Coast and the area off the East Coast of the USA are
characterised by Jurassic/Cretaceous evaporites. In the
North Sea the extension of Zechstein evaporites forms
an important boundary which is reflected in the
porewater composition of overlying Mesozoic
sediments. In addition to chemical analysis, isotope
composition (
13 C=
12 C and
18 O=
16 O) can provide valuable information concerning the origin of the
porewater.
Many of the dissolved ions in porewater are in
equilibrium with the minerals present. They are then
not very useful as indicators of the origin of the water.
ClÀ and BrÀ are better tracers for fluid flow as they do
not react very much with the minerals present.
The water produced from drill stem tests may be
strongly contaminated by the drilling mud filtrate and
the composition of the mud should be considered
when using the analyses of such waters. Water produced during production is less likely to be strongly
contaminated because of the larger volume involved.
The results of the analyses may be expressed as % or
ppm.
mg=l ¼ ppm/density of water
meq=l ¼ mg=l  valence/mol.weight
The main anions in subsurface waters are ClÀ,
HCO
À
3 and SO
2À
4 , and the main cations are Na
þ
; K
þ
and Ca
++ .
Meteoric water is characterised initially by low
ionic strength but it then reacts with minerals and
also amorphous phases like opal A. Unless meteoric
water flows through evaporites the chlorinity will
remain very low and the main anions will be bicarbonate HCO 3
ð
Þ or carbonate CO
2À
3
À
Á
. Sulphate SO
2À
4
À
Á
may form in meteoric water due to oxidation of
sulphides in rocks and of sulphur in organic matter,
but the sulphate will tend to be reduced to sulphides by
sulphate-reducing bacteria.
Porewater of marine origin naturally starts with the
composition of seawater but only a few centimetres
below the seafloor most of the free oxygen is removed
from the porewater due to oxidation of organic matter
in the sediments. A few metres below the seabed
nearly all the sulphate which was in the seawater has
been consumed by sulphate-reducing bacteria. It is
therefore a characteristic of so-called connate water
that it has very low sulphate content. The chlorinity,
however, remains practically unchanged because ClÀ
is not consumed by any significant diagenetic process.
Steep concentration gradients and a strong drive for
transport by diffusion exist across the redox boundary
because of the difference in solubility of many ions
between the seawater, which is normally oxidised, and
the porewater below. Sulphate SO
2À
4
À
Á
is transported
downwards and is reduced to sulphides below the
redox boundary. Reduced sulphur reacts with ironbearing minerals including iron oxides (haematite)
to form pyrite (FeS 2 ). Reduced manganese and
iron (Mn
2+ , Fe
2+ ) will be transported upwards and
precipitated above the redox boundary. While much
of the iron will be trapped in the reduced state as
sulphides, manganese sulphides are rather soluble and
very little Mn will therefore be trapped below the redox
boundary. Ferrous iron (Fe
2+ ) may also be trapped as
carbonate such as siderite (FeCO 3 ). Manganese is
therefore transported upwards more efficiently than
iron and may form large deposits (as manganese
nodules) in deepwater environments where the sedimentation rate is low.
The composition of subsurface water is strongly
influenced by the dissolution of evaporites where
these are present but the effect can often be shown to
be rather local (radius <1 km). Meteoric water can
dilute the chlorinity of porewater but mixing of
porewater is not very efficient in sedimentary basins.
This is because the flow is slow and laminar and
pore waters with different salinities have different
densities, which also tends to inhibit mixing. Transport by diffusion from high to lower salinity may
nevertheless be significant over distances of a few
hundred metres, depending on the diffusion constant
of the sediment matrix. Low permeability shales also
have low diffusion constants. Dehydration of minerals
such as gypsum or clay minerals like smectite, kaolinite and gibbsite also causes reductions in salinity
because pure crystal-bound water is released into the
porewater.
Higher salinities than seawater are in most cases
due to the dissolution of evaporites.
