cooling batholith as a whole the fluid flow may be more
uniform, depending on the rate of cooling and on the
circulation of groundwater by thermal convection. It
has also been argued that compaction-driven flow may
be episodic when the fracture pressure is reached.
Fluids flow through the fractures produced by hydrofracturing but as they escape, pressure will be reduced
and the fractures will be reduced or close. It is not so
clear if the fracture will stay closed for some time and
then leak again or whether it will continue to transmit
fluids at a variable rate corresponding to the rate of
compaction. At depths greater than about 3 km where
most of the compaction is chemical, the rate of compaction will be slow and relatively independent of
changes in the stress field. When considering larger
volumes of rocks they can not be heated or cooled
rapidly, at least not in a non-hydrothermal environment, because of the high specific heat capacity of the
rocks. The rate of compaction and the resulting fluid
flow will then be relatively uniform over a limited time.
10.12 Formation of Overpressure
(Abnormal Pressure)
Overpressure is a term used for subsurface pressures that
significantly exceed the hydrostatic pressure. This
implies that the flow of porewater to the surface during
compaction is resisted to a considerable degree, so that
the pressure gradients are increased in the least permeable part of the sediments. Overpressure may be produced by different mechanisms. The simplest type of
overpressure is due to the pressure (head) of an elevated
groundwater table connected to the basin through an
aquifer. Rainwater infiltration into the ground will then
help to maintain the pressure even if the aquitards (seals)
do not have very low permeability. The overpressure
will nevertheless decrease away from the area of
recharge. This is also expressed by decreasing piezometric surfaces along the direction of flow. For overpressure to develop due to compaction the permeability
in the seal must be many orders of magnitude lower
because the fluid flux is very much lower than in the case
of meteoric water flow (Bjørlykke 1993). In a meteoric
water aquifer the flux may be up to 0.1–1.0 m/year,
while the compaction-driven flux is usually less than the
sedimentarion rate which may typically be 0.1 mm/year.
During compaction of sediments there must always
be a slight overpressure because there must be sufficient pressure gradients for the excess porewater to
flow out so that the porosity can be reduced. The
Darcy equation shows that the pressure gradient must
be an inverse function of the effective permeability of
the rocks forming the seal. When we have very low
permeabilities a high pressure gradient will build up,
which will drive the water out. The term disequilibrium compaction has been used to describe the development of overpressure because the permeability is
too low for the water to be expelled at lower pressure
gradients. A disequilibrieum is always required for
compaction to take place, but if the permeablities are
not very low, only a slight overpressure is required for
the explusion of porewater during compaction. The
build-up of overpressure reduces the effective stress
with the effect of stopping or at least reducing
mechanical compaction (Fig. 10.11). Overpressure
thus provides a negative feedback on mechanical compaction. Chemical compaction in siliceous rocks
involving quartz cementation will still continue as a
function of temperature also during uplift as long as
the temperature exceeds 70-80
C but at a lower rate.
The pore pressure can then build up to fracture
pressure.
From the Darcy equation we see that the pressure
gradient (P) is:
rP ¼ F Á μ=k
It is clear that increases in the fluid flux (F) could
cause high overpressure, but the pressure gradient is
very sensitive to variation in permeability.
High sedimentation rates and basin subsidence will
increase the compaction-driven fluid flux (F) and will
contribute to the build-up of overpressure if the permeability is considered to be constant. As shown
above, the average upwards flow of porewater is
equal to the integrated change in the porosity/depth
curve in the underlying sediments.
In addition there is a fluid flux driven by the release
of crystal-bound water which in sediments with high
contents of water-bearing minerals may be significant.
When porewater is heated the thermal expansion of
water will also add to the fluid flux but calculations
show that this is not very significant.
10 Subsurface Water and Fluid Flow in Sedimentary Basins
297
uniform, depending on the rate of cooling and on the
circulation of groundwater by thermal convection. It
has also been argued that compaction-driven flow may
be episodic when the fracture pressure is reached.
Fluids flow through the fractures produced by hydrofracturing but as they escape, pressure will be reduced
and the fractures will be reduced or close. It is not so
clear if the fracture will stay closed for some time and
then leak again or whether it will continue to transmit
fluids at a variable rate corresponding to the rate of
compaction. At depths greater than about 3 km where
most of the compaction is chemical, the rate of compaction will be slow and relatively independent of
changes in the stress field. When considering larger
volumes of rocks they can not be heated or cooled
rapidly, at least not in a non-hydrothermal environment, because of the high specific heat capacity of the
rocks. The rate of compaction and the resulting fluid
flow will then be relatively uniform over a limited time.
10.12 Formation of Overpressure
(Abnormal Pressure)
Overpressure is a term used for subsurface pressures that
significantly exceed the hydrostatic pressure. This
implies that the flow of porewater to the surface during
compaction is resisted to a considerable degree, so that
the pressure gradients are increased in the least permeable part of the sediments. Overpressure may be produced by different mechanisms. The simplest type of
overpressure is due to the pressure (head) of an elevated
groundwater table connected to the basin through an
aquifer. Rainwater infiltration into the ground will then
help to maintain the pressure even if the aquitards (seals)
do not have very low permeability. The overpressure
will nevertheless decrease away from the area of
recharge. This is also expressed by decreasing piezometric surfaces along the direction of flow. For overpressure to develop due to compaction the permeability
in the seal must be many orders of magnitude lower
because the fluid flux is very much lower than in the case
of meteoric water flow (Bjørlykke 1993). In a meteoric
water aquifer the flux may be up to 0.1–1.0 m/year,
while the compaction-driven flux is usually less than the
sedimentarion rate which may typically be 0.1 mm/year.
During compaction of sediments there must always
be a slight overpressure because there must be sufficient pressure gradients for the excess porewater to
flow out so that the porosity can be reduced. The
Darcy equation shows that the pressure gradient must
be an inverse function of the effective permeability of
the rocks forming the seal. When we have very low
permeabilities a high pressure gradient will build up,
which will drive the water out. The term disequilibrium compaction has been used to describe the development of overpressure because the permeability is
too low for the water to be expelled at lower pressure
gradients. A disequilibrieum is always required for
compaction to take place, but if the permeablities are
not very low, only a slight overpressure is required for
the explusion of porewater during compaction. The
build-up of overpressure reduces the effective stress
with the effect of stopping or at least reducing
mechanical compaction (Fig. 10.11). Overpressure
thus provides a negative feedback on mechanical compaction. Chemical compaction in siliceous rocks
involving quartz cementation will still continue as a
function of temperature also during uplift as long as
the temperature exceeds 70-80
C but at a lower rate.
The pore pressure can then build up to fracture
pressure.
From the Darcy equation we see that the pressure
gradient (P) is:
rP ¼ F Á μ=k
It is clear that increases in the fluid flux (F) could
cause high overpressure, but the pressure gradient is
very sensitive to variation in permeability.
High sedimentation rates and basin subsidence will
increase the compaction-driven fluid flux (F) and will
contribute to the build-up of overpressure if the permeability is considered to be constant. As shown
above, the average upwards flow of porewater is
equal to the integrated change in the porosity/depth
curve in the underlying sediments.
In addition there is a fluid flux driven by the release
of crystal-bound water which in sediments with high
contents of water-bearing minerals may be significant.
When porewater is heated the thermal expansion of
water will also add to the fluid flux but calculations
show that this is not very significant.
10 Subsurface Water and Fluid Flow in Sedimentary Basins
297