13.2 Compaction, Flow Mechanisms
13.2.4 Pore Water Pressure, Overpressure
As mentioned earlier, compaction flow is delayed or
prevented if the permeability of the overlying beds
drops to such low values that the released pore water
cannot escape. It then becomes overpressured with
respect to the normal hydrostatic pressure, Ph' because
it has to carry part of or almost the entire load of solid
matter exerting lithostatic press ure (Fig. 13.l1a).
Hence, the state of underconsolidation of subsurface
layers can be determined from the pressure of their
pore fluids. The ratio between the pressure of the pore
fluid, Pf (Pf=Ph +u; U = excess pore pressure ), and the
lithostatic pressure, PI' is called geostatic ratio. Values
for this ratio vary between 0.47 (no excess pore pressure) and in excess of 0.9, indicating very high
overpressure. In the latter case, the fluid pressure can
exceed the minimum horizontal stress and thus cause
fracturing andfluid escape (Fig. 13.11b).
Apart from sediment-Ioading of low-permeability
strata, overpressure can be generated bythe exertion of
tectonic lateral stress, thermal expansion of pore fluids
and/or dehydration of c1ay minerals, and by the
buildup of a hydraulic head in weIl sealed, shallowly
buried sediments.
For further details see, e.g., Osbome and Swabrick (1997),
Gordon and Flemmings (1998). In non-compressional settings, fracturing preferentially occurs in the depth range of
1000 to 3000 m, where horizontal stresses are considerably
lower than vertical stresses (e.g. Rouchet 1978).
The existence of overpressure signifies that the section is
more or less sealed by overlying low-permeability layers.
Consequently, there is a high likelihood that hydrocarbons,
possibly released from the sequence, are not lost by migration to the land surface or sea bottom. Similarly, one can assume that the primary pore water (connate water or chemically altered formation water) was not recently mixed with
meteoric water (see below).
Some sedimentary basins exhibit subnormal pressures in
drillholes of some depth. These pressures are less than the
hydrostatic pressure. Such low pressures can only occur in
porous rocks that are separated from circulating groundwater
by a low permeable barrier. Under this condition, subnormal
pressure can be generated by extracting fluids or gas from the
reservoir. Other causes for the reduction in pressure may be
an increase in pore volume by fracturing and decompression,
a decrease in the subsurface temperature, or osmotic effects.
13.2.5 Refraction of Pore Water Flow
So far, vertically ascending pore water in homogeneous sediments has been considered. The direction of
compaction flow is, however, often significantly modified by the intercalation ofwell-permeable layers, sediment wedges, and irregular sediment bodies. These in
addition may cause differential compaction and inc1ined bedding (see above). If vertically upstreaming
pore fluid in low-permeable muds reaches, forexampIe, an inc1ined porous sand bed or sand wedge, its
663
direction offlow is refracted and thus forced to follow
more or less the dip ofthe sandy bed (Fig. 13.10d and
e). Similarly, reef detritus can act as conduit for compaction flow (Fig. 13.10f).
In the South Caspian basin, for example, subsurface pressure
data indicate that sands are acting as drains not only for the
underlying, but also for the overlying compacting shales
(Bredehoeft et al. 1988). The highly permeable sands therefore display flow velocities considerably higher than those
calculated for vertically ascending pore waters. The lateral
movement of compaction water may allow subdivision of a
sequence into several units which are hydraulically more or
less isolated from one another.
In conclusion, the compactional flow regime is characterized by upward and outward expulsion of pore waters from growing basin fill. Lithostatic loading or
compressive tectonic stress provides the driving mechanism for flow. The zone of compaction flow mayaiso
inc1ude burial depths where the fluid pressure exceeds
hydrostatic pressure (Fig. 13.11). Compaction flow is
substantial in thick sequences and may contribute significantlyto diagenetic processes. Neverthe1ess its volurne is finite and limited by the water content of the
sediments involved. Growth faults frequently act as
barriers for the updip migration of compaction water
(including hydrocarbons).
13.2.6 Other Subsurface Flow Regimes
In growing sedimentary sequences, compaction flow is
the most obvious result of pore space reduction with
increasing burial depth. However, sedimentary basins
are also affected by other flow mechanisIDS. Inc1uding
compaction flow, we have to consider at least four different types of subsurface flow regimes (Fig. 13.12):
- Meteoric flow (gravity-driven flow).
- Compaction flow (pressure-driven flow, discussed
above).
- Thermobaric flow (temperature/pressure driven
flow).
- Convection (density driven flow).
All these flow regimes may occur simultaneously in
various basin settings which actively accumulate sediment. The model ofFigure 13.12 refers to a repeatedly
prograding shelf-slope depositional system as is common on passive continental margins with high terrestrial sediment input.
These flow regimes have been discussed in some detail, e.g.,
by Bjerlykke (1984), Galloway (1984), Hanor (1987), Land
(1997), Morse et al. (1997), and many others. Their role in
diagenesis is described further in Sect. 11.3.
