where the problem of seismic imaging is restricted to
seeing through the salt as such. It is still fair to say that
the structural geology of evaporites has attracted much
attention, partly because the high mobility of such
deposits poses intriguing structural geological
problems, and even more so because salt structures
are associated with a variety of structural and stratigraphic traps of significance for the petroleum industry. The structural geology of salt very much reflects
the local tectonic environment, be it extensional, contractional or strike-slip. This implies that salt bodies
come in an almost infinite variety of shapes, some of
which even challenge the limits of the imagination.
This variety of shapes, combined with the acoustic
properties of salt, poses great challenges for reflection
seismic imaging of salt bodies and the strata beneath
and close to them.
Gravity-driven deformation at continental margins
is generated by the regional gradient of the margin
itself and is characterised by upslope extension and
downslope contraction. Salt and its overlying sedimentary pile spread in a seaward direction due to
regional tilting in response to lithosphere cooling,
whereas synkinematic sedimentation induces loading
instabilities. At basin scale, thin-skinned deformation
may induce extreme upslope salt thinning, leading to
the formation of salt welds, as well as massive downslope salt thickening. Most often, the extensional
domain can be divided into three sub-domains, which
are, in a seaward direction, the sealed tilted block,
growth
fault/rollover,
and
diapir
domains
(Fig. 12.16a). The upper domain is characterised by
tilted blocks that are sealed early by synkinematic
sedimentation. The rollover domain displays a large
amount of extension, whereas the domain of diapirs is
generally considered as gently translating, accommodating small amounts of extension. Diapirs correspond
to weak zones and are easily and often squeezed.
Downslope of the margin, contractional structures balance the amount of upslope stretching.
The domain of shortening is also divided into three
sub-domains. In a seaward direction they are composed of diapirs squeezed at late stage, polyharmonic
folds and thrust faults developed at early stage, and
folds and thrusts developed at late stage. Contractional
structures are initiated in a domain located at a distance from the initial salt edge. Compression remains
localised in this domain during the initial stages of
evolution by continued deformation. The upslope
migration of contraction can then reach the extensional domain and squeeze the diapirs.
Analogue experiments show that the overall structural zoning is mainly controlled by the initial condition (salt basin) and the basal slope angle, whereas the
type of structures in the structural domains strongly
depends on sedimentation rate (Fig. 12.16b).
An early attempt to classify salt structures systematically and to set this into a dynamic context was
made by Trusheim in 1960. He suggested that salt
impiercements grow from elongated low-profile ridges
(anticlines and rollers) triggered by gravitational
contrasts, developing into rows of pillows, diapirs
and eventually into walls and sheets of salt. The
diapirs come in a variety of shapes from regular massive stocks, via irregular masses to elegant
mushrooms. This geometric classification is undoubtedly valid for a tectonically stable, evenly subsiding
basin. But even this relatively predictable kinematic
growth pattern of salt structures causes great problems
in seismic imaging due to the complex pattern of
internal flow in the salt structure itself, including horizontal displacement, affiliated with the development
of overhanging or even horizontal walls.
12.7.1 Hydrocarbon Prospectivity in
Salt-filled Basins
Deep parts of basins where salt structures tend to be
situated may be excellent sediment traps. The growth
of diapirs contributes to the development of local
depocentres and the areas around salt diapirs may
accumulate large volumes of reservoir rocks of good
quality and be associated with excellent structural and
stratigraphic traps. However, due to the capacity of
salt to flow horizontally at shallow levels and develop
overhanging bulges and sheets, and even to become
detached from its deeper sources, the detailed geometric configuration around the stem of the salt structure,
including its diameter, is commonly disguised and the
diameter of the stem itself may be impossible to determine from reflection seismic data.
