is 10 kPa/m (0.1 bar/m). Expressed in psi (pounds per
square inch) the equivalent gradient for freshwater is
0.434 psi/ft. In basins like the North Sea and the Gulf
Coast the water density varies significantly, and typical Gulf Coast pressure gradients are 0.465 psi/ft or
10.71 kPa/m (Dickey 1979).
If a standpipe (well) is installed in a normally
pressured sediment (no overpressure) the water
would rise in the pipe to the sea level or groundwater
table. This is called a piezometric or potensiometric
surface. Artesian overpressures may be due to meteoric water flow from, for instance, a mountain lake
into a sedimentary basin. If fluid pressure is higher
than the fluid pressure corresponding to the weight of
the fluid above, the water in the standpipe would rise
ΔH m above the local water table depending on the
degree of overpressure.
During burial and basin subsidence (compaction,
compression) the pore pressure is above hydrostatic
(transient overpressure) and the water flows out of the
sediments as they compact. Unless the permeability
in the sediments is very low, only very small
overpressures are required for the expulsion of water
during compaction. Then the rate of porewater flow is
a direct function of the rate of compaction (porosity
reduction). If there are low-permeability barriers to
flow in all directions, high overpressures may develop
during burial because it takes a long time for the pore
pressures caused by the added overburden to dissipate/
drain. There are also other processes that may lead to
overpressure. High sedimentation rates will cause
higher rates of compaction and compaction-driven
flux. Overpressure will retard mechanical compaction
because the effective stress is reduced. The porosity
reduction is however very much a function of time and
temperature in the case of chemical compaction. Since
chemical compaction in siliceous sediments is mainly
a function of temperature, compaction will continue
even at high overpressures and reduced effective
stress.
Lower than hydrostatic pressures (underpressure)
can also develop but are less common and are usually
formed during uplift in the sedimentary basin. Below
are listed three ways in which underpressure may
develop:
(1) Tectonic extension may slightly increase porosity
and create fractures which need to be filled with
fluids, thus lowering the fluid pressure. As water
with no gas bubbles has low bulk compressibility
(4.10
À4 MPa
À1 ), a small increase in the porosity
caused by the creation of new fractures will produce a significant lowering of pressure. During
uplift the sediments no longer compact and water
can therefore not flow in from the rock matrix to
fill the fractures without lowering the pressure.
Extension during uplift will thus tend to draw in
meteoric water from above, but if the fractures are
not connected so that the water can flow up to the
surface, the flow will be rather limited. Compressional tectonics or strike slip tectonics may produce episodes of rapid fluid flow along fractures,
often referred to as seismic pumping.
(2) Condensation of gas to liquid petroleum may
cause reduced fluid volume and lower pore pressure. This may, however, often be compensated
for by the expansion of dry gas and the release of
gas from porewater.
(3) Cooling and contraction of water (the opposite of
aquathermal pressuring) may cause lowering of
fluid pressure below hydrostatic.
11.2 Normally Consolidated Versus
Overconsolidated Sediments
A layer in a sediment sequence that never before in its
geological history has been subjected to higher vertical effective stress than at present, is called normally
consolidated (NC). If, on the other hand, the sediment
has been subjected to higher effective stresses, e.g. by
previous glacial loading, by higher overburden that
subsequently has been eroded, and/or by pore
pressures in the past that were lower than at present,
the sediment is called overconsolidated (OC) as it has
been preloaded. The ratio between the past maximum
effective vertical stress and the present stress is commonly called the overconsolidation ratio (OCR).
At relatively shallow depths in a sedimentary basin
(less than 2À3 km, <70À90
C), the mechanical compaction processes dominate over the chemical compaction in siliceous sediments. At higher temperature
(deeper burial) chemical compaction processes
become dominant in controlling the rate of compaction. Carbonate sediments may, however, become
cemented and highly consolidated at shallow depth.
