uncommon that shortening is accommodated by
hinterland-directed thrusting. In cases where the single
faults do not become joined along a roof fault, a
system of “blind” faults may develop, terminating at
a tip-line. Alternatively, the faults may break the surface. In rarer cases the faults climb down-section in the
direction of transport. This is an indication that the
strain rate is greater along the roof-fault than it is along
the floor fault.
The regions where contractional faults climb upsection are termed ramps and the total geometry of the
fault is that of a ramp-flat-ramp. In such cases, the pure
geometry of the fault planes forces the strata inside the
horses to become folded. The folds reflect the geometry and steepness of the fault plane because the front of
the horse depends on the cut-out angle of the original
ramp. Ramps parallel to the transport direction may
also influence the development and the geometry of
the thrust system. Such features may potentially separate subunits of contrasting deformational style. In
cases where strain rates are not similar across the
ramp, shear and strong rotation occur, and when the
structures propagate to affect the surface topography
they may strongly influence the depositional systems
associated with the mountain chain.
It is obvious that the tectonic processes described
above produce a variety of structural traps, among
which anticlines with along-strike closure and horses
delineated by sealing faults may be the most obvious.
Because the subsurface structuring also per definition
affects the topography during mountain building, different types of subtle and stratigraphic traps are also
likely to be generated (Fig. 12.14a).
12.5.2 Hydrocarbon Prospectivity of
Contractional Regimes
The very dynamic character of contractional systems
obviously produces a variety of sedimentary systems
and structural and stratigraphic traps. The relief
associated with orogens normally is measured in
kilometres. Strong erosional forces, gravitational
instability and climatic influences are important
parameters in the development of mountain chains.
The system is flooded with a variety of clastic erosional products, the mineralogical composition of
which reflects the types of rocks that are involved in
the orogen in the first place.
This implies that reservoir rocks and hydrocarbon
traps of all kinds are abundant. Because the mountain
chain necessarily is uplifted, however, organic-rich
marine deposits of the kind that would produce the
source rock, would be rare. An exception to this would
be cases where a source rock deposited before the
contraction started becomes involved in the orogen.
In such cases, the critical factors would be the depth of
tectonic burial of the source rock, the geothermal
gradient of the greater orogeny and the positioning of
the source rock relative to the reservoirs. In the
dynamic environment of a nappe pile, it must be
taken into consideration that units now separated by
tens of kilometres may have been juxtaposed at the
time of maturation and migration.
Three principally different basin settings can be
distinguished. Intramontane basins are collapse
structures or structural lows generated by folding and
thrusting inside the realm of the mountain chain.
Those active at the peak tectonic activity are likely
to trap large amounts of coarse clastics over a short
period of time. They are in most cases of restricted size
and source rocks are rarely associated with them.
Foreland basins are far more interesting from a hydrocarbon exploration point of view. These are stabilised
as accommodation areas for sediments due to the
gravity load of the progressing orogen, and may trap
the bulk of the sediments eroded from the rising
mountains and transported towards the orogenic
front. The central parts of foreland basins may reach
thousands of metres in depth and constitute deep
marine depositional systems. Thus, the foreland
basin may offer a whole range of sedimentary
environments from fluvial, via shallow marine to
deep marine. The structuring in the foreland basin
position is moderate, but increasing during the progressive development of the orogen. Thus, the basin
will eventually become overrun by the advancing
deformation front and cannibalised. In this process, a
variety of structures are developed from gravitational
extensional to contractional complexes of folds, thrust
sheets and duplexes. In basins related to subduction
zones, different types of accretionary prisms may provide source rocks and reservoir rocks, as well as traps,
both stratigraphic and structutral. However, these are
very dynamic systems, sometimes too dynamic to
provide low-risk exploration targets. Also, the
sediments in such systems are likely to be too finegrained to provide good reservoirs.
342
R.H. Gabrielsen
hinterland-directed thrusting. In cases where the single
faults do not become joined along a roof fault, a
system of “blind” faults may develop, terminating at
a tip-line. Alternatively, the faults may break the surface. In rarer cases the faults climb down-section in the
direction of transport. This is an indication that the
strain rate is greater along the roof-fault than it is along
the floor fault.
The regions where contractional faults climb upsection are termed ramps and the total geometry of the
fault is that of a ramp-flat-ramp. In such cases, the pure
geometry of the fault planes forces the strata inside the
horses to become folded. The folds reflect the geometry and steepness of the fault plane because the front of
the horse depends on the cut-out angle of the original
ramp. Ramps parallel to the transport direction may
also influence the development and the geometry of
the thrust system. Such features may potentially separate subunits of contrasting deformational style. In
cases where strain rates are not similar across the
ramp, shear and strong rotation occur, and when the
structures propagate to affect the surface topography
they may strongly influence the depositional systems
associated with the mountain chain.
It is obvious that the tectonic processes described
above produce a variety of structural traps, among
which anticlines with along-strike closure and horses
delineated by sealing faults may be the most obvious.
Because the subsurface structuring also per definition
affects the topography during mountain building, different types of subtle and stratigraphic traps are also
likely to be generated (Fig. 12.14a).
12.5.2 Hydrocarbon Prospectivity of
Contractional Regimes
The very dynamic character of contractional systems
obviously produces a variety of sedimentary systems
and structural and stratigraphic traps. The relief
associated with orogens normally is measured in
kilometres. Strong erosional forces, gravitational
instability and climatic influences are important
parameters in the development of mountain chains.
The system is flooded with a variety of clastic erosional products, the mineralogical composition of
which reflects the types of rocks that are involved in
the orogen in the first place.
This implies that reservoir rocks and hydrocarbon
traps of all kinds are abundant. Because the mountain
chain necessarily is uplifted, however, organic-rich
marine deposits of the kind that would produce the
source rock, would be rare. An exception to this would
be cases where a source rock deposited before the
contraction started becomes involved in the orogen.
In such cases, the critical factors would be the depth of
tectonic burial of the source rock, the geothermal
gradient of the greater orogeny and the positioning of
the source rock relative to the reservoirs. In the
dynamic environment of a nappe pile, it must be
taken into consideration that units now separated by
tens of kilometres may have been juxtaposed at the
time of maturation and migration.
Three principally different basin settings can be
distinguished. Intramontane basins are collapse
structures or structural lows generated by folding and
thrusting inside the realm of the mountain chain.
Those active at the peak tectonic activity are likely
to trap large amounts of coarse clastics over a short
period of time. They are in most cases of restricted size
and source rocks are rarely associated with them.
Foreland basins are far more interesting from a hydrocarbon exploration point of view. These are stabilised
as accommodation areas for sediments due to the
gravity load of the progressing orogen, and may trap
the bulk of the sediments eroded from the rising
mountains and transported towards the orogenic
front. The central parts of foreland basins may reach
thousands of metres in depth and constitute deep
marine depositional systems. Thus, the foreland
basin may offer a whole range of sedimentary
environments from fluvial, via shallow marine to
deep marine. The structuring in the foreland basin
position is moderate, but increasing during the progressive development of the orogen. Thus, the basin
will eventually become overrun by the advancing
deformation front and cannibalised. In this process, a
variety of structures are developed from gravitational
extensional to contractional complexes of folds, thrust
sheets and duplexes. In basins related to subduction
zones, different types of accretionary prisms may provide source rocks and reservoir rocks, as well as traps,
both stratigraphic and structutral. However, these are
very dynamic systems, sometimes too dynamic to
provide low-risk exploration targets. Also, the
sediments in such systems are likely to be too finegrained to provide good reservoirs.
342
R.H. Gabrielsen