the deformation. Thus, a mildly deformed province may
contain sets of symmetrical buckle-folds (Figs. 12.1a and
12.16b), whereas a province of advanced contractional
deformation may be dominated by systems of complex,
overturned fault-propagation folds and thrusts
(Figs. 12.1a and 12.15).
Complex folds are also common in strike-slip
regimes, developed either as fold-trains formed orthogonally to the contractional axis of the strike-slip system (Figs. 12.1c and 12.14) in areas of constraining
bends (Fig. 12.14c), or in connection with transpressional
segments of the strike-slip system. Here, folds may
grade into flower structures. (Figs. 12.1a and 12.14c).
Many petroleum provinces occur in extensional basin
settings, where the rotated fault block and affiliated
accommodation structures provide the most common
trap type (Figs. 12.1b, 12.8, 12.10 and 12.11). For example some of the major hydrocarbon fields in the Norwegian continental shelf, like the Statfjord and the Oseberg
fields, occur in such settings. The rotated fault block
itself becomes tilted according to the amount of extension and the geometry of the principal fault plane (planar
or listric). Secondary traps may occur due to elastic
uplift of the footwall, roll-overs in the hangingwall due
to a strongly listric fault plane, or forced folds associated
with ramps in the detachment (Fig. 12.1e).
Halokinesis (the processes associated with the salt
movements) occurs because salt has a specific gravity
of 2.2 g/cm
3 ; as opposed to fully consolidated (low
porosity) sedimentary rocks that have specific
gravities of 2.5–2.7 g/cm
3 ), creating a buoyancy
effect. This is further supported by the fact that on a
geological timescale, salt flows plastically. A number
of pronounced structural types result, from pillows to
diapirs of salt. Strong structuring may also be
associated with mud diapirism. In a classical study of
mild to extensive salt movements, Halbouty (1979)
identified no less than nine different structural trap
types (Figs. 12.1 and 12.17). The general experience
is that larger structures are affiliated with early-stage
salt structures, like pillows, whereas a much more
complex structural pattern and dismembering of the
structural traps are typical for the more advanced
stages of deformation, including diapirism.
In the following sections, trap types and risks
associated with the different trap types are investigated.
12.1.1 Principal Stress Regimes and Types
of Stress
The concept of plate tectonics offers a useful framework for structural geological analysis on all relevant
scales in petroleum geology, from regional in the
exploration stage, to local in the reservoir evaluation
and production stages. This is natural, because the
principal geological stress systems are ruled by processes in the deep Earth like mantle convection and
lithosphere subduction, the secondary effects of which
are manifested at the base of the lithosphere and along
plate margins. Based on these concepts, the basic
dynamics of the lithosphere can be quantified, which
is a prerequisite for the evaluation and calculation of
the state of stress at any point. As seen in the perspective of the petroleum structural geologist, understanding and quantifying the stress situation at the plate
margins is a prerequisite for understanding the state
of stress in any basin system and in any reservoir.
We term principal stresses originated at plate
margins far-field or contemporary stresses. The stress
situation in a basin or a reservoir may be a sum of
several far-field stresses combined with a local stress,
which may be related to burial, erosion, geothermal
gradients, topography, basement relief and structural
inhomogeneities in the substratum. In other words, the
plate tectonic framework provides a basic and general
concept on which any structural geological analysis of
a sedimentary basin rests, but it must be supplied with
information on the local stress system that is
superimposed on it. In the context of the far-field
plate tectonic stress, we distinguish between the plate
boundary and the intra-plate component.
ä ä ä
Fig. 12.1 (continued) the development of fault propagation
fold and a fully developed thrust. See Figs. 12.2 (for structural)
and 12.15 (for stratigraphic) traps associated with contractional
or inverted terranes. (b) Typical structural trap type associated
with strike-slip (positive and negative flower structures and
associated folds. See Fig. 12.13 for possible stratigraphic traps
in strike-slip regimes. (c) Extensional, rotated fault-blocks
above a decollement (floor fault). See Figs. 12.11 and 12.12
for structural, stratigraphic and unconformity traps in such
systems. (d) Different types of salt structures and associated
trap types. (See also Fig. 12.16)
12 The Structure and Hydrocarbon Traps of Sedimentary Basins
321
contain sets of symmetrical buckle-folds (Figs. 12.1a and
12.16b), whereas a province of advanced contractional
deformation may be dominated by systems of complex,
overturned fault-propagation folds and thrusts
(Figs. 12.1a and 12.15).
