(Chuhan et al. 2003) (Fig 4.5d). During faulting coarse
grained sand may produce grain crushing at about
1 km depth (10 MP effective normal stress). Fine
grained sand requires higher stresses (>15 MPa).
Poorly sorted sandstones and sand with rock (lithic)
fragments lose much of their porosity at rather shallow
(1–2 km) depth. Quartz cementation strengthens the
rocks at a faster rate than the increase in vertical stress
from the overburden. Only 2–4% quartz cement will in
most cases effectively shut down mechanical compaction in sandstones, so that further compaction is
mainly chemically controlled by the rate of mineral
dissolution and precipitation (Fig. 4.6). The mechanical compaction is important because it determines the
intergranular volume (IGV) which is the porosity prior
to quartz cementation. This is typically 25–30% or
even more for well sorted quartz-rich sandstones.
Sand with even relatively small amounts of detrital
clay will compact more than clean sand. Lithic (rock)
fragments also compact more readily (Pittman and
Larese 1989) and this is reflected in their lower IGV.
Generally, in relatively well-sorted quartz arenites
and feldspathic sandstones the porosity is to a large
extent destroyed by quartz cementation (Fig. 4.7a).
The amount of quartz cement is mainly a function
of the grain surfaces available for quartz precipitation
and the time-temperature integral (Walderhaug 1994).
High geothermal gradients and slow subsidence rates
will therefore tend to increase the amount of quartz
cement at a specific depth.
In several of the Upper Jurassic reservoir rocks
from the North Sea, amorphous silica from Rhaxella
sponges dissolved to produce high supersaturation of
silica relative to quartz. This caused precipitation of a
coating of minute quartz crystals on the surface of
clastic quartz grains (Fig. 4.7b, c). This coating of
microquartz has prevented or retarded the precipitation of later quartz cement and is the main reason for
high porosity and good reservoir quality at great depth
(up to 5 km) in these reservoir rocks. The microquartz
precipitated at low temperature (60–65
C), when the
porewater was highly supersaturated with respect to
quartz through the dissolution of opal A or opal CT
and while the quartz growth rate was low. At higher
temperatures when unstable silicates like opal A, opal
CT and smectite have dissolved, the porewater will
only be slightly supersaturated with respect to quartz,
insufficient to precipitate quartz on the microquartz
surfaces which requires higher supersaturation than
normal quartz (Aase et al. 1996). Rhaxella had not
evolved before the Upper Jurassic and so older
sandstones like the Middle Jurassic Brent sandstones
do not have this type of microquartz.
At temperatures above about 100–120
C some of
what we have called kaolinite has recystallised to
dickite, which has the same chemical composition.
Dickite often occurs as slightly thicker crystals and
can also be distinguished from kaolinite on XRD
scans. Kaolin or kandite may be used as a common
name for these clay minerals.
Carbonate-cemented intervals may be effective
barriers to fluid flow. This can be detrimental to the
reservoir quality, though in some cases may be useful
if they are laterally extensive. Such low permeability
layers may then prevent the flow of gas from below the
oil/water contact and from above the gas/oil contact
into an oil-producing well. This is called coning.
The replacement of K-feldspar or plagioclase by
albite is often observed in sandstone buried to about
3 km or more and is referred to as albitisation. Albite
becomes more stable than K-feldspar because Na
+ is
normally the dominant cation in the porewater while
the potassium concentration is reduced due to removal
by the clay mineral reactions. Albitisation is normally
observed as a partial replacement of K-feldspar or
plagioclase grains, which does not change the reservoir properties very much. Albitisation of plagioclase
will, however, release some Ca
2+ that may then precipitate as calcite (Boles 1982), though the amount is
rather limited.
Smectite may be present in some muddy, and particularly volcanic, sandstones which have been flushed
with limited amounts of meteoric water. At
temperatures from about 70 to 80
C smectite dissolves
and is replaced by mixed-layer minerals and illite.
Sandstones containing smectite normally have poor
reservoir quality.
Dissolution of smectite and precipitation of illite
and quartz will cause a sharp increase in the seismic
velocity and rock density and this mineral transition
may therefore show up as a horizontal reflector on
seismic and could be mistaken for a fluid contact i.e.
gas/oil or oil/water contact (Thyberg et al. 2010).
