formation water salinity is in most cases close to that
of seawater, or less saline (brackish) due probably to a
component of meteoric water. Near evaporites the
porewater is also often rich in calcium due to gypsum
or anhydrite, with a corresponding reduction in
sodium, so that CaCl 2 is an important dissolved salt.
Dissolved NaCl may be transported away from the
evaporites by diffusion or by porewater flow (advection). In both cases the salinity will be reduced away
from the salt deposit and it is unlikely that the
dissolved salt would have become sufficiently
concentrated for halite to be precipitated again.
The halite and also gypsum commonly observed in
small amounts in sandstone cores is due to evaporation
in the core store; they are not diagenetic minerals as
has sometimes been reported. During tectonic uplift
and erosion, hydration of minerals like anhydrite
causes increased salinity but this is normally diluted
with meteoric water. Near the surface the salinity can
of course increase by evaporation. In the seawater at
shallow water depth the pH is relatively high because
the water is often at least nearly saturated with respect
to calcite and the pCO 2 is low. At greater water depth
in the ocean and also below the seafloor the solubility
of CO 2 increases, lowering the pH. Carbon dioxide is
consumed near the ocean surface by photosynthesis
and released by oxidation of organic matter sinking
towards the ocean floor. Just below the seafloor in the
sulphate reduction zone more CO 2 is produced during
fomentation of organic matter. Then at greater burial
depth, thermal maturation of kerogen releases CO 2 .
Organic acids are also generated from the source rocks
and probably also from the oil. Porewater is, however,
like seawater a buffered solution and organic acids are
weak acids. Compared to the total buffering system of,
firstly, the silicate mineral system and, secondly, the
carbonate system, the addition of relatively small
amounts of comparatively weak acids (organic acids)
will not change the pH of the porewater significantly
(Hutcheon 1989). These different types of CO 2 have
characteristic ranges in δ
13 C composition.
Formation analyses from sedimentary basins show
that the pore waters are frequently crudely stratified
with respect to salinity and this puts constraints on
porewater flow. Also the oxygen isotope compositions
may vary with depth, probably at least partly due to
diagenetic reactions. At shallow depth and low temperature the porewater is normally out of equilibrium
with respect to the silicate minerals because of the
slow reaction rates. Meteoric water is usually highly
supersaturated with respect to quartz because it does
not precipitate at low temperatures (<70–80
C). Seawater, on the other hand, is usually very much undersaturated with respect to quartz because silica is taken
out of seawater by siliceous organisms, mainly
diatoms.
Amorphous silica from organisms (opal A) may
survive during burial down to 1.5–2 km (60–80
C)
before dissolving. Opal A is replaced first by opal
CT, which is unstable and will be replaced by quartz.
As long as opal A and opal CT exist in the sediments
the porewater is supersaturated with respect to quartz.
At higher temperatures the porewater becomes closer
to equilibrium with quartz and other minerals present.
The isotopic composition of subsurface water
reflects to a large extent the initial composition.
Porewater with a marine origin has characteristic
values close to Standard Mean Ocean Water
(SMOW). More negative values may indicate the
introduction of meteoric water into a marine basin.
At greater burial depth the composition of porewater
is more influenced by the reactions of the minerals. As
a general rule dissolution of minerals formed at low
temperature, i.e. during weathering (kaolinite, smectite), causes the pore waters to become more positive
when they dissolve at greater depth (higher
temperatures). This is because the minerals that
precipitated at a higher temperature, in this case illite,
will contain less
18 O. Dissolution and precipitation of
clastic quartz, which was originally precipitated at
high temperature, will shift the porewater in a negative
direction when low temperature quartz, with more
18 O, is precipitated.
We now have considerable data on the composition
of porewater in sedimentary basins from exploration
and production wells. It is clear that the porewater
composition varies greatly over distances of a few
hundred metres and this is evidence of very limited
mixing by advection. Except around salt domes the
porewater seems to be crudely stratified with respect to
both salinity and isotopic composition δ
18 O
À
Á
. This
puts important constraints on the transport of solids in
solution by fluid flow. The saline porewater is not
transported very far from the salt. With increasing
temperature the density of porewater is reduced,
while it is increased by increasing salinity.