In addition to the bulge above the salt
empiercement itself, which will reflect the geometry
of the upper layers of the empiercement, the main
types of features that may constitute structural hydrocarbon traps adjacent to salt diapirs are the rim
syncline system (sometimes several generations),
12 The Structure and Hydrocarbon Traps of Sedimentary Basins
345
seeing through the salt as such. It is still fair to say that
the structural geology of evaporites has attracted much
attention, partly because the high mobility of such
deposits poses intriguing structural geological
problems, and even more so because salt structures
are associated with a variety of structural and stratigraphic traps of significance for the petroleum industry. The structural geology of salt very much reflects
the local tectonic environment, be it extensional, contractional or strike-slip. This implies that salt bodies
come in an almost infinite variety of shapes, some of
which even challenge the limits of the imagination.
This variety of shapes, combined with the acoustic
properties of salt, poses great challenges for reflection
seismic imaging of salt bodies and the strata beneath
and close to them.
Gravity-driven deformation at continental margins
is generated by the regional gradient of the margin
itself and is characterised by upslope extension and
downslope contraction. Salt and its overlying sedimentary pile spread in a seaward direction due to
regional tilting in response to lithosphere cooling,
whereas synkinematic sedimentation induces loading
instabilities. At basin scale, thin-skinned deformation
may induce extreme upslope salt thinning, leading to
the formation of salt welds, as well as massive downslope salt thickening. Most often, the extensional
domain can be divided into three sub-domains, which
are, in a seaward direction, the sealed tilted block,
growth
fault/rollover,
and
diapir
domains
(Fig. 12.16a). The upper domain is characterised by
tilted blocks that are sealed early by synkinematic
sedimentation. The rollover domain displays a large
amount of extension, whereas the domain of diapirs is
generally considered as gently translating, accommodating small amounts of extension. Diapirs correspond
to weak zones and are easily and often squeezed.
Downslope of the margin, contractional structures balance the amount of upslope stretching.
The domain of shortening is also divided into three
sub-domains. In a seaward direction they are composed of diapirs squeezed at late stage, polyharmonic
folds and thrust faults developed at early stage, and
folds and thrusts developed at late stage. Contractional
structures are initiated in a domain located at a distance from the initial salt edge. Compression remains
localised in this domain during the initial stages of
evolution by continued deformation. The upslope
migration of contraction can then reach the extensional domain and squeeze the diapirs.
Analogue experiments show that the overall structural zoning is mainly controlled by the initial condition (salt basin) and the basal slope angle, whereas the
type of structures in the structural domains strongly
depends on sedimentation rate (Fig. 12.16b).
An early attempt to classify salt structures systematically and to set this into a dynamic context was
made by Trusheim in 1960. He suggested that salt
impiercements grow from elongated low-profile ridges
(anticlines and rollers) triggered by gravitational
contrasts, developing into rows of pillows, diapirs
and eventually into walls and sheets of salt. The
diapirs come in a variety of shapes from regular massive stocks, via irregular masses to elegant
mushrooms. This geometric classification is undoubtedly valid for a tectonically stable, evenly subsiding
basin. But even this relatively predictable kinematic
growth pattern of salt structures causes great problems
in seismic imaging due to the complex pattern of
internal flow in the salt structure itself, including horizontal displacement, affiliated with the development
of overhanging or even horizontal walls.
12.7.1 Hydrocarbon Prospectivity in
Salt-filled Basins
Deep parts of basins where salt structures tend to be
situated may be excellent sediment traps. The growth
of diapirs contributes to the development of local
depocentres and the areas around salt diapirs may
accumulate large volumes of reservoir rocks of good
quality and be associated with excellent structural and
stratigraphic traps. However, due to the capacity of
salt to flow horizontally at shallow levels and develop
overhanging bulges and sheets, and even to become
detached from its deeper sources, the detailed geometric configuration around the stem of the salt structure,
including its diameter, is commonly disguised and the
diameter of the stem itself may be impossible to determine from reflection seismic data.
In addition to the bulge above the salt
empiercement itself, which will reflect the geometry
of the upper layers of the empiercement, the main
types of features that may constitute structural hydrocarbon traps adjacent to salt diapirs are the rim
syncline system (sometimes several generations),
12 The Structure and Hydrocarbon Traps of Sedimentary Basins
345