porous sandstones (aquifers) against shales, rendering
permeable sandstones a dead-end in terms of fluid
flow. Faults that cut through sandstones may have a
clay smear from adjacent shales or from mica or
authigenic kaolinite inside the sandstone, and this
may significantly reduce the permeability, in some
cases sufficiently to form an oil trap.
Flow along fault planes requires that they are kept
open to some degree. A force equivalent to the horizontal stress acts on the fault plane, trying to close it;
an equivalent overpressure is required to counteract
this in order for the fracture to remain open
(Fig. 10.11). However, this corresponds closely to
the fracture pressure and even without the presence
of a fault the rocks would fracture. At such high
overpressure there are very low effective stresses and
the sediments are unable to compact mechanically.
We must also consider the source of the fluids. When
there is no compaction (porosity reduction) in the
adjacent sediments, water can not flow from the rock
matrix into the fractures. Well-cemented sedimentary
rocks and basement rocks have high shear strength and
may produce rock fragments (brecciation) during
faulting. These rock fragments may wedge the fault
plane, helping to resist the horizontal stress.
Brecciated fault planes may therefore be important
conduits for fluid flow but over time the permeability
will gradually be reduced by cementation.
The cements will in most cases not be due to
precipitation from advective flow but form by diffusion from the adjacent rock matrix. Carbonate and
silicate minerals next to the faults are under lithostatic
stress and therefore more readily soluble than when
unstressed. In the case of carbonate cement this is
fairly clear because upwards (cooling) flow will dissolve calcite rather than precipitate it, due to its retrograde solubility. Brecciated faults contain broken rock
fragments and quartz grains that are good nucleation
sites for quartz cement formation. A thermodynamic
drive towards dissolution of the minerals under stress
is therefore likely, with ensuing precipitation in the
fault plane. Renewed fracturing is therefore required
for the faults to remain permeable.
10.10 Seismic Pumping
In crystalline and well-cemented sedimentary rocks
tectonic shear may result in an extensional shear failure which opens up fractures. Increases in the tectonic
stress may reduce the opening of such fractures and
Pressure
Water
Tight (low permeability)
shale (cap rock).
Reservoir sandstone.
Depth
Lithostatic pressure
∇P = ΔP/H
ΔP
F = Fluid flux
(m
3
/m
2 /s)
Hydrostatic pressure
(ρ w gh)
Fracture pressure
Fig. 10.11 Illustration of fluid pressure and rock pressure
(lithostatic). If the permeability in shales (seals) is low enough
overpressure will build up. The pressure can not exceed the
fracture pressure which is equal to the horizontal stress, as the
seal will then leak
10 Subsurface Water and Fluid Flow in Sedimentary Basins
295
permeable sandstones a dead-end in terms of fluid
flow. Faults that cut through sandstones may have a
clay smear from adjacent shales or from mica or
authigenic kaolinite inside the sandstone, and this
may significantly reduce the permeability, in some
cases sufficiently to form an oil trap.
Flow along fault planes requires that they are kept
open to some degree. A force equivalent to the horizontal stress acts on the fault plane, trying to close it;
an equivalent overpressure is required to counteract
this in order for the fracture to remain open
(Fig. 10.11). However, this corresponds closely to
the fracture pressure and even without the presence
of a fault the rocks would fracture. At such high
overpressure there are very low effective stresses and
the sediments are unable to compact mechanically.
We must also consider the source of the fluids. When
there is no compaction (porosity reduction) in the
adjacent sediments, water can not flow from the rock
matrix into the fractures. Well-cemented sedimentary
rocks and basement rocks have high shear strength and
may produce rock fragments (brecciation) during
faulting. These rock fragments may wedge the fault
plane, helping to resist the horizontal stress.
Brecciated fault planes may therefore be important
conduits for fluid flow but over time the permeability
will gradually be reduced by cementation.
The cements will in most cases not be due to
precipitation from advective flow but form by diffusion from the adjacent rock matrix. Carbonate and
silicate minerals next to the faults are under lithostatic
stress and therefore more readily soluble than when
unstressed. In the case of carbonate cement this is
fairly clear because upwards (cooling) flow will dissolve calcite rather than precipitate it, due to its retrograde solubility. Brecciated faults contain broken rock
fragments and quartz grains that are good nucleation
sites for quartz cement formation. A thermodynamic
drive towards dissolution of the minerals under stress
is therefore likely, with ensuing precipitation in the
fault plane. Renewed fracturing is therefore required
for the faults to remain permeable.
10.10 Seismic Pumping
In crystalline and well-cemented sedimentary rocks
tectonic shear may result in an extensional shear failure which opens up fractures. Increases in the tectonic
stress may reduce the opening of such fractures and
Pressure
Water
Tight (low permeability)
shale (cap rock).
Reservoir sandstone.
Depth
Lithostatic pressure
∇P = ΔP/H
ΔP
F = Fluid flux
(m
3
/m
2 /s)
Hydrostatic pressure
(ρ w gh)
Fracture pressure
Fig. 10.11 Illustration of fluid pressure and rock pressure
(lithostatic). If the permeability in shales (seals) is low enough
overpressure will build up. The pressure can not exceed the
fracture pressure which is equal to the horizontal stress, as the
seal will then leak
10 Subsurface Water and Fluid Flow in Sedimentary Basins
295