be about 2 km (Harrison and Summa 1991). The depth
of penetration does not depend only on the head of the
meteoric water but also on compaction processes in
the sediments, which can generate overpressures that
may exceed the meteoric water head. Even slight
overpressures due to compaction will strongly reduce
the depth of meteoric water penetration.
The rainfall, the catchment area and the percentage
of infiltration into the groundwater determine the
upper limit of meteoric water flow. Meteoric water
which flows down into a sedimentary basin must eventually flow up to the surface, in order to maintain a
continuous flow. Permeable sandstones are hydraulically almost dead ends if they pinch out into very low
permeability mudstones. Pressure will then build up in
the aquifer and reduce the meteoric water inflow,
forcing water to flow through the overlying
mudstones. We must remember that even if the fluid
flux is small, the large area of contact between a sand
layer which serves as aquifer and the overlying mud
will allow relatively high volumes of porewater to
escape upwards through the mud. Even if the flow
per area (flux) is small through the mudstones the
area for vertical flow is large compared to the vertical
cross-section. At shallow depths (<500 m) prior to
severe compaction, overpressure is rarely developed
as the permeability is much higher than in compacted
mudstones and shales.
The degree of meteoric water flushing is highly
dependent on climate and facies. The land surface
and vegetation determine the percentage of rainfall
which infiltrates down to the groundwater. Fluvial,
deltaic and nearshore shallow marine sediments will
be flushed by meteoric water shortly after deposition.
The flux is then likely to be high and the porewater is
still very much undersaturated with respect to feldspar
and mica. The total volume of water flowing through
each volume of sediment is inversely related to sedimentation rates. At high sedimentation rates the
sediments spend less time in the zone of meteoric
water flushing. Sands deposited in more distal shelf
facies (nearer the shelf edge) and turbidites (on the
slopes) are normally less well connected to the main
groundwater wedge, so that the flux is lower and the
porewater is closer to equilibrium with respect to the
mineral phases.
In the North Sea basin it has been demonstrated that
reservoir sandstones deposited in fluvial and shallow
marine environments have been subjected to more
feldspar dissolution (secondary porosity) and contain
more authigenic kaolinite than sandstones representing turbidite facies (Bjørlykke and Aagaard
1992). Sediments in sedimentary basins like the
North Sea may be intensively flushed by meteoric
water immediately after deposition and also after
uplift episodes and erosion. When tectonic uplift
results in subaerial exposure and the formation of
islands, meteoric water is collected on land and driven
into the subsurface around the islands and adjacent to
other land areas. The sediments most strongly affected
by meteoric water leaching are constantly being
removed by erosion, however. This may explain why
there is not always much evidence of feldspar leaching
and high kaolinite contents immediately below
unconformities.
Good examples of clay mineral diagenesis related
to modern groundwater systems have been observed
down to 3–400 m depth in the Mississippi Gulf coastal
plain (Hanor and Mcmanus 1988). In Canada there is
isotopic evidence of recent meteoric water diagenesis
extending several hundred metres below the land surface (Longstaffe 1984). It must be stressed that the
isotopic composition of the porewater acquires a meteoric signature as soon as a volume of meteoric water
has displaced the marine (connate) porewater. New
minerals (like calcite, kaolinite and quartz) precipitated in this porewater will reflect that isotopic
signature.
10.6 Porewater Flow Driven by Thermal
Convection
Thermal convection is an effective mechanism for the
mass transfer of dissolved material in sedimentary
basins, because the same water can be used over and
over again (Wood and Hewett 1982, Davis et al. 1985).
The limitation of fluid (water) supply, which constrains
compaction-driven flow, is then eliminated. Thermal
convection may occur because the density of water
decreases with depth in a sedimentary basin as the
temperature increases, due to the thermal expansion
of water. This creates an inverse density gradient
which may be unstable. If the isotherms (lines of
equal temperature) are horizontal, the density as a
function of temperature will not vary horizontally and
there is no flow unless the water overturns. The denser
upper layers of porewater may start to overturn and
290
K. Bjørlykke
of penetration does not depend only on the head of the
meteoric water but also on compaction processes in
the sediments, which can generate overpressures that
may exceed the meteoric water head. Even slight
overpressures due to compaction will strongly reduce
the depth of meteoric water penetration.
