10.1 Composition of Formation Water
In most cases the formation water is saline and Cl
À is
by far the dominant anion, so that the Na
+
=Cl
À ratio is
less than 1. The rest of the positive charge is mostly
made up of Mg
2+ and Ca
2+ . The composition of
porewater is a function of its primary origin, modified
by the mineral composition of the sediments, the
temperature and the quantity of dissolved gases, particularly CO 2 .
Pure water of meteoric origin derived from fresh
groundwater may gradually mix with more saline
water and become more brackish. Meteoric water usually has less than 10,000 ppm dissolved material compared to seawater, which has 35,000 ppm and contains
more bicarbonate (HCO
À
3 ) and small amounts of magnesium, sodium and calcium. Because meteoric water
comes from a land surface, it brings with it oxygen and
bacteria which can break down hydrocarbons (see
“Biodegradation”). In sediments with even small
amounts of organic matter the porewater quickly
becomes reducing as oxygen is consumed by breakdown of the organic matter. The composition of meteoric water alters as it reacts with the more readily
soluble minerals in the sediments. Small amounts of
carbonate makes the meteoric water basic and will act
as a buffer with respect to pH. Meteoric water will
flow from land areas where the groundwater table is
elevated above sea level through sandstones serving as
aquifers far offshore, depending on the pressure head
of the groundwater table.
Drilling on the continental shelves, for example off
the east coast of the USA and also offshore Africa,
shows that meteoric water aquifers are widespread and
that water which is virtually fresh is sometimes found
below the seabed as far out as 100 km from the coast.
In the northern North Sea there is also isotopic evidence that the formation water in shallow reservoirs is
partly of meteoric origin. This also shows that the
porewater is often stratified and there is very limited
vertical mixing of porewater with different salinities.
Marine porewater, which is present in sediments
when they are deposited, will initially have an approximately normal salinity but sulphate is removed by
sulphate reduction just below the seafloor. Clay
minerals act as ion exchangers, absorbing cations
like K
+ and Mg
2+ from the porewater. Compacted
clay and mud may function as a membrane. Clay
minerals are normally negatively charged on the surface and particularly at the ends, where there are
broken silicate bonds. Negatively charged ions are
repulsed and held back by the membrane due to the
negative charges of the clay minerals. In order for the
charges on both sides to equalise, the small ions,
particularly H
+ , must move in the opposite direction,
and as a result there is a higher H
+ concentration
(lower pH). This process, which will concentrate salt,
is called salt sieving. Membranes also discriminate
selectively amongst different cations, depending on
their size and charge. Those alkali ions which are
strongly hydrated (Na
+ , Li
+
) and also Mg
2+ , will to a
lesser degree be adsorbed onto the surface of the clays
and will be more mobile than for example K
+ and Rb
+
,
which are less hydrated. At higher temperatures the
hydration becomes less effective and ions like Mg
2+
are more available to be adsorbed on clay minerals and
form carbonate minerals like dolomite.
At 70À100
C, smectite will dissolve and form
illite and water. At 120–140
C kaolinite will become
unstable and form illite, quartz and water. This process binds cations, particularly K
+ , and releases pure
water, thus reducing the salinity of the porewater. As
a result the porewater in shales often has a lower
salinity than that in sandstones at the same depth.
The composition of porewater in sandstones varies
greatly, depending on whether they contain meteoric
water or not.
The quantity of dissolved solids in porewater
increases as a function of depth in most cases.
Concentrations of 100,000–300,000 ppm of dissolved
matter (total dissolved solids, TDS) are typical for
basins with evaporite beds. With increasing temperature, the kinetic obstacles to mineral solution and
precipitation reactions are reduced. At temperatures
above about 80
C the porewater will tend to be in
equilibrium with most of the minerals present. In sedimentary basins with evaporites, these will greatly
influence the composition of the porewater in
overlying formations. Dissolved salts move upwards
through compaction-driven porewater flow; diffusion
due to high concentration gradients also plays a major
role. Sedimentary basins along the Atlantic Coast in
areas where Mesozoic evaporites are deposited have
porewater compositions which are essentially different from those in areas north of this palaeoclimatic
belt. The South American continental shelf, the Gulf
10 Subsurface Water and Fluid Flow in Sedimentary Basins
281
In most cases the formation water is saline and Cl
À is
by far the dominant anion, so that the Na
+
=Cl
À ratio is
less than 1. The rest of the positive charge is mostly
made up of Mg
2+ and Ca
2+ . The composition of
porewater is a function of its primary origin, modified
by the mineral composition of the sediments, the
temperature and the quantity of dissolved gases, particularly CO 2 .
