imaging to be straightforward. When it comes to the
fault systems and the drag structures, these occur close
to the stem of the diapir and are likely to be covered
and completely obscured by the overhanging diapir
bulb.
One particular difficulty in seismic interpretation
may occur in cases where dynamic salt interacts with
faulting. At the crests of salt diapirs and stocks a
combination of ring-shaped and radial fault systems
is commonly found, but these are unproblematic in
seismic imaging and interpretation. In addition,
numerous examples exist of fault activity promoted
by the underlying active salt, providing a substratum
for detachment. Again, such structural relations are
also clearly displayed in reflection seismic data.
Finally, though, due to transfer of larger volumes of
salt towards the basin axis, smaller pillow-shaped
volumes of salt are frequently left and trapped along
basin margins, where they interact with the basin margin fault system. In such cases different configurations
are sometimes developed in the hangingwall and the
footwall of the salt-involved fault, causing complex
and contrasting sedimentary conditions across the
fault so that significant problems arise in sequence
correlation. Where salt has intruded along the faultplane, interpretation of seismic data may be hampered
by reduction of the general data quality.
In more complex tectonic environments (strike-slip
and contraction), the complexity and variety of salt
body configuration is commonly much greater,
because the final geometry of the salt body will be
determined by directed flow reflecting varying differential stress and strain. For example, the importance of
evaporite sequences in the development of many
thrust belts like the Pyrenees and the West Spitsbergen
thrust-and-fold belt is well documented. In such
settings seismic imaging may be complicated by salt
being involved as an extensive, continuous or
disrupted unit during the thrusting, and also because
it may have accumulated unevenly and become
integrated in contraction structures like fold cores
and duplexes, and as intrusions along fault planes.
The quality of seismic imaging performance in
areas of salt has been greatly improved in recent
years. Still, the days of surprises are not yet over
when results from drilling become available. The
effort in improving seismic imaging techniques must
therefore continue. And it should go hand in hand with
field study and analogue modelling.
12.8 Faults and Fault Architecture
Fractures (faults and joints) are found in practically all
hydrocarbon reservoirs and are crucial elements
because they both influence the migration of
hydrocarbons within the reservoir and contribute to
the entrapment of fluids. Due to great variation in
fault rocks and fracture types and their distribution,
the influence of fractures on reservoir communication
is not easily predictable. It is therefore natural that the
analysis of single faults and fracture systems is receiving increasing attention.
For the assessment of the architecture of faults and
fracture systems one can, in principle, chose between
stochastic and deterministic or a combination of the
two. Given the complexity and generalised architecture of larger faults, however, stochastic methods are
less favourable in analysis of such features. This also
seems to be the case for fracture systems generated in
stress situations where σ 1 is distinctly different from
σ 3 . To provide input to a deterministic reservoir model
that aims at taking the complexity of larger faults into
consideration, we have performed field studies in
order to constrain realistic characteristics for the
units that commonly can be defined within the realm
of a fault zone.
Since mesoscopic and macroscopic faults affect
volumes of rock, and accordingly should be described
as composite rock bodies that include a complex system of structures (fault rock, folds and fractures), the
term “fault zone” is applied here. In the following we
use “fracture” as a general term that includes faults,
deformation bands and joints, and we distinguish
between shear fractures (microscale) and faults
(meso- and megascale). Also, we use the term “highstrain zone” for the parts of the fault core where shear
is concentrated.
12.8.1 The Structural Elements of the Fault
Zone
It is well established that faults are commonly zoned
and composed of several units with distinct deformation styles (Fig. 12.17). These include the fault core,
where most of the displacement is accommodated, and
its associated damage zones that are geometrically and
mechanically related to the development of the fault.
12 The Structure and Hydrocarbon Traps of Sedimentary Basins
347
fault systems and the drag structures, these occur close
to the stem of the diapir and are likely to be covered
and completely obscured by the overhanging diapir
bulb.
One particular difficulty in seismic interpretation
may occur in cases where dynamic salt interacts with
faulting. At the crests of salt diapirs and stocks a
combination of ring-shaped and radial fault systems
is commonly found, but these are unproblematic in
seismic imaging and interpretation. In addition,
numerous examples exist of fault activity promoted
by the underlying active salt, providing a substratum
for detachment. Again, such structural relations are
also clearly displayed in reflection seismic data.
Finally, though, due to transfer of larger volumes of
salt towards the basin axis, smaller pillow-shaped
volumes of salt are frequently left and trapped along
basin margins, where they interact with the basin margin fault system. In such cases different configurations
are sometimes developed in the hangingwall and the
footwall of the salt-involved fault, causing complex
and contrasting sedimentary conditions across the
fault so that significant problems arise in sequence
correlation. Where salt has intruded along the faultplane, interpretation of seismic data may be hampered
by reduction of the general data quality.
In more complex tectonic environments (strike-slip
and contraction), the complexity and variety of salt
body configuration is commonly much greater,
because the final geometry of the salt body will be
determined by directed flow reflecting varying differential stress and strain. For example, the importance of
evaporite sequences in the development of many
thrust belts like the Pyrenees and the West Spitsbergen
thrust-and-fold belt is well documented. In such
settings seismic imaging may be complicated by salt
being involved as an extensive, continuous or
disrupted unit during the thrusting, and also because
it may have accumulated unevenly and become
integrated in contraction structures like fold cores
and duplexes, and as intrusions along fault planes.
The quality of seismic imaging performance in
areas of salt has been greatly improved in recent
years. Still, the days of surprises are not yet over
when results from drilling become available. The
effort in improving seismic imaging techniques must
therefore continue. And it should go hand in hand with
field study and analogue modelling.
12.8 Faults and Fault Architecture
Fractures (faults and joints) are found in practically all
hydrocarbon reservoirs and are crucial elements
because they both influence the migration of
hydrocarbons within the reservoir and contribute to
the entrapment of fluids. Due to great variation in
fault rocks and fracture types and their distribution,
the influence of fractures on reservoir communication
is not easily predictable. It is therefore natural that the
analysis of single faults and fracture systems is receiving increasing attention.
For the assessment of the architecture of faults and
fracture systems one can, in principle, chose between
stochastic and deterministic or a combination of the
two. Given the complexity and generalised architecture of larger faults, however, stochastic methods are
less favourable in analysis of such features. This also
seems to be the case for fracture systems generated in
stress situations where σ 1 is distinctly different from
σ 3 . To provide input to a deterministic reservoir model
that aims at taking the complexity of larger faults into
consideration, we have performed field studies in
order to constrain realistic characteristics for the
units that commonly can be defined within the realm
of a fault zone.
Since mesoscopic and macroscopic faults affect
volumes of rock, and accordingly should be described
as composite rock bodies that include a complex system of structures (fault rock, folds and fractures), the
term “fault zone” is applied here. In the following we
use “fracture” as a general term that includes faults,
deformation bands and joints, and we distinguish
between shear fractures (microscale) and faults
(meso- and megascale). Also, we use the term “highstrain zone” for the parts of the fault core where shear
is concentrated.
12.8.1 The Structural Elements of the Fault
Zone
It is well established that faults are commonly zoned
and composed of several units with distinct deformation styles (Fig. 12.17). These include the fault core,
where most of the displacement is accommodated, and
its associated damage zones that are geometrically and
mechanically related to the development of the fault.
12 The Structure and Hydrocarbon Traps of Sedimentary Basins
347