into the water column except when there is local
focusing of the flow. During periods with no sedimentation (hiatus), the deposited sediments continue
to compact and porewater flows across the redox
boundary just below the seabed and dissolved ions
like Fe
2+ and Mn
2+ may precipitate as Fe(OH) 3 and
Mn(OH) 4 .
Very high degrees of focusing are required to
obtain high flow rates. It is then easy to understand
why compaction-driven flow is several orders of
magnitude lower than meteoric water flow, at least in
the shallow parts of the basin near land. The slow
porewater flow rates also imply that this water has
time to approach thermal and chemical equilibrium
with the minerals. Porewater will always transport
some heat but, compared to the background heat flow
by conduction, this is very small and in most cases can
be ignored (Bethke 1985, Ludvigsen 1992).
Modelling compaction-driven flow in the Gulf
Basin, Harrison and Summa (1991) found that the
maximum rate of the vertical component is 2 mm/
year (2 km/million years), which is approximately
equivalent to the maximum sedimentation rate. This
flow rate is too slow to contribute significantly to the
heat flow. The temperature distribution in the Gulf
Basin at the present day does not reflect compactiondriven porewater flow. Most of the heat transport is by
conduction. The average compaction-driven flow
within a basin is more or less independent of the
permeability because it is a function of the rate of
loss of porosity in the underlying sediments. During
mechanical compaction, low permeability sediments
may result in overpressure and a certain reduction of
fluid flow, but at greater depth where the compaction is
mostly chemical, the fluid flux is independent of the
permeability although variations in permeability may
focus the flow.
10.8 Constraints on Water Flow in
Sedimentary Basins by Porewater
Chemistry
Many sedimentary basins contain evaporites, often
occupying the basal part, having formed during the
initial rifting. Very high salinity is then typically found
in a zone of a few hundred metres adjacent to the salt.
This is the case with the Zechstein salt in the North Sea
basin. The shallower parts of a basin and in particular
near tectonically uplifted areas, may contain
porewater of meteoric origin with very low salinity.
The isotopic composition of the porewater often shows
a very clear stratification. Oxygen isotopes are often
negative in the shallow parts of the basin due to the
inflow of meteoric water, while they may be positive at
greater depth due to diagenetic reactions.
During subsidence there are generally no open
fractures because of the ductile properties of subsiding sediments (Bjørlykke and Høeg 1997).
The permeabilities in shales are very low, probably
less than a nanodarcy (Leonard 1993). Samples
measured in the laboratory may show erroneously
high values because of fracturing resulting from
unloading during core retrieval. Increasing the effective stress during laboratory measurements to compensate for this has been shown to lower the
permeabilities by two orders of magnitude (Katsube
et al. 1991). In sediments which have been subject to
uplift, however, fracture permeability may be important. Laboratory measurements of shale permeabilities
may be several orders of magnitude larger than the
results of well tests in the field (Oelkers 1996).
In subsiding basins the effective permeabilities on a
large scale must be low, not above about 10
À9 Darcy,
to maintain overpressures over time. Calculating the
porewater flux due to compaction and using observed
pressure gradients, the effective permeability of thick
shale sequences can be calculated. Using data from
Haltenbanken offshore Norway this method gave
permeabilities between 10
À9 and 10
À10 Darcy (Olstad
et al. 1997). These calculations are based on one
dimensional models and the results are most probably
minimum values for the permeability values of the cap
rock since lateral drainage is not included. Lateral flow
would reduce the vertical flow and the permeability
would have had to be lower to maintain the observed
vertical pressure gradients. Increasing the vertical
fluid flux by focussing the flow would imply that the
permeability was higher.
10.9 The Importance of Faults
Faults may greatly affect fluid flow in sedimentary
basins and may serve either as conduits or barriers,
depending on the situation. A clear distinction must be
made between flow along, and across, the fault plane.
