. 12.11, trap type 10a). In principle, this trap type may form in all parts of the graben
system, but it is most likely to develop along the
graben shoulders.
On-lapping unconformity traps are found above the
unconformity, and are dependent upon whether erosion on nearby highs has taken place or not
(Fig. 12.11, trap type 10b). The relief across master
faults may be considerable at this stage, and it is likely
that there is significant erosion and reworking of sands
from the marginal highs at this stage. Since the fault
activity is retarded, the graben relief will diminish
during this process, and the traps may be of considerable lateral extent and contain large volumes of sand.
12.3.1.4 Stratigraphic Traps Related to
Thermal Subsidence and Sediment
Loading
At this stage sediment transport may be both axial and
transverse within the graben system, but it is likely that
the transverse systems will dominate in the basin margin areas close to major hinterland relief. Minor
continued fault activity should still be expected along
the master faults of the graben margins due to isostatic
adjustments. These areas will also act as pivots during
the shift in subsidence pattern. Altogether this implies
relatively smooth graben slopes, with an increased
possibilty to develop thick and extensive sandsheets
with axes oriented transversely to the graben axis.
Because of the high rate of subsidence of the graben
floor at this stage, deposition is likely to take place in a
marine environment and the axial basin may be
starved of sediment. As the thermal gradients of the
system approach equilibrium, the basin will fill in and
finally level out the relief completely.
Basin-margin fans (Fig. 12.11, trap type 11a) represent a well-described trap type. These are true stratigraphic or palaeotopographic in type, and will
normally have great lateral extent. Their thickness
will depend upon the degree to which the graben relief
was levelled out when deposition took place.
As the graben is expected to be filled by water,
transport agents may be gravity mass flows and turbidity currents (Fig. 12.11, trap type 11b). Large transport
distances are therefore possible and the submarine
fans may be completely separated from the delta
systems along basin margins, particularly in periods
with low-stand of sea level (Fig. 12.11, trap type 12).
Examples here include some of the main reservoirs in
the North Sea, like the Frigg, Forties and Bruce fields.
The platform-vergent fans are closely related to the
graben-vergent fans, but occur between crests of
rotated fault-blocks (Fig. 12.11, trap type 13). During
infilling of the graben, sedimentary packages in areas
with the thicker sedimentary fill will tend to suffer a
stronger compaction than areas with thin packages.
This results in development of a hangingwall compaction syncline, which, if it has surface expression, may
act as a local sediment trap.
The platform-vergent fans will, however, be more
restricted than the basin-vergent ones, and are more
dependent on a local sediment source. These
circumstances make this trap type less attractive due
to small potential sediment volumes.
12.4 Strike-Slip Systems
The strike-slip structural regime is characterised by
horizontal orientations of σ 1 and σ 3 , whereas σ 2 is
vertical (Fig. 12.2). Hence, the orientation of the
plane of τ max is vertical, which is also the orientation
of the master faults. The displacement along the master fault will be in the horizontal plane (parallel to
strike) and the faulting includes initiation of a complex
system of secondary fractures.
The general development of shear systems can
conveniently be analysed by the use of analogue
mechanical experiments. Strike-slip systems are
highly dynamic, and the geometry of the initial stages
is very different from that of the mature stages of
development. Figure 12.13a shows the relation
between the structural elements at the initial stage of
strain for a right-lateral (dextral) shear system. To
analyse the shear system, one decomposes the shearforces into compressional and tensional vectors by
constructing a vector parallelogram. The dominant
features define a system of conjugate fractures
(Riedel- and Riedel’-shears) that are related to the
compressive component of the shear. Of these, the
12 The Structure and Hydrocarbon Traps of Sedimentary Basins
337