separates the subplatform from the interior graben,
together with the extra-marginal fault complex, are
the most profound fault zones of the basin, and the
two are likely to be linked along the principal detachment found within the lower crust. The subplatform is
heavily faulted and encompasses a number of secondary rotated fault blocks that again may be criss-crossed
by a third order of faults. There are examples that the
second order faults flatten along local detachments at
shallower levels than both the primary master faults of
the marginal fault complex and the inner and outer
marginal fault systems. The inner marginal fault system also coincides with the axis of basinward rotation
as activated during the post-rift stage.
The interior basin is the unit of the basin which is
underlain by the most extensively thinned crust and
where the maximum post-rift subsidence occurs. In
the case of a symmetrical (pure-shear) basin, it is
delineated on both margins by inner margin fault
systems, whereas in the case of an asymmetrical
basin (simple shear) only one of the margins has this
status. Even the deepest part of the basin is underlain
by rotated fault blocks, initiated during the active
stretching stage of the basin formation.
It should be noted that the structural elements
described above are not likely to be present along the
entire basin margin. Thus, in some segments, the platform may be in direct contact with the interior basin,
whereas in other segments the platform or the platform
marginal high may be missing. This inconsistency
may reflect the influence of structural or lithological
inhomogeneities in the basement, varying strain rates
or uneven bulk extension along the basin axis. Indeed,
it is common for large rifts that the basin is divided
into several sub-basins or basin units, each distinguished by its particular geometry and even polarity.
12.3.1 The Structural Influence on Reservoir
and Source Rock Distribution in
Extensional Basins
The types of traps related to the different stages in
graben formation are illustrated in Fig. 12.11. The
numbered trap types mentioned in the following
sections refer to this figure.
12.3.1.1 The Initial Stage
According to the model presented above, sedimentation during the initial stage will occur in a broad,
shallow basin with moderate surface gradients. Due
to the moderate stretching at this stage, sedimentation
will generally keep pace with subsidence. Local
depocentres would be related to few, steep normal
faults with attached accommodation structures and
shallow catchments. Because of the moderate surface
gradients and only few and minor fault escarpments, it
is reasonable to assume that source areas for sand must
be sought outside the marginal platform fault system.
Traps generated at this stage can be buried to very
great depths (several kilometres) during the total
graben subsidence. In addition to potential overmaturation due to deep burial, the scarcity of source
rocks may be a general problem for prospectivity
of traps related to this stage, because terrestrial
conditions are likely to prevail. However, exceptions
to this are for example the West African rift basin
lacustrine shales.
In cases where later extension has caused significant rotation of the pre-rift succession, the burial problem may be locally avoided, with pre-rift strata riding
structurally high, for example along the marginal areas
of the Viking Graben tilt blocks. As a consequence of
their early establishment in the subsidence history,
traps of this type may become faulted and fragmented
by later movements. Even though the topography is
influenced by active faults to a limited degree, traps
related to such structures still may occur.
The general three-stage model suggests that axial
transport dominates in the initial graben stage and
stratigraphic traps might be generated by the axial
fluvial system (Fig. 12.11, trap type 4). The modest
tectonic subsidence characteristic of this stage would
favour large lateral extension and good continuity of
the sand sheet. The stratigraphic traps would be related
to meandering or braided river systems and shallow
lakes, and the geometry of the system would to a large
extent be ruled by the subsidence rates in the incipient
central graben. Examples of axially transported
sandstones of this type in the primitive Viking Graben
include the Statfjord Formation and Lomvi Formation.
These sheet sands contrast with the more lenticular
sand bodies of the Lunde and Teist formations,
332
R.H. Gabrielsen
together with the extra-marginal fault complex, are
the most profound fault zones of the basin, and the
two are likely to be linked along the principal detachment found within the lower crust. The subplatform is
heavily faulted and encompasses a number of secondary rotated fault blocks that again may be criss-crossed
by a third order of faults. There are examples that the
second order faults flatten along local detachments at
shallower levels than both the primary master faults of
the marginal fault complex and the inner and outer
marginal fault systems. The inner marginal fault system also coincides with the axis of basinward rotation
as activated during the post-rift stage.
The interior basin is the unit of the basin which is
underlain by the most extensively thinned crust and
where the maximum post-rift subsidence occurs. In
the case of a symmetrical (pure-shear) basin, it is
delineated on both margins by inner margin fault
systems, whereas in the case of an asymmetrical
basin (simple shear) only one of the margins has this
status. Even the deepest part of the basin is underlain
by rotated fault blocks, initiated during the active
stretching stage of the basin formation.
It should be noted that the structural elements
described above are not likely to be present along the
entire basin margin. Thus, in some segments, the platform may be in direct contact with the interior basin,
whereas in other segments the platform or the platform
marginal high may be missing. This inconsistency
may reflect the influence of structural or lithological
inhomogeneities in the basement, varying strain rates
or uneven bulk extension along the basin axis. Indeed,
it is common for large rifts that the basin is divided
into several sub-basins or basin units, each distinguished by its particular geometry and even polarity.
12.3.1 The Structural Influence on Reservoir
and Source Rock Distribution in
Extensional Basins
The types of traps related to the different stages in
graben formation are illustrated in Fig. 12.11. The
numbered trap types mentioned in the following
sections refer to this figure.
12.3.1.1 The Initial Stage
According to the model presented above, sedimentation during the initial stage will occur in a broad,
shallow basin with moderate surface gradients. Due
to the moderate stretching at this stage, sedimentation
will generally keep pace with subsidence. Local
depocentres would be related to few, steep normal
faults with attached accommodation structures and
shallow catchments. Because of the moderate surface
gradients and only few and minor fault escarpments, it
is reasonable to assume that source areas for sand must
be sought outside the marginal platform fault system.
Traps generated at this stage can be buried to very
great depths (several kilometres) during the total
graben subsidence. In addition to potential overmaturation due to deep burial, the scarcity of source
rocks may be a general problem for prospectivity
of traps related to this stage, because terrestrial
conditions are likely to prevail. However, exceptions
to this are for example the West African rift basin
lacustrine shales.
In cases where later extension has caused significant rotation of the pre-rift succession, the burial problem may be locally avoided, with pre-rift strata riding
structurally high, for example along the marginal areas
of the Viking Graben tilt blocks. As a consequence of
their early establishment in the subsidence history,
traps of this type may become faulted and fragmented
by later movements. Even though the topography is
influenced by active faults to a limited degree, traps
related to such structures still may occur.
The general three-stage model suggests that axial
transport dominates in the initial graben stage and
stratigraphic traps might be generated by the axial
fluvial system (Fig. 12.11, trap type 4). The modest
tectonic subsidence characteristic of this stage would
favour large lateral extension and good continuity of
the sand sheet. The stratigraphic traps would be related
to meandering or braided river systems and shallow
lakes, and the geometry of the system would to a large
extent be ruled by the subsidence rates in the incipient
central graben. Examples of axially transported
sandstones of this type in the primitive Viking Graben
include the Statfjord Formation and Lomvi Formation.
These sheet sands contrast with the more lenticular
sand bodies of the Lunde and Teist formations,
332
R.H. Gabrielsen