viscous and ductile, and sometimes combinations like
elastico-plastic.
Unfortunately, these terms are not always used in a
consequent manner and are therefore liable to cause
confusion when taken out of context or not precisely
defined. This particularly concerns the term “brittle”,
because it is used in a double sense, namely as a
deformation mechanism and a deformation style.
When applied in the context of deformation
mechanisms, brittle deformation implies that existing
bonds are physically broken between mineral grains,
or that fracturing of the individual grains themselves
takes place. As a consequence, the rock loses its
cohesion and (potentially) physically falls apart. In
contrast, the plastic deformation mechanism implies
that deformation takes place by the transfer of
dislocations on the atomic scale. This means that the
mineral can change its shape without loss of cohesion. Generally, plastic deformation occurs at higher
p,T-conditions than those accompanying brittle
deformation.
Concerning deformation style, the term brittle is
used about localised strain, like that associated with
jointing and faulting, and particularly in cases when
the rock loses its cohesion and where the deformation
occurs at lower p,T-conditions (though not necessarily
so). The ductile deformation style characterises strain
also by low stress, which is homogeneously distributed
over a wider area, as commonly observed in connection
with folding and meso- and mega-scale shear-zones. A
ductile style of deformation is predominant at high p,Tconditions, but may also occur under very low p,Tconditions, if it involves weak materials like sand and
clay. In such cases, however, displacement takes place
along grain boundaries or along borders between rock
bodies and not by dislocation creep or other atomicscale mechanisms that characterise the plastic deformation mechanism.
12.2 Petroleum Systems in Extensional
Regimes
Areas of extension are affiliated with horizontal divergent stress and are found in association with constructive or passive plate boundaries and in intra-plate
settings. Thus, extensional stress regimes either are
associated with subsidence and basin formation (in
intra-plate settings) or characterise active break-up of
continents (along constructive or passive margins).
Although the conditions for development of petroleum systems in areas of active spreading may be
meagre, the remnants of the earlier stages of breakup, now situated in passive margins settings, fulfil all
the requirements that characterise productive petroleum provinces. This is because such tectonic regimes
have undergone crustal thinning and associated subsidence, which involves all the processes essential for
petroleum to be generated, trapped and accumulated in
sufficient volumes and concentrations for petroleum
fields to be commercially interesting. Accordingly,
such settings frequently display an attractive combination and distribution of source, reservoir and cap
rocks, structural and stratigraphic traps and the
conditions for maturation, expulsion, migration and
accumulation of hydrocarbons.
12.2.1 Extensional Basins
The formation of extensional basins may be seen as
the first stage of the Wilson Cycle, which begins with
thinning, stretching and rifting of the continental crust
followed by continental break-up and mid-oceanic
spreading. The concept of the Wilson cycle predicts
that this process becomes reversed, causing closure of
the ocean, collision between the adjacent continental
plates, and hence the construction of a mountain chain
along the zone of collision (Fig. 12.3). The junction
between the continental plates defines the suture
between the two.
If we use the present North Atlantic as one example, the highly hydrocarbon-rich northern North Sea
basin system is situated in a passive continental margin configuration, where the extensional basin system
developed during continental break-up. In contrast,
Iceland, where petroleum resources are less abundant,
is situated on the top of the mid-oceanic spreading
ridge. However, if one looks more closely at the structural configuration at depth, one finds that the northern
North Sea basin system, which includes the Viking
Graben that developed in Jurassic-Cretaceous times, is
324
R.H. Gabrielsen
elastico-plastic.
Unfortunately, these terms are not always used in a
consequent manner and are therefore liable to cause
confusion when taken out of context or not precisely
defined. This particularly concerns the term “brittle”,
because it is used in a double sense, namely as a
deformation mechanism and a deformation style.
When applied in the context of deformation
mechanisms, brittle deformation implies that existing
bonds are physically broken between mineral grains,
or that fracturing of the individual grains themselves
takes place. As a consequence, the rock loses its
cohesion and (potentially) physically falls apart. In
contrast, the plastic deformation mechanism implies
that deformation takes place by the transfer of
dislocations on the atomic scale. This means that the
mineral can change its shape without loss of cohesion. Generally, plastic deformation occurs at higher
p,T-conditions than those accompanying brittle
deformation.
Concerning deformation style, the term brittle is
used about localised strain, like that associated with
jointing and faulting, and particularly in cases when
the rock loses its cohesion and where the deformation
occurs at lower p,T-conditions (though not necessarily
so). The ductile deformation style characterises strain
also by low stress, which is homogeneously distributed
over a wider area, as commonly observed in connection
with folding and meso- and mega-scale shear-zones. A
ductile style of deformation is predominant at high p,Tconditions, but may also occur under very low p,Tconditions, if it involves weak materials like sand and
clay. In such cases, however, displacement takes place
along grain boundaries or along borders between rock
bodies and not by dislocation creep or other atomicscale mechanisms that characterise the plastic deformation mechanism.
12.2 Petroleum Systems in Extensional
Regimes
Areas of extension are affiliated with horizontal divergent stress and are found in association with constructive or passive plate boundaries and in intra-plate
settings. Thus, extensional stress regimes either are
associated with subsidence and basin formation (in
intra-plate settings) or characterise active break-up of
continents (along constructive or passive margins).
Although the conditions for development of petroleum systems in areas of active spreading may be
meagre, the remnants of the earlier stages of breakup, now situated in passive margins settings, fulfil all
the requirements that characterise productive petroleum provinces. This is because such tectonic regimes
have undergone crustal thinning and associated subsidence, which involves all the processes essential for
petroleum to be generated, trapped and accumulated in
sufficient volumes and concentrations for petroleum
fields to be commercially interesting. Accordingly,
such settings frequently display an attractive combination and distribution of source, reservoir and cap
rocks, structural and stratigraphic traps and the
conditions for maturation, expulsion, migration and
accumulation of hydrocarbons.
12.2.1 Extensional Basins
The formation of extensional basins may be seen as
the first stage of the Wilson Cycle, which begins with
thinning, stretching and rifting of the continental crust
followed by continental break-up and mid-oceanic
spreading. The concept of the Wilson cycle predicts
that this process becomes reversed, causing closure of
the ocean, collision between the adjacent continental
plates, and hence the construction of a mountain chain
along the zone of collision (Fig. 12.3). The junction
between the continental plates defines the suture
between the two.
If we use the present North Atlantic as one example, the highly hydrocarbon-rich northern North Sea
basin system is situated in a passive continental margin configuration, where the extensional basin system
developed during continental break-up. In contrast,
Iceland, where petroleum resources are less abundant,
is situated on the top of the mid-oceanic spreading
ridge. However, if one looks more closely at the structural configuration at depth, one finds that the northern
North Sea basin system, which includes the Viking
Graben that developed in Jurassic-Cretaceous times, is
324
R.H. Gabrielsen