Meteoric water circulation. Meteoric water circulation (groundwater flow) is common in permeable sedi-
13.2.4 Pore Water Pressure, Overpressure
As mentioned earlier, compaction flow is delayed or
prevented if the permeability of the overlying beds
drops to such low values that the released pore water
cannot escape. It then becomes overpressured with
respect to the normal hydrostatic pressure, Ph' because
it has to carry part of or almost the entire load of solid
matter exerting lithostatic press ure (Fig. 13.l1a).
Hence, the state of underconsolidation of subsurface
layers can be determined from the pressure of their
pore fluids. The ratio between the pressure of the pore
fluid, Pf (Pf=Ph +u; U = excess pore pressure ), and the
lithostatic pressure, PI' is called geostatic ratio. Values
for this ratio vary between 0.47 (no excess pore pressure) and in excess of 0.9, indicating very high
overpressure. In the latter case, the fluid pressure can
exceed the minimum horizontal stress and thus cause
fracturing andfluid escape (Fig. 13.11b).
Apart from sediment-Ioading of low-permeability
strata, overpressure can be generated bythe exertion of
tectonic lateral stress, thermal expansion of pore fluids
and/or dehydration of c1ay minerals, and by the
buildup of a hydraulic head in weIl sealed, shallowly
buried sediments.
For further details see, e.g., Osbome and Swabrick (1997),
Gordon and Flemmings (1998). In non-compressional settings, fracturing preferentially occurs in the depth range of
1000 to 3000 m, where horizontal stresses are considerably
lower than vertical stresses (e.g. Rouchet 1978).
The existence of overpressure signifies that the section is
more or less sealed by overlying low-permeability layers.
Consequently, there is a high likelihood that hydrocarbons,
possibly released from the sequence, are not lost by migration to the land surface or sea bottom. Similarly, one can assume that the primary pore water (connate water or chemically altered formation water) was not recently mixed with
meteoric water (see below).
Some sedimentary basins exhibit subnormal pressures in
drillholes of some depth. These pressures are less than the
hydrostatic pressure. Such low pressures can only occur in
porous rocks that are separated from circulating groundwater
by a low permeable barrier. Under this condition, subnormal
pressure can be generated by extracting fluids or gas from the
reservoir. Other causes for the reduction in pressure may be
an increase in pore volume by fracturing and decompression,
a decrease in the subsurface temperature, or osmotic effects.
13.2.5 Refraction of Pore Water Flow
So far, vertically ascending pore water in homogeneous sediments has been considered. The direction of
compaction flow is, however, often significantly modified by the intercalation ofwell-permeable layers, sediment wedges, and irregular sediment bodies. These in
addition may cause differential compaction and inc1ined bedding (see above). If vertically upstreaming
pore fluid in low-permeable muds reaches, forexampIe, an inc1ined porous sand bed or sand wedge, its
663
direction offlow is refracted and thus forced to follow
more or less the dip ofthe sandy bed (Fig. 13.10d and
e). Similarly, reef detritus can act as conduit for compaction flow (Fig. 13.10f).
In the South Caspian basin, for example, subsurface pressure
data indicate that sands are acting as drains not only for the
underlying, but also for the overlying compacting shales
(Bredehoeft et al. 1988). The highly permeable sands therefore display flow velocities considerably higher than those
calculated for vertically ascending pore waters. The lateral
movement of compaction water may allow subdivision of a
sequence into several units which are hydraulically more or
less isolated from one another.
In conclusion, the compactional flow regime is characterized by upward and outward expulsion of pore waters from growing basin fill. Lithostatic loading or
compressive tectonic stress provides the driving mechanism for flow. The zone of compaction flow mayaiso
inc1ude burial depths where the fluid pressure exceeds
hydrostatic pressure (Fig. 13.11). Compaction flow is
substantial in thick sequences and may contribute significantlyto diagenetic processes. Neverthe1ess its volurne is finite and limited by the water content of the
sediments involved. Growth faults frequently act as
barriers for the updip migration of compaction water
(including hydrocarbons).
13.2.6 Other Subsurface Flow Regimes
In growing sedimentary sequences, compaction flow is
the most obvious result of pore space reduction with
increasing burial depth. However, sedimentary basins
are also affected by other flow mechanisIDS. Inc1uding
compaction flow, we have to consider at least four different types of subsurface flow regimes (Fig. 13.12):
- Meteoric flow (gravity-driven flow).
- Compaction flow (pressure-driven flow, discussed
above).
- Thermobaric flow (temperature/pressure driven
flow).
- Convection (density driven flow).
All these flow regimes may occur simultaneously in
various basin settings which actively accumulate sediment. The model ofFigure 13.12 refers to a repeatedly
prograding shelf-slope depositional system as is common on passive continental margins with high terrestrial sediment input.
These flow regimes have been discussed in some detail, e.g.,
by Bjerlykke (1984), Galloway (1984), Hanor (1987), Land
(1997), Morse et al. (1997), and many others. Their role in
diagenesis is described further in Sect. 11.3.
Meteoric water circulation. Meteoric water circulation (groundwater flow) is common in permeable sedi-