Complex folds are also common in strike-slip
regimes, developed either as fold-trains formed orthogonally to the contractional axis of the strike-slip system (Figs. 12.1c and 12.14) in areas of constraining
bends (Fig. 12.14c), or in connection with transpressional
segments of the strike-slip system. Here, folds may
grade into flower structures. (Figs. 12.1a and 12.14c).
Many petroleum provinces occur in extensional basin
settings, where the rotated fault block and affiliated
accommodation structures provide the most common
trap type (Figs. 12.1b, 12.8, 12.10 and 12.11). For example some of the major hydrocarbon fields in the Norwegian continental shelf, like the Statfjord and the Oseberg
fields, occur in such settings. The rotated fault block
itself becomes tilted according to the amount of extension and the geometry of the principal fault plane (planar
or listric). Secondary traps may occur due to elastic
uplift of the footwall, roll-overs in the hangingwall due
to a strongly listric fault plane, or forced folds associated
with ramps in the detachment (Fig. 12.1e).
Halokinesis (the processes associated with the salt
movements) occurs because salt has a specific gravity
of 2.2 g/cm
3 ; as opposed to fully consolidated (low
porosity) sedimentary rocks that have specific
gravities of 2.5–2.7 g/cm
3 ), creating a buoyancy
effect. This is further supported by the fact that on a
geological timescale, salt flows plastically. A number
of pronounced structural types result, from pillows to
diapirs of salt. Strong structuring may also be
associated with mud diapirism. In a classical study of
mild to extensive salt movements, Halbouty (1979)
identified no less than nine different structural trap
types (Figs. 12.1 and 12.17). The general experience
is that larger structures are affiliated with early-stage
salt structures, like pillows, whereas a much more
complex structural pattern and dismembering of the
structural traps are typical for the more advanced
stages of deformation, including diapirism.
In the following sections, trap types and risks
associated with the different trap types are investigated.
12.1.1 Principal Stress Regimes and Types
of Stress
The concept of plate tectonics offers a useful framework for structural geological analysis on all relevant
scales in petroleum geology, from regional in the
exploration stage, to local in the reservoir evaluation
and production stages. This is natural, because the
principal geological stress systems are ruled by processes in the deep Earth like mantle convection and
lithosphere subduction, the secondary effects of which
are manifested at the base of the lithosphere and along
plate margins. Based on these concepts, the basic
dynamics of the lithosphere can be quantified, which
is a prerequisite for the evaluation and calculation of
the state of stress at any point. As seen in the perspective of the petroleum structural geologist, understanding and quantifying the stress situation at the plate
margins is a prerequisite for understanding the state
of stress in any basin system and in any reservoir.
We term principal stresses originated at plate
margins far-field or contemporary stresses. The stress
situation in a basin or a reservoir may be a sum of
several far-field stresses combined with a local stress,
which may be related to burial, erosion, geothermal
gradients, topography, basement relief and structural
inhomogeneities in the substratum. In other words, the
plate tectonic framework provides a basic and general
concept on which any structural geological analysis of
a sedimentary basin rests, but it must be supplied with
information on the local stress system that is
superimposed on it. In the context of the far-field
plate tectonic stress, we distinguish between the plate
boundary and the intra-plate component.
ä ä ä
Fig. 12.1 (continued) the development of fault propagation
fold and a fully developed thrust. See Figs. 12.2 (for structural)
and 12.15 (for stratigraphic) traps associated with contractional
or inverted terranes. (b) Typical structural trap type associated
with strike-slip (positive and negative flower structures and
associated folds. See Fig. 12.13 for possible stratigraphic traps
in strike-slip regimes. (c) Extensional, rotated fault-blocks
above a decollement (floor fault). See Figs. 12.11 and 12.12
for structural, stratigraphic and unconformity traps in such
systems. (d) Different types of salt structures and associated
trap types. (See also Fig. 12.16)
12 The Structure and Hydrocarbon Traps of Sedimentary Basins
321