130
K. Bjørlykke and J. Jahren
grained sand may produce grain crushing at about
1 km depth (10 MP effective normal stress). Fine
grained sand requires higher stresses (>15 MPa).
Poorly sorted sandstones and sand with rock (lithic)
fragments lose much of their porosity at rather shallow
(1–2 km) depth. Quartz cementation strengthens the
rocks at a faster rate than the increase in vertical stress
from the overburden. Only 2–4% quartz cement will in
most cases effectively shut down mechanical compaction in sandstones, so that further compaction is
mainly chemically controlled by the rate of mineral
dissolution and precipitation (Fig. 4.6). The mechanical compaction is important because it determines the
intergranular volume (IGV) which is the porosity prior
to quartz cementation. This is typically 25–30% or
even more for well sorted quartz-rich sandstones.
Sand with even relatively small amounts of detrital
clay will compact more than clean sand. Lithic (rock)
fragments also compact more readily (Pittman and
Larese 1989) and this is reflected in their lower IGV.
Generally, in relatively well-sorted quartz arenites
and feldspathic sandstones the porosity is to a large
extent destroyed by quartz cementation (Fig. 4.7a).
The amount of quartz cement is mainly a function
of the grain surfaces available for quartz precipitation
and the time-temperature integral (Walderhaug 1994).
High geothermal gradients and slow subsidence rates
will therefore tend to increase the amount of quartz
cement at a specific depth.
In several of the Upper Jurassic reservoir rocks
from the North Sea, amorphous silica from Rhaxella
sponges dissolved to produce high supersaturation of
silica relative to quartz. This caused precipitation of a
coating of minute quartz crystals on the surface of
clastic quartz grains (Fig. 4.7b, c). This coating of
microquartz has prevented or retarded the precipitation of later quartz cement and is the main reason for
high porosity and good reservoir quality at great depth
(up to 5 km) in these reservoir rocks. The microquartz
precipitated at low temperature (60–65
C), when the
porewater was highly supersaturated with respect to
quartz through the dissolution of opal A or opal CT
and while the quartz growth rate was low. At higher
temperatures when unstable silicates like opal A, opal
CT and smectite have dissolved, the porewater will
only be slightly supersaturated with respect to quartz,
insufficient to precipitate quartz on the microquartz
surfaces which requires higher supersaturation than
normal quartz (Aase et al. 1996). Rhaxella had not
evolved before the Upper Jurassic and so older
sandstones like the Middle Jurassic Brent sandstones
do not have this type of microquartz.
At temperatures above about 100–120
C some of
what we have called kaolinite has recystallised to
dickite, which has the same chemical composition.
Dickite often occurs as slightly thicker crystals and
can also be distinguished from kaolinite on XRD
scans. Kaolin or kandite may be used as a common
name for these clay minerals.
Carbonate-cemented intervals may be effective
barriers to fluid flow. This can be detrimental to the
reservoir quality, though in some cases may be useful
if they are laterally extensive. Such low permeability
layers may then prevent the flow of gas from below the
oil/water contact and from above the gas/oil contact
into an oil-producing well. This is called coning.
The replacement of K-feldspar or plagioclase by
albite is often observed in sandstone buried to about
3 km or more and is referred to as albitisation. Albite
becomes more stable than K-feldspar because Na
+ is
normally the dominant cation in the porewater while
the potassium concentration is reduced due to removal
by the clay mineral reactions. Albitisation is normally
observed as a partial replacement of K-feldspar or
plagioclase grains, which does not change the reservoir properties very much. Albitisation of plagioclase
will, however, release some Ca
2+ that may then precipitate as calcite (Boles 1982), though the amount is
rather limited.
Smectite may be present in some muddy, and particularly volcanic, sandstones which have been flushed
with limited amounts of meteoric water. At
temperatures from about 70 to 80
C smectite dissolves
and is replaced by mixed-layer minerals and illite.
Sandstones containing smectite normally have poor
reservoir quality.
Dissolution of smectite and precipitation of illite
and quartz will cause a sharp increase in the seismic
velocity and rock density and this mineral transition
may therefore show up as a horizontal reflector on
seismic and could be mistaken for a fluid contact i.e.
gas/oil or oil/water contact (Thyberg et al. 2010).
130
K. Bjørlykke and J. Jahren