Uranium is highly soluble in seawater as uranyl
(UO
2+
) and precipitates as reduced uranium oxide
(UO 2 ) on organic matter in the water column and
below the redox boundary. There is thus a strong concentration gradient transporting uranium from the seawater down into the sediments. The adsorption of
uranium onto organic matter settling on the seafloor
coupled with restricted ventilation of the water above
the sediment at the time of deposition, makes source
rocks like the Kimmeridge shale strongly enriched in
uranium, causing peaks on the gamma ray well log curve.
Iron and manganese may be transported upwards
through the sediments in the reduced state by diffusion
and then precipitate on the seafloor as oxides because of
their reduced solubility in the oxidised state. Iron may
also be precipitated below the redox boundary as iron
sulphides or iron carbonate (siderite), though iron
carbonates will not form during sulphate reduction.
This is because all the iron will be precipitated as
sulphide, which has a much lower solubility than siderite. Manganese is not precipitated as sulphides because
of the high solubility of Mn-sulphides, but may be
precipitated as Mn-carbonate in the reduced zone.
At a depth where there is practically no more free
dissolved oxygen in the porewater, sulphates can be
used by sulphate-reducing bacteria. The reduction of
sulphates produces sulphides such as pyrite.
The composition of clastic sediments is modified
by the addition of new components produced locally
within the basin:
(1) Biogenic carbonates and silica.
(2) Authigenic minerals precipitating near the seabed
such as carbonates, phosphates, glauconite,
chamosite, sulphides and iron and manganese
minerals.
(3) Meteoric water-flushing causing leaching of feldspar and mica and precipitation of kaolinite
beneath the seafloor.
4.5
Importance of Biogenic Activity
Bioturbation plays an important role in changing the
textural composition of the sediments after deposition.
The burrowing organisms eat mud and thereby oxidise
organic matter and physically destroy the primary lamination. Sediments overturned by bioturbation become
more exposed to oxidation at the sea or lake bottom.
Bioturbation may reduce the porosity and permeability
of sandy laminae by mixing clay with clean sand.
Bioturbation will also destroy thin clay laminae,
which may significantly increase the vertical permeability and this may be very significant for reservoir
quality. Undisturbed primary lamination may be evidence of rather rapid sedimentation giving little time
for a burrowing bottom fauna to become established,
or alternatively indicate strongly reducing conditions
restricting the fauna. Black shales usually have well
preserved lamination due to lack of burrowing
organisms. The presence or absence of burrowing
also influences the physical properties, particularly
the difference in velocity and resistivity parallel and
vertical to bedding (anisotropy) and this may be
important for geophysical modelling.
Burrowing worms produce faecal material which
may develop into smectite-rich clays, which in turn
may develop into chlorite coatings, thus improving
reservoir quality. Early diagenetic formation of
coatings on quartz grains is extremely important due
to its role in preserving porosity at greater depth.
Most clastic depositional environments have some
organisms producing organic matter which, at least in
part, is incorporated within the sediments. Both
sandstones and mudstones nearly always contain significant amounts of biogenic material from calcareous,
and sometimes also siliceous, organisms and this may
later be an important source of carbonate and silica
cement at greater burial depth.
Marine organisms composed of aragonite dissolve
during relatively shallow burial and calcite
precipitates either as replacements within the fossils
(neomorphism) or as cement in pore space between the
grains.
Carbonate cement in sandstones may form layers or
concretions and in most cases is derived from biogenic
carbonate, particularly from organisms composed of
aragonite. Siliceous organisms composed of opal (e.g.
diatoms or siliceous sponges) may be an important
source of microquartz coatings on quartz grains at
greater depth.
Carbonate cements in both mudstones and
sandstones are mostly due to dissolution and
reprecipitation of biogenic carbonate or early aragonite cement. There are usually no other major sources
of carbonate cement. In the sulphate-reducing zone,
carbonate concretions form, often with a negative δ
13 C
due to the CO 2 produced during sulphate reduction.