Faults are also important because they may offset
294
K. Bjørlykke
focusing of the flow. During periods with no sedimentation (hiatus), the deposited sediments continue
to compact and porewater flows across the redox
boundary just below the seabed and dissolved ions
like Fe
2+ and Mn
2+ may precipitate as Fe(OH) 3 and
Mn(OH) 4 .
Very high degrees of focusing are required to
obtain high flow rates. It is then easy to understand
why compaction-driven flow is several orders of
magnitude lower than meteoric water flow, at least in
the shallow parts of the basin near land. The slow
porewater flow rates also imply that this water has
time to approach thermal and chemical equilibrium
with the minerals. Porewater will always transport
some heat but, compared to the background heat flow
by conduction, this is very small and in most cases can
be ignored (Bethke 1985, Ludvigsen 1992).
Modelling compaction-driven flow in the Gulf
Basin, Harrison and Summa (1991) found that the
maximum rate of the vertical component is 2 mm/
year (2 km/million years), which is approximately
equivalent to the maximum sedimentation rate. This
flow rate is too slow to contribute significantly to the
heat flow. The temperature distribution in the Gulf
Basin at the present day does not reflect compactiondriven porewater flow. Most of the heat transport is by
conduction. The average compaction-driven flow
within a basin is more or less independent of the
permeability because it is a function of the rate of
loss of porosity in the underlying sediments. During
mechanical compaction, low permeability sediments
may result in overpressure and a certain reduction of
fluid flow, but at greater depth where the compaction is
mostly chemical, the fluid flux is independent of the
permeability although variations in permeability may
focus the flow.
10.8 Constraints on Water Flow in
Sedimentary Basins by Porewater
Chemistry
Many sedimentary basins contain evaporites, often
occupying the basal part, having formed during the
initial rifting. Very high salinity is then typically found
in a zone of a few hundred metres adjacent to the salt.
This is the case with the Zechstein salt in the North Sea
basin. The shallower parts of a basin and in particular
near tectonically uplifted areas, may contain
porewater of meteoric origin with very low salinity.
The isotopic composition of the porewater often shows
a very clear stratification. Oxygen isotopes are often
negative in the shallow parts of the basin due to the
inflow of meteoric water, while they may be positive at
greater depth due to diagenetic reactions.
During subsidence there are generally no open
fractures because of the ductile properties of subsiding sediments (Bjørlykke and Høeg 1997).
The permeabilities in shales are very low, probably
less than a nanodarcy (Leonard 1993). Samples
measured in the laboratory may show erroneously
high values because of fracturing resulting from
unloading during core retrieval. Increasing the effective stress during laboratory measurements to compensate for this has been shown to lower the
permeabilities by two orders of magnitude (Katsube
et al. 1991). In sediments which have been subject to
uplift, however, fracture permeability may be important. Laboratory measurements of shale permeabilities
may be several orders of magnitude larger than the
results of well tests in the field (Oelkers 1996).
In subsiding basins the effective permeabilities on a
large scale must be low, not above about 10
À9 Darcy,
to maintain overpressures over time. Calculating the
porewater flux due to compaction and using observed
pressure gradients, the effective permeability of thick
shale sequences can be calculated. Using data from
Haltenbanken offshore Norway this method gave
permeabilities between 10
À9 and 10
À10 Darcy (Olstad
et al. 1997). These calculations are based on one
dimensional models and the results are most probably
minimum values for the permeability values of the cap
rock since lateral drainage is not included. Lateral flow
would reduce the vertical flow and the permeability
would have had to be lower to maintain the observed
vertical pressure gradients. Increasing the vertical
fluid flux by focussing the flow would imply that the
permeability was higher.
10.9 The Importance of Faults
Faults may greatly affect fluid flow in sedimentary
basins and may serve either as conduits or barriers,
depending on the situation. A clear distinction must be
made between flow along, and across, the fault plane.
Faults are also important because they may offset
294
K. Bjørlykke