mudrocks and shales may vary greatly and have very
different physical properties, depending on the clay
mineral composition and on the content of silt and
sand. Mudstones and shales may have high contents
of silt and sand and also carbonate and may from
transitions into poorly sorted sandstones and impure
limestones.
The most common clay minerals are smectite, illite,
chlorite and kaolinite. When the silt and sand content
exceeds 40–50% there may be a grain-supported structure, and this marks the transition into clay-rich
siltstones and sandstones. A sediment of sand and silt
grains floating in a matrix of clay has for many purposes
the same properties as the clay fraction, but the density
is higher and so is the seismic velocity. This is because
the stiffness is determined by the load-bearing grains.
Poorly sorted clays like glacial clays compact readily at relatively low effective stresses because the clay
particles are packed in between the silt and sand
grains. Data on compaction of clay and mudstones
may be obtained from measuring the degree of compaction where the burial history and maximum effective stress can be estimated, or by experimental
compaction in the laboratory.
In fine-grained sediments like clays and mudstones
the total overburden stress is distributed over a very
large number of grain contacts and the stress per grain
contact may be quite low. Relatively small amounts of
minerals precipitated as cement between the primary
grains can then cause a very significant increase in
stiffness and seismic velocity even at shallow burial.
Most commonly this involves carbonate cement,
which makes soft clay grade into marls and calcareous
mudstones with higher bulk moduli. Quartz cementation requires higher temperatures (>80
C)
corresponding to 2–2.5 km in basins with normal
geothermal gradients.
11.5.2.2 Experimental Compaction of Clays
Experimental compaction of clays is difficult. It
requires very careful sample preparation, and compaction tests up to 50 MPa stress may take 5–6 weeks for
smectite-rich clays. This is because the permeability is
so low that it takes a long time for the excess water to
drain. Time is also required to allow for the slight
compaction at constant stress (creep) which may also
be referred to as secondary compaction.
Compaction of mud to mudstones and shales is the
result of natural processes during burial, usually over
several million years. We can determine the resultant
rock properties by analysing natural rock samples in
the laboratory. It is nevertheless still difficult to estimate the effective stress and temperatures to which
these rocks have been subjected. This is particularly
true in the case of samples exposed on land after
substantial uplift. Samples from offshore wells in subsiding basins are much better constrained with respect
to the burial history but representative mudstones are
rarely cored. Cuttings can be analysed mineralogically, but it is difficult to test their mechanical
properties without reconstituting the samples.
There is often a need to predict the compaction of
sediments (soils) including clays in an engineering
context and they are then tested in the laboratory to
measure the strain (compaction) as a function of effective stress and other soil and rock mechanical
parameters. To simulate natural burial in sedimentary
basins we use rather high stresses, up to 50 MPa or
more, corresponding to 4–5 km of overburden. In most
cases, though, chemical compaction becomes dominant at shallower depth.
By testing artificial mixtures of clays we can measure their physical properties as a function of clay
mineralogy and silt and sand content. Kaolinitic
clays compact much more readily than smectite,
which is the most fine-grained clay mineral and has
very low compressibility (Mondol et al. 2007). At
about 20 MPa effective stress, corresponding to
about 2 km of burial, pure smectite has more than
40% porosity while kaolinite has less than 20%
(Fig. 11.7). Even at 50 MPa corresponding to
4–5 km burial depth at hydrostatic pressure the porosity may still exceed 40%. Clay minerals compact more
when wet than dry (Mondol et al. 2007). This is
probably because the friction between the grains is
higher in dry clays (Fig. 11.7).
In the case of smectite the large surface area and the
water which is bound to these clay surfaces make it
difficult to define the proportion of free water, and
hence determine the exact porosity, which will also
depend on the composition (electrolytic strength) of
the porewater. Clay minerals tend to have a negative
charge, causing repulsion between the clay particles.
However, the negative charges will adsorb cations like
Na
+ and K
+
, thus neutralising this repulsion. This is
what causes flocculation when river-borne clays enter
the sea. Marine clays therefore have a more stable clay
mineral fabric and higher shear strength than
314
K. Bjørlykke et al.
different physical properties, depending on the clay
mineral composition and on the content of silt and
sand. Mudstones and shales may have high contents
of silt and sand and also carbonate and may from
transitions into poorly sorted sandstones and impure
limestones.