The rainfall, the catchment area and the percentage
of infiltration into the groundwater determine the
upper limit of meteoric water flow. Meteoric water
which flows down into a sedimentary basin must eventually flow up to the surface, in order to maintain a
continuous flow. Permeable sandstones are hydraulically almost dead ends if they pinch out into very low
permeability mudstones. Pressure will then build up in
the aquifer and reduce the meteoric water inflow,
forcing water to flow through the overlying
mudstones. We must remember that even if the fluid
flux is small, the large area of contact between a sand
layer which serves as aquifer and the overlying mud
will allow relatively high volumes of porewater to
escape upwards through the mud. Even if the flow
per area (flux) is small through the mudstones the
area for vertical flow is large compared to the vertical
cross-section. At shallow depths (<500 m) prior to
severe compaction, overpressure is rarely developed
as the permeability is much higher than in compacted
mudstones and shales.
The degree of meteoric water flushing is highly
dependent on climate and facies. The land surface
and vegetation determine the percentage of rainfall
which infiltrates down to the groundwater. Fluvial,
deltaic and nearshore shallow marine sediments will
be flushed by meteoric water shortly after deposition.
The flux is then likely to be high and the porewater is
still very much undersaturated with respect to feldspar
and mica. The total volume of water flowing through
each volume of sediment is inversely related to sedimentation rates. At high sedimentation rates the
sediments spend less time in the zone of meteoric
water flushing. Sands deposited in more distal shelf
facies (nearer the shelf edge) and turbidites (on the
slopes) are normally less well connected to the main
groundwater wedge, so that the flux is lower and the
porewater is closer to equilibrium with respect to the
mineral phases.
In the North Sea basin it has been demonstrated that
reservoir sandstones deposited in fluvial and shallow
marine environments have been subjected to more
feldspar dissolution (secondary porosity) and contain
more authigenic kaolinite than sandstones representing turbidite facies (Bjørlykke and Aagaard
1992). Sediments in sedimentary basins like the
North Sea may be intensively flushed by meteoric
water immediately after deposition and also after
uplift episodes and erosion. When tectonic uplift
results in subaerial exposure and the formation of
islands, meteoric water is collected on land and driven
into the subsurface around the islands and adjacent to
other land areas. The sediments most strongly affected
by meteoric water leaching are constantly being
removed by erosion, however. This may explain why
there is not always much evidence of feldspar leaching
and high kaolinite contents immediately below
unconformities.
Good examples of clay mineral diagenesis related
to modern groundwater systems have been observed
down to 3–400 m depth in the Mississippi Gulf coastal
plain (Hanor and Mcmanus 1988). In Canada there is
isotopic evidence of recent meteoric water diagenesis
extending several hundred metres below the land surface (Longstaffe 1984). It must be stressed that the
isotopic composition of the porewater acquires a meteoric signature as soon as a volume of meteoric water
has displaced the marine (connate) porewater. New
minerals (like calcite, kaolinite and quartz) precipitated in this porewater will reflect that isotopic
signature.
10.6 Porewater Flow Driven by Thermal
Convection
Thermal convection is an effective mechanism for the
mass transfer of dissolved material in sedimentary
basins, because the same water can be used over and
over again (Wood and Hewett 1982, Davis et al. 1985).
The limitation of fluid (water) supply, which constrains
compaction-driven flow, is then eliminated. Thermal
convection may occur because the density of water
decreases with depth in a sedimentary basin as the
temperature increases, due to the thermal expansion
of water. This creates an inverse density gradient
which may be unstable. If the isotherms (lines of
equal temperature) are horizontal, the density as a
function of temperature will not vary horizontally and
there is no flow unless the water overturns. The denser
upper layers of porewater may start to overturn and
290
K. Bjørlykke