creates microfractures which develop at a lower pressure than that required to form proper fracturing.
These microfractures should theoretically develop
when the pressure exceeds the sum of the least
horizontal stress and the tensional strength of the
rock. Since the pressure required for large-scale
fractures to form (fracture pressure) is higher than
the LOT test value, the tensional strength which
allows microfractures to form is lower than when
forming proper hydrofracturing.
In the case of microfractures (LOT tests) these
probably deform the rock in a different way, so that
the rock can heal once the pressure is released.
Fractures developed by hydrofracturing during
leakage of oil are likely to be vertical because they
develop parallel to the direction of maximum stress
which is normally vertical in subsiding basins with
little external tectonic stress. As a fracture opens, the
permeability along it is increased. This reduces the
pressure gradient along the fault plane to less than
the fracture gradient. The top of the fracture may
therefore be above fracture pressure, while the lowest
part is below fracture pressure and subject to effective
stress trying to close it. The fractures produced by
hydrofracturing must therefore propagate upwards
and are of limited vertical extent. They develop first
in the least permeable parts of the shale (cap rock)
which may only contain water because of low capillary entry pressure. The pressure in these very small
water-saturated pores should not be influenced by the
pressure in the petroleum phase in the sandstones
(Fig. 15.4).
The excess pressure in the petroleum phase compared to the water pressure, is held by the capillary
forces and does not influence the pressure causing
onset of hydrofracturing (Bjørkum et. al. 1998). However once the first fracturing has occurred the petroleum will be the continuous phase along the fracture,
and it is the pressure in the hydrocarbon phase which
causes leakage when the horizontal stress is exceeded.
In the laboratory, water has been shown to flow
though a cap rock while oil has been retained by the
capillary pressures (Teige et al. 2005).
If we consider migration in two dimensions it is
clear that it is not only the source rock that will
fracture. All the overlying shales that could serve as
potential cap rocks could reach fracture pressure and
leak petroleum if there are no lateral drainage paths.
This is because the fracture pressure gradient is steeper
than the fluid pressure gradient. Accumulation of
petroleum in a trap capped by shale which does not
fracture, requires that the pressure be reduced by lateral flow of water to maintain pressure below fracture
pressure (Fig. 15.5).
15.5 Migration Through Tectonically
Fractured Rocks
A clear distinction must be made between different
types of tectonic fracturing and hydrofracturing,
although these processes can occur together. It is
important also to distinguish between faults formed
during subsidence and those formed during uplift.
Some of the fractures observed may have been
formed during soft sediment deformation just after
deposition or be associated with growth faulting, and
thus unrelated to tectonic stress.
During subsidence and uplift the direction of stress
may have changed several times, producing fractures
with different orientations. We must therefore not
assume that all the fractures were open or closed at
the same time. Fractures produced during late stages
of uplift are most likely to be open, but they have very
little relevance to the conditions during oil migration.
However, rocks that are now outcropping have usually
had a long history of deformation.
Numerous studies have been carried out on the
pattern of fracturing in outcrops on land to serve as
analogues for subsurface fracture patterns which can
not readily be mapped from seismic or cores. This has
then served as a basis for sophisticated fluid modelling
and its consequence for oil migration, assuming in
some cases that the fractures are open and more permeable than the matrix, in others that they are less
permeable than the matrix.
15.6 Trapping of Petroleum Below a Cap
Rock
A cap rock traps petroleum if the flow into the trap
exceeds the flow out of the trap. A trap may leak
petroleum through the matrix of the seal or through
fractures produced by overpressure or tectonically. If
there is leakage through the matrix of a shale it is
because the capillary forces are not high enough to
resist the buoyancy of the petroleum. As we have seen,
15 Petroleum Migration
379
These microfractures should theoretically develop
when the pressure exceeds the sum of the least
horizontal stress and the tensional strength of the
rock. Since the pressure required for large-scale
fractures to form (fracture pressure) is higher than
the LOT test value, the tensional strength which
allows microfractures to form is lower than when
forming proper hydrofracturing.
In the case of microfractures (LOT tests) these
probably deform the rock in a different way, so that
the rock can heal once the pressure is released.
Fractures developed by hydrofracturing during
leakage of oil are likely to be vertical because they
develop parallel to the direction of maximum stress
which is normally vertical in subsiding basins with
little external tectonic stress. As a fracture opens, the
permeability along it is increased. This reduces the
pressure gradient along the fault plane to less than
the fracture gradient. The top of the fracture may
therefore be above fracture pressure, while the lowest
part is below fracture pressure and subject to effective
stress trying to close it. The fractures produced by
hydrofracturing must therefore propagate upwards
and are of limited vertical extent. They develop first
in the least permeable parts of the shale (cap rock)
which may only contain water because of low capillary entry pressure. The pressure in these very small
water-saturated pores should not be influenced by the
pressure in the petroleum phase in the sandstones
(Fig. 15.4).
The excess pressure in the petroleum phase compared to the water pressure, is held by the capillary
forces and does not influence the pressure causing
onset of hydrofracturing (Bjørkum et. al. 1998). However once the first fracturing has occurred the petroleum will be the continuous phase along the fracture,
and it is the pressure in the hydrocarbon phase which
causes leakage when the horizontal stress is exceeded.
In the laboratory, water has been shown to flow
though a cap rock while oil has been retained by the
capillary pressures (Teige et al. 2005).
If we consider migration in two dimensions it is
clear that it is not only the source rock that will
fracture. All the overlying shales that could serve as
potential cap rocks could reach fracture pressure and
leak petroleum if there are no lateral drainage paths.
This is because the fracture pressure gradient is steeper
than the fluid pressure gradient. Accumulation of
petroleum in a trap capped by shale which does not
fracture, requires that the pressure be reduced by lateral flow of water to maintain pressure below fracture
pressure (Fig. 15.5).
15.5 Migration Through Tectonically
Fractured Rocks
A clear distinction must be made between different
types of tectonic fracturing and hydrofracturing,
although these processes can occur together. It is
important also to distinguish between faults formed
during subsidence and those formed during uplift.
Some of the fractures observed may have been
formed during soft sediment deformation just after
deposition or be associated with growth faulting, and
thus unrelated to tectonic stress.
During subsidence and uplift the direction of stress
may have changed several times, producing fractures
with different orientations. We must therefore not
assume that all the fractures were open or closed at
the same time. Fractures produced during late stages
of uplift are most likely to be open, but they have very
little relevance to the conditions during oil migration.
However, rocks that are now outcropping have usually
had a long history of deformation.
Numerous studies have been carried out on the
pattern of fracturing in outcrops on land to serve as
analogues for subsurface fracture patterns which can
not readily be mapped from seismic or cores. This has
then served as a basis for sophisticated fluid modelling
and its consequence for oil migration, assuming in
some cases that the fractures are open and more permeable than the matrix, in others that they are less
permeable than the matrix.
15.6 Trapping of Petroleum Below a Cap
Rock
A cap rock traps petroleum if the flow into the trap
exceeds the flow out of the trap. A trap may leak
petroleum through the matrix of the seal or through
fractures produced by overpressure or tectonically. If
there is leakage through the matrix of a shale it is
because the capillary forces are not high enough to
resist the buoyancy of the petroleum. As we have seen,
15 Petroleum Migration
379