Pure water of meteoric origin derived from fresh
groundwater may gradually mix with more saline
water and become more brackish. Meteoric water usually has less than 10,000 ppm dissolved material compared to seawater, which has 35,000 ppm and contains
more bicarbonate (HCO
À
3 ) and small amounts of magnesium, sodium and calcium. Because meteoric water
comes from a land surface, it brings with it oxygen and
bacteria which can break down hydrocarbons (see
“Biodegradation”). In sediments with even small
amounts of organic matter the porewater quickly
becomes reducing as oxygen is consumed by breakdown of the organic matter. The composition of meteoric water alters as it reacts with the more readily
soluble minerals in the sediments. Small amounts of
carbonate makes the meteoric water basic and will act
as a buffer with respect to pH. Meteoric water will
flow from land areas where the groundwater table is
elevated above sea level through sandstones serving as
aquifers far offshore, depending on the pressure head
of the groundwater table.
Drilling on the continental shelves, for example off
the east coast of the USA and also offshore Africa,
shows that meteoric water aquifers are widespread and
that water which is virtually fresh is sometimes found
below the seabed as far out as 100 km from the coast.
In the northern North Sea there is also isotopic evidence that the formation water in shallow reservoirs is
partly of meteoric origin. This also shows that the
porewater is often stratified and there is very limited
vertical mixing of porewater with different salinities.
Marine porewater, which is present in sediments
when they are deposited, will initially have an approximately normal salinity but sulphate is removed by
sulphate reduction just below the seafloor. Clay
minerals act as ion exchangers, absorbing cations
like K
+ and Mg
2+ from the porewater. Compacted
clay and mud may function as a membrane. Clay
minerals are normally negatively charged on the surface and particularly at the ends, where there are
broken silicate bonds. Negatively charged ions are
repulsed and held back by the membrane due to the
negative charges of the clay minerals. In order for the
charges on both sides to equalise, the small ions,
particularly H
+ , must move in the opposite direction,
and as a result there is a higher H
+ concentration
(lower pH). This process, which will concentrate salt,
is called salt sieving. Membranes also discriminate
selectively amongst different cations, depending on
their size and charge. Those alkali ions which are
strongly hydrated (Na
+ , Li
+
) and also Mg
2+ , will to a
lesser degree be adsorbed onto the surface of the clays
and will be more mobile than for example K
+ and Rb
+
,
which are less hydrated. At higher temperatures the
hydration becomes less effective and ions like Mg
2+
are more available to be adsorbed on clay minerals and
form carbonate minerals like dolomite.
At 70À100
C, smectite will dissolve and form
illite and water. At 120–140
C kaolinite will become
unstable and form illite, quartz and water. This process binds cations, particularly K
+ , and releases pure
water, thus reducing the salinity of the porewater. As
a result the porewater in shales often has a lower
salinity than that in sandstones at the same depth.
The composition of porewater in sandstones varies
greatly, depending on whether they contain meteoric
water or not.
The quantity of dissolved solids in porewater
increases as a function of depth in most cases.
Concentrations of 100,000–300,000 ppm of dissolved
matter (total dissolved solids, TDS) are typical for
basins with evaporite beds. With increasing temperature, the kinetic obstacles to mineral solution and
precipitation reactions are reduced. At temperatures
above about 80
C the porewater will tend to be in
equilibrium with most of the minerals present. In sedimentary basins with evaporites, these will greatly
influence the composition of the porewater in
overlying formations. Dissolved salts move upwards
through compaction-driven porewater flow; diffusion
due to high concentration gradients also plays a major
role. Sedimentary basins along the Atlantic Coast in
areas where Mesozoic evaporites are deposited have
porewater compositions which are essentially different from those in areas north of this palaeoclimatic
belt. The South American continental shelf, the Gulf
10 Subsurface Water and Fluid Flow in Sedimentary Basins
281