The most common clay minerals are smectite, illite,
chlorite and kaolinite. When the silt and sand content
exceeds 40–50% there may be a grain-supported structure, and this marks the transition into clay-rich
siltstones and sandstones. A sediment of sand and silt
grains floating in a matrix of clay has for many purposes
the same properties as the clay fraction, but the density
is higher and so is the seismic velocity. This is because
the stiffness is determined by the load-bearing grains.
Poorly sorted clays like glacial clays compact readily at relatively low effective stresses because the clay
particles are packed in between the silt and sand
grains. Data on compaction of clay and mudstones
may be obtained from measuring the degree of compaction where the burial history and maximum effective stress can be estimated, or by experimental
compaction in the laboratory.
In fine-grained sediments like clays and mudstones
the total overburden stress is distributed over a very
large number of grain contacts and the stress per grain
contact may be quite low. Relatively small amounts of
minerals precipitated as cement between the primary
grains can then cause a very significant increase in
stiffness and seismic velocity even at shallow burial.
Most commonly this involves carbonate cement,
which makes soft clay grade into marls and calcareous
mudstones with higher bulk moduli. Quartz cementation requires higher temperatures (>80
C)
corresponding to 2–2.5 km in basins with normal
geothermal gradients.
11.5.2.2 Experimental Compaction of Clays
Experimental compaction of clays is difficult. It
requires very careful sample preparation, and compaction tests up to 50 MPa stress may take 5–6 weeks for
smectite-rich clays. This is because the permeability is
so low that it takes a long time for the excess water to
drain. Time is also required to allow for the slight
compaction at constant stress (creep) which may also
be referred to as secondary compaction.
Compaction of mud to mudstones and shales is the
result of natural processes during burial, usually over
several million years. We can determine the resultant
rock properties by analysing natural rock samples in
the laboratory. It is nevertheless still difficult to estimate the effective stress and temperatures to which
these rocks have been subjected. This is particularly
true in the case of samples exposed on land after
substantial uplift. Samples from offshore wells in subsiding basins are much better constrained with respect
to the burial history but representative mudstones are
rarely cored. Cuttings can be analysed mineralogically, but it is difficult to test their mechanical
properties without reconstituting the samples.
There is often a need to predict the compaction of
sediments (soils) including clays in an engineering
context and they are then tested in the laboratory to
measure the strain (compaction) as a function of effective stress and other soil and rock mechanical
parameters. To simulate natural burial in sedimentary
basins we use rather high stresses, up to 50 MPa or
more, corresponding to 4–5 km of overburden. In most
cases, though, chemical compaction becomes dominant at shallower depth.
By testing artificial mixtures of clays we can measure their physical properties as a function of clay
mineralogy and silt and sand content. Kaolinitic
clays compact much more readily than smectite,
which is the most fine-grained clay mineral and has
very low compressibility (Mondol et al. 2007). At
about 20 MPa effective stress, corresponding to
about 2 km of burial, pure smectite has more than
40% porosity while kaolinite has less than 20%
(Fig. 11.7). Even at 50 MPa corresponding to
4–5 km burial depth at hydrostatic pressure the porosity may still exceed 40%. Clay minerals compact more
when wet than dry (Mondol et al. 2007). This is
probably because the friction between the grains is
higher in dry clays (Fig. 11.7).
In the case of smectite the large surface area and the
water which is bound to these clay surfaces make it
difficult to define the proportion of free water, and
hence determine the exact porosity, which will also
depend on the composition (electrolytic strength) of
the porewater. Clay minerals tend to have a negative
charge, causing repulsion between the clay particles.
However, the negative charges will adsorb cations like
Na
+ and K
+
, thus neutralising this repulsion. This is
what causes flocculation when river-borne clays enter
the sea. Marine clays therefore have a more stable clay
mineral fabric and higher shear strength than
314
K. Bjørlykke et al.