Carbonate concretions in cores may be mistaken for
continuous carbonate layers but it is possible to
4 Sandstones and Sandstone Reservoirs
123
(UO
2+
) and precipitates as reduced uranium oxide
(UO 2 ) on organic matter in the water column and
below the redox boundary. There is thus a strong concentration gradient transporting uranium from the seawater down into the sediments. The adsorption of
uranium onto organic matter settling on the seafloor
coupled with restricted ventilation of the water above
the sediment at the time of deposition, makes source
rocks like the Kimmeridge shale strongly enriched in
uranium, causing peaks on the gamma ray well log curve.
Iron and manganese may be transported upwards
through the sediments in the reduced state by diffusion
and then precipitate on the seafloor as oxides because of
their reduced solubility in the oxidised state. Iron may
also be precipitated below the redox boundary as iron
sulphides or iron carbonate (siderite), though iron
carbonates will not form during sulphate reduction.
This is because all the iron will be precipitated as
sulphide, which has a much lower solubility than siderite. Manganese is not precipitated as sulphides because
of the high solubility of Mn-sulphides, but may be
precipitated as Mn-carbonate in the reduced zone.
At a depth where there is practically no more free
dissolved oxygen in the porewater, sulphates can be
used by sulphate-reducing bacteria. The reduction of
sulphates produces sulphides such as pyrite.
The composition of clastic sediments is modified
by the addition of new components produced locally
within the basin:
(1) Biogenic carbonates and silica.
(2) Authigenic minerals precipitating near the seabed
such as carbonates, phosphates, glauconite,
chamosite, sulphides and iron and manganese
minerals.
(3) Meteoric water-flushing causing leaching of feldspar and mica and precipitation of kaolinite
beneath the seafloor.
4.5
Importance of Biogenic Activity
Bioturbation plays an important role in changing the
textural composition of the sediments after deposition.
The burrowing organisms eat mud and thereby oxidise
organic matter and physically destroy the primary lamination. Sediments overturned by bioturbation become
more exposed to oxidation at the sea or lake bottom.
Bioturbation may reduce the porosity and permeability
of sandy laminae by mixing clay with clean sand.
Bioturbation will also destroy thin clay laminae,
which may significantly increase the vertical permeability and this may be very significant for reservoir
quality. Undisturbed primary lamination may be evidence of rather rapid sedimentation giving little time
for a burrowing bottom fauna to become established,
or alternatively indicate strongly reducing conditions
restricting the fauna. Black shales usually have well
preserved lamination due to lack of burrowing
organisms. The presence or absence of burrowing
also influences the physical properties, particularly
the difference in velocity and resistivity parallel and
vertical to bedding (anisotropy) and this may be
important for geophysical modelling.
Burrowing worms produce faecal material which
may develop into smectite-rich clays, which in turn
may develop into chlorite coatings, thus improving
reservoir quality. Early diagenetic formation of
coatings on quartz grains is extremely important due
to its role in preserving porosity at greater depth.
Most clastic depositional environments have some
organisms producing organic matter which, at least in
part, is incorporated within the sediments. Both
sandstones and mudstones nearly always contain significant amounts of biogenic material from calcareous,
and sometimes also siliceous, organisms and this may
later be an important source of carbonate and silica
cement at greater burial depth.
Marine organisms composed of aragonite dissolve
during relatively shallow burial and calcite
precipitates either as replacements within the fossils
(neomorphism) or as cement in pore space between the
grains.
Carbonate cement in sandstones may form layers or
concretions and in most cases is derived from biogenic
carbonate, particularly from organisms composed of
aragonite. Siliceous organisms composed of opal (e.g.
diatoms or siliceous sponges) may be an important
source of microquartz coatings on quartz grains at
greater depth.
Carbonate cements in both mudstones and
sandstones are mostly due to dissolution and
reprecipitation of biogenic carbonate or early aragonite cement. There are usually no other major sources
of carbonate cement. In the sulphate-reducing zone,
carbonate concretions form, often with a negative δ
13 C
due to the CO 2 produced during sulphate reduction.
Carbonate concretions in cores may be mistaken for
continuous carbonate layers but it is possible to
4 Sandstones and Sandstone Reservoirs
123