10 Subsurface Water and Fluid Flow in Sedimentary Basins
283
of seawater, or less saline (brackish) due probably to a
component of meteoric water. Near evaporites the
porewater is also often rich in calcium due to gypsum
or anhydrite, with a corresponding reduction in
sodium, so that CaCl 2 is an important dissolved salt.
Dissolved NaCl may be transported away from the
evaporites by diffusion or by porewater flow (advection). In both cases the salinity will be reduced away
from the salt deposit and it is unlikely that the
dissolved salt would have become sufficiently
concentrated for halite to be precipitated again.
The halite and also gypsum commonly observed in
small amounts in sandstone cores is due to evaporation
in the core store; they are not diagenetic minerals as
has sometimes been reported. During tectonic uplift
and erosion, hydration of minerals like anhydrite
causes increased salinity but this is normally diluted
with meteoric water. Near the surface the salinity can
of course increase by evaporation. In the seawater at
shallow water depth the pH is relatively high because
the water is often at least nearly saturated with respect
to calcite and the pCO 2 is low. At greater water depth
in the ocean and also below the seafloor the solubility
of CO 2 increases, lowering the pH. Carbon dioxide is
consumed near the ocean surface by photosynthesis
and released by oxidation of organic matter sinking
towards the ocean floor. Just below the seafloor in the
sulphate reduction zone more CO 2 is produced during
fomentation of organic matter. Then at greater burial
depth, thermal maturation of kerogen releases CO 2 .
Organic acids are also generated from the source rocks
and probably also from the oil. Porewater is, however,
like seawater a buffered solution and organic acids are
weak acids. Compared to the total buffering system of,
firstly, the silicate mineral system and, secondly, the
carbonate system, the addition of relatively small
amounts of comparatively weak acids (organic acids)
will not change the pH of the porewater significantly
(Hutcheon 1989). These different types of CO 2 have
characteristic ranges in δ
13 C composition.
Formation analyses from sedimentary basins show
that the pore waters are frequently crudely stratified
with respect to salinity and this puts constraints on
porewater flow. Also the oxygen isotope compositions
may vary with depth, probably at least partly due to
diagenetic reactions. At shallow depth and low temperature the porewater is normally out of equilibrium
with respect to the silicate minerals because of the
slow reaction rates. Meteoric water is usually highly
supersaturated with respect to quartz because it does
not precipitate at low temperatures (<70–80
C). Seawater, on the other hand, is usually very much undersaturated with respect to quartz because silica is taken
out of seawater by siliceous organisms, mainly
diatoms.
Amorphous silica from organisms (opal A) may
survive during burial down to 1.5–2 km (60–80
C)
before dissolving. Opal A is replaced first by opal
CT, which is unstable and will be replaced by quartz.
As long as opal A and opal CT exist in the sediments
the porewater is supersaturated with respect to quartz.
At higher temperatures the porewater becomes closer
to equilibrium with quartz and other minerals present.
The isotopic composition of subsurface water
reflects to a large extent the initial composition.
Porewater with a marine origin has characteristic
values close to Standard Mean Ocean Water
(SMOW). More negative values may indicate the
introduction of meteoric water into a marine basin.
At greater burial depth the composition of porewater
is more influenced by the reactions of the minerals. As
a general rule dissolution of minerals formed at low
temperature, i.e. during weathering (kaolinite, smectite), causes the pore waters to become more positive
when they dissolve at greater depth (higher
temperatures). This is because the minerals that
precipitated at a higher temperature, in this case illite,
will contain less
18 O. Dissolution and precipitation of
clastic quartz, which was originally precipitated at
high temperature, will shift the porewater in a negative
direction when low temperature quartz, with more
18 O, is precipitated.
We now have considerable data on the composition
of porewater in sedimentary basins from exploration
and production wells. It is clear that the porewater
composition varies greatly over distances of a few
hundred metres and this is evidence of very limited
mixing by advection. Except around salt domes the
porewater seems to be crudely stratified with respect to
both salinity and isotopic composition δ
18 O
À
Á
. This
puts important constraints on the transport of solids in
solution by fluid flow. The saline porewater is not
transported very far from the salt. With increasing
temperature the density of porewater is reduced,
while it is increased by increasing salinity.
10 Subsurface Water and Fluid Flow in Sedimentary Basins
283