feldspar leaching could then be used as an indication
of the conductivity in the reservoir.
River water and groundwater are usually supersaturated with respect to quartz but undersaturated with
respect to amorphous silica. About 10–30 ppm
dissolved silica is common in groundwater and
shallow porewater while the solubility of quartz at
20
C is only 4–5 ppm, showing that quartz does not
form at low temperatures. In very alkaline water (i.e.
E. African lakes), quartz may precipitate at low
temperatures.
The early burial history of sandstones is not well
studied for the simple reason that cores are not normally taken at depths shallower than 1–1.5 km in
offshore basins, while onshore, erosion may have
removed most of the youngest sequence. Looking at
sandstone thin sections one can often get the impression that kaolinite is precipitated at a relatively late
stage because it is a pore-filling mineral that may
subsequently be surrounded by quartz cement. More
detailed textural studies and isotopic evidence indicate
that the kaolinite is formed early and may be enclosed
in quart cement. Pore-filling kaolinite must, however,
have been hanging to the pore wall and may have been
pushed aside by growing quartz cement.
The isotopic composition ðδ
18 OÞ of kaolinite
suggests that it precipitated at relatively low
temperatures, in the range 30–60
C, depending on
the assumptions made about the isotopic composition
of the porewater (Glasmann 1989). These
temperatures are little higher than should be expected
during meteoric water flushing and it is possible that
some of the kaolinite may be recrystallised at a higher
temperature, resetting the isotopic composition. Much
of what has been described or analysed as kaolinite has
turned out to be dickite, which has the same composition but often with thicker, more blocky crystals.
Studies have shown that dickite often replaces some
of the kaolinite when temperatures exceed 100
C.
Another possibility is that kaolinite may form diagenetically from other precursor minerals such as
gibbsite (Al(OH) 3 ) or amorphous aluminium
compounds. Kaolinite could then form without meteoric water flushing since such reactions do not produce
any other cations like K
+ which would have to be
removed. In the North Sea basin abundant authigenic
kaolinite is found in the shallowest reservoirs
(1.5–2 km) where there is very little or no quartz
cement in sandstones and this is the best evidence
that most of the kaolinite formed early at shallow
depth. The fact that kaolinite is much more abundant
in shallow marine and deltaic sandstones than those
deposited on deeper submarine slopes is also evidence
that kaolinite forms at shallow depth.
4.7
Consequences for Reservoir Quality
Meteoric water flushing dissolves feldspar and mica
and precipitates authigenic clay minerals, most commonly kaolinite. This dissolution produces holes
which are secondary pore spaces (secondary porosity)
but the precipitation of clay minerals like kaolinite
reduces the porosity, so that there is little net gain in
pore space. Authigenic kaolinite tends to occur as
pore-filling minerals and this reduces the permeability.
Clean well-sorted sand may increase its specific surface and pore size distribution due to the authigenic
kaolinite. The smaller pores (<0.005 mm) in between
the authigenic kaolinite crystals may be too small to be
filled with oil because of the high capillary entry
pressure necessary to infiltrate these pores. The total
water saturation will consequently then be higher in
the reservoir rock.
Authigenic kaolinite usually occurs as clusters and
is rarely pervasive through the sandstones, allowing
oil to flow between and around the most densely
kaolinite-cemented pores. However, if the kaolinite
is altered to illite at greater depth, the damage to the
reservoir may be much more severe, due to permeability reduction.
4.8
Mechanical Compaction of Loose
Sand
During the first part of its burial history (0–2 km) wellsorted sand is generally still loose if it is not carbonatecemented. Mechanical compaction may nevertheless
be very significant. Experimental compaction of loose
sand with an initial porosity 40–42% shows that,
depending on grain strength and grain size, the porosity may be reduced to 35–25% at stresses of
20–30 MPa corresponding to 2–3 km of burial for
normally pressured rocks (Fig. 4.5). The experimental
data show that well sorted coarse-grained sand is more
compressible than fine-grained sand (Chuhan et al.