High salinity porewater is found in the central
North Sea above the Permian evaporites in the Central
Graben, while in the northern North Sea the porewater
is of normal salinity or brackish composition. In the
northern North Sea where there are no evaporites,
282
K. Bjørlykke
characterised by Jurassic/Cretaceous evaporites. In the
North Sea the extension of Zechstein evaporites forms
an important boundary which is reflected in the
porewater composition of overlying Mesozoic
sediments. In addition to chemical analysis, isotope
composition (
13 C=
12 C and
18 O=
16 O) can provide valuable information concerning the origin of the
porewater.
Many of the dissolved ions in porewater are in
equilibrium with the minerals present. They are then
not very useful as indicators of the origin of the water.
ClÀ and BrÀ are better tracers for fluid flow as they do
not react very much with the minerals present.
The water produced from drill stem tests may be
strongly contaminated by the drilling mud filtrate and
the composition of the mud should be considered
when using the analyses of such waters. Water produced during production is less likely to be strongly
contaminated because of the larger volume involved.
The results of the analyses may be expressed as % or
ppm.
mg=l ¼ ppm/density of water
meq=l ¼ mg=l  valence/mol.weight
The main anions in subsurface waters are ClÀ,
HCO
À
3 and SO
2À
4 , and the main cations are Na
þ
; K
þ
and Ca
++ .
Meteoric water is characterised initially by low
ionic strength but it then reacts with minerals and
also amorphous phases like opal A. Unless meteoric
water flows through evaporites the chlorinity will
remain very low and the main anions will be bicarbonate HCO 3
ð
Þ or carbonate CO
2À
3
À
Á
. Sulphate SO
2À
4
À
Á
may form in meteoric water due to oxidation of
sulphides in rocks and of sulphur in organic matter,
but the sulphate will tend to be reduced to sulphides by
sulphate-reducing bacteria.
Porewater of marine origin naturally starts with the
composition of seawater but only a few centimetres
below the seafloor most of the free oxygen is removed
from the porewater due to oxidation of organic matter
in the sediments. A few metres below the seabed
nearly all the sulphate which was in the seawater has
been consumed by sulphate-reducing bacteria. It is
therefore a characteristic of so-called connate water
that it has very low sulphate content. The chlorinity,
however, remains practically unchanged because ClÀ
is not consumed by any significant diagenetic process.
Steep concentration gradients and a strong drive for
transport by diffusion exist across the redox boundary
because of the difference in solubility of many ions
between the seawater, which is normally oxidised, and
the porewater below. Sulphate SO
2À
4
À
Á
is transported
downwards and is reduced to sulphides below the
redox boundary. Reduced sulphur reacts with ironbearing minerals including iron oxides (haematite)
to form pyrite (FeS 2 ). Reduced manganese and
iron (Mn
2+ , Fe
2+ ) will be transported upwards and
precipitated above the redox boundary. While much
of the iron will be trapped in the reduced state as
sulphides, manganese sulphides are rather soluble and
very little Mn will therefore be trapped below the redox
boundary. Ferrous iron (Fe
2+ ) may also be trapped as
carbonate such as siderite (FeCO 3 ). Manganese is
therefore transported upwards more efficiently than
iron and may form large deposits (as manganese
nodules) in deepwater environments where the sedimentation rate is low.
The composition of subsurface water is strongly
influenced by the dissolution of evaporites where
these are present but the effect can often be shown to
be rather local (radius <1 km). Meteoric water can
dilute the chlorinity of porewater but mixing of
porewater is not very efficient in sedimentary basins.
This is because the flow is slow and laminar and
pore waters with different salinities have different
densities, which also tends to inhibit mixing. Transport by diffusion from high to lower salinity may
nevertheless be significant over distances of a few
hundred metres, depending on the diffusion constant
of the sediment matrix. Low permeability shales also
have low diffusion constants. Dehydration of minerals
such as gypsum or clay minerals like smectite, kaolinite and gibbsite also causes reductions in salinity
because pure crystal-bound water is released into the
porewater.
Higher salinities than seawater are in most cases
due to the dissolution of evaporites.
High salinity porewater is found in the central
North Sea above the Permian evaporites in the Central
Graben, while in the northern North Sea the porewater
is of normal salinity or brackish composition. In the
northern North Sea where there are no evaporites,
282
K. Bjørlykke