Since this consolidation (compaction) is not due to
mechanical compaction but to chemical processes it
is sometimes referred to as “pseudo overconsolidation”. Since we refer to the maximum effective
stress sediments can not be undercompacted. Poorly
304
K. Bjørlykke et al.
square inch) the equivalent gradient for freshwater is
0.434 psi/ft. In basins like the North Sea and the Gulf
Coast the water density varies significantly, and typical Gulf Coast pressure gradients are 0.465 psi/ft or
10.71 kPa/m (Dickey 1979).
If a standpipe (well) is installed in a normally
pressured sediment (no overpressure) the water
would rise in the pipe to the sea level or groundwater
table. This is called a piezometric or potensiometric
surface. Artesian overpressures may be due to meteoric water flow from, for instance, a mountain lake
into a sedimentary basin. If fluid pressure is higher
than the fluid pressure corresponding to the weight of
the fluid above, the water in the standpipe would rise
ΔH m above the local water table depending on the
degree of overpressure.
During burial and basin subsidence (compaction,
compression) the pore pressure is above hydrostatic
(transient overpressure) and the water flows out of the
sediments as they compact. Unless the permeability
in the sediments is very low, only very small
overpressures are required for the expulsion of water
during compaction. Then the rate of porewater flow is
a direct function of the rate of compaction (porosity
reduction). If there are low-permeability barriers to
flow in all directions, high overpressures may develop
during burial because it takes a long time for the pore
pressures caused by the added overburden to dissipate/
drain. There are also other processes that may lead to
overpressure. High sedimentation rates will cause
higher rates of compaction and compaction-driven
flux. Overpressure will retard mechanical compaction
because the effective stress is reduced. The porosity
reduction is however very much a function of time and
temperature in the case of chemical compaction. Since
chemical compaction in siliceous sediments is mainly
a function of temperature, compaction will continue
even at high overpressures and reduced effective
stress.
Lower than hydrostatic pressures (underpressure)
can also develop but are less common and are usually
formed during uplift in the sedimentary basin. Below
are listed three ways in which underpressure may
develop:
(1) Tectonic extension may slightly increase porosity
and create fractures which need to be filled with
fluids, thus lowering the fluid pressure. As water
with no gas bubbles has low bulk compressibility
(4.10
À4 MPa
À1 ), a small increase in the porosity
caused by the creation of new fractures will produce a significant lowering of pressure. During
uplift the sediments no longer compact and water
can therefore not flow in from the rock matrix to
fill the fractures without lowering the pressure.
Extension during uplift will thus tend to draw in
meteoric water from above, but if the fractures are
not connected so that the water can flow up to the
surface, the flow will be rather limited. Compressional tectonics or strike slip tectonics may produce episodes of rapid fluid flow along fractures,
often referred to as seismic pumping.
(2) Condensation of gas to liquid petroleum may
cause reduced fluid volume and lower pore pressure. This may, however, often be compensated
for by the expansion of dry gas and the release of
gas from porewater.
(3) Cooling and contraction of water (the opposite of
aquathermal pressuring) may cause lowering of
fluid pressure below hydrostatic.
11.2 Normally Consolidated Versus
Overconsolidated Sediments
A layer in a sediment sequence that never before in its
geological history has been subjected to higher vertical effective stress than at present, is called normally
consolidated (NC). If, on the other hand, the sediment
has been subjected to higher effective stresses, e.g. by
previous glacial loading, by higher overburden that
subsequently has been eroded, and/or by pore
pressures in the past that were lower than at present,
the sediment is called overconsolidated (OC) as it has
been preloaded. The ratio between the past maximum
effective vertical stress and the present stress is commonly called the overconsolidation ratio (OCR).
At relatively shallow depths in a sedimentary basin
(less than 2À3 km, <70À90
C), the mechanical compaction processes dominate over the chemical compaction in siliceous sediments. At higher temperature
(deeper burial) chemical compaction processes
become dominant in controlling the rate of compaction. Carbonate sediments may, however, become
cemented and highly consolidated at shallow depth.
Since this consolidation (compaction) is not due to
mechanical compaction but to chemical processes it
is sometimes referred to as “pseudo overconsolidation”. Since we refer to the maximum effective
stress sediments can not be undercompacted. Poorly
304
K. Bjørlykke et al.