4 Sandstones and Sandstone Reservoirs
127
of the conductivity in the reservoir.
River water and groundwater are usually supersaturated with respect to quartz but undersaturated with
respect to amorphous silica. About 10–30 ppm
dissolved silica is common in groundwater and
shallow porewater while the solubility of quartz at
20
C is only 4–5 ppm, showing that quartz does not
form at low temperatures. In very alkaline water (i.e.
E. African lakes), quartz may precipitate at low
temperatures.
The early burial history of sandstones is not well
studied for the simple reason that cores are not normally taken at depths shallower than 1–1.5 km in
offshore basins, while onshore, erosion may have
removed most of the youngest sequence. Looking at
sandstone thin sections one can often get the impression that kaolinite is precipitated at a relatively late
stage because it is a pore-filling mineral that may
subsequently be surrounded by quartz cement. More
detailed textural studies and isotopic evidence indicate
that the kaolinite is formed early and may be enclosed
in quart cement. Pore-filling kaolinite must, however,
have been hanging to the pore wall and may have been
pushed aside by growing quartz cement.
The isotopic composition ðδ
18 OÞ of kaolinite
suggests that it precipitated at relatively low
temperatures, in the range 30–60
C, depending on
the assumptions made about the isotopic composition
of the porewater (Glasmann 1989). These
temperatures are little higher than should be expected
during meteoric water flushing and it is possible that
some of the kaolinite may be recrystallised at a higher
temperature, resetting the isotopic composition. Much
of what has been described or analysed as kaolinite has
turned out to be dickite, which has the same composition but often with thicker, more blocky crystals.
Studies have shown that dickite often replaces some
of the kaolinite when temperatures exceed 100
C.
Another possibility is that kaolinite may form diagenetically from other precursor minerals such as
gibbsite (Al(OH) 3 ) or amorphous aluminium
compounds. Kaolinite could then form without meteoric water flushing since such reactions do not produce
any other cations like K
+ which would have to be
removed. In the North Sea basin abundant authigenic
kaolinite is found in the shallowest reservoirs
(1.5–2 km) where there is very little or no quartz
cement in sandstones and this is the best evidence
that most of the kaolinite formed early at shallow
depth. The fact that kaolinite is much more abundant
in shallow marine and deltaic sandstones than those
deposited on deeper submarine slopes is also evidence
that kaolinite forms at shallow depth.
4.7
Consequences for Reservoir Quality
Meteoric water flushing dissolves feldspar and mica
and precipitates authigenic clay minerals, most commonly kaolinite. This dissolution produces holes
which are secondary pore spaces (secondary porosity)
but the precipitation of clay minerals like kaolinite
reduces the porosity, so that there is little net gain in
pore space. Authigenic kaolinite tends to occur as
pore-filling minerals and this reduces the permeability.
Clean well-sorted sand may increase its specific surface and pore size distribution due to the authigenic
kaolinite. The smaller pores (<0.005 mm) in between
the authigenic kaolinite crystals may be too small to be
filled with oil because of the high capillary entry
pressure necessary to infiltrate these pores. The total
water saturation will consequently then be higher in
the reservoir rock.
Authigenic kaolinite usually occurs as clusters and
is rarely pervasive through the sandstones, allowing
oil to flow between and around the most densely
kaolinite-cemented pores. However, if the kaolinite
is altered to illite at greater depth, the damage to the
reservoir may be much more severe, due to permeability reduction.
4.8
Mechanical Compaction of Loose
Sand
During the first part of its burial history (0–2 km) wellsorted sand is generally still loose if it is not carbonatecemented. Mechanical compaction may nevertheless
be very significant. Experimental compaction of loose
sand with an initial porosity 40–42% shows that,
depending on grain strength and grain size, the porosity may be reduced to 35–25% at stresses of
20–30 MPa corresponding to 2–3 km of burial for
normally pressured rocks (Fig. 4.5). The experimental
data show that well sorted coarse-grained sand is more
compressible than fine-grained sand (Chuhan et al.
4 Sandstones and Sandstone Reservoirs
127