tectonic deformation like faulting. During production
of reservoir rocks the stain rates may become high.
In sedimentary basins down to depths of 1.5–2 km
mechanical compaction caused by the increase of
effective vertical stresses is the dominating compaction process and has the greatest influence on the
sediment’s hydro-mechanical properties. However, at
greater depths where temperatures are higher
(>70–80
C), it is mainly chemical compaction that
contributes to the volume change and to the hydromechanical properties through the effects of dissolution, precipitation and cementation.
11.5.1 Sands and Sandstones
The effective stress of the overburden is transmitted
through a framework of load-bearing grains. These
grain-to-grain contact stresses may become very
much higher than the average effective stress. Not all
grains will be subjected to high stresses because they
may be shielded by other grains within the loadbearing grain framework. The stress from the overburden will thus be concentrated on these other grains,
which then may fracture. The small areas of grain
contact make the stress at these contacts very high,
even at moderate burial depth.
Natural sand grains of quartz and feldspar are
mostly blocky rather than spherical and have an irregular surface. This means that the area of contact is
likely to be very small even for the larger grains,
resulting in much higher contact stresses than for
smaller grains. If sand grains had been perfectly spherical the contact would be controlled by the elasticity,
and the stress would then be independent of the grain
size.
Experimental compaction of well sorted sand
reveals that coarse-grained sand aggregates are
subjected to significant grain fracturing and compaction at 20–30 MPa effective stresses while fine-grained
sand does not fracture, and compacts much less at the
same stress levels (Fig. 11.6) (Chuhan et al. 2002,
2003, Bjørlykke et al. 2004).
Well sorted sand (e.g. beach sand) with an intitial
porosity of 40–45% may compact mechanically to
30–38% porosity, depending on the stress level and
the grain size. Poorly sorted sand and sand with high
mud contents will compact much more at even lower
stresses.
With increasing compaction due to grain rearrangement or breakage, and cementation, the rock becomes
less porous, less compressible and stronger. This process increases both the number and area of grain
contacts.
Loose uncemented sands subjected to shear deformation may develop thin deformation bands. Such
shear bands may be composed of densely packed
grains where smaller silt-sized grains have been
packed between larger grains during the shearing. If
there is little clay the shear strength of the shear band
will exceed the shear strength of the matrix and the
shear deformation will shift laterally to an area where
there has been no strain (deformation). This is called
strain-hardening and results in a network of shear
bands which have only been subjected to small offsets.
Large displacements recorded on seismics may in
reality consist of a broad zone of deformation bands.
Shear deformation in sand may also result in grain
crushing. Experimental deformation of sand suggests,
however, that the effective stress normal to the shear
band must be at least 10 MPa for coarse sand to be
crushed. In the case of normal faults the horizontal
stress must have been 10 MPa and the vertical stress
20–25% higher, which corresponds to burial depths of
1–1.5 km.
Precipitation of quartz or other cements increases
the stiffness of sand and reduces its compressibility,
transforming loose sand into indurated sandstones.
Only relatively small amounts of quartz cement, probably only 2–4%, are required to effectively stop the
mechanical compaction that is due to rearrangement of
grains. As a result the velocity, and particularly the
shear velocity, will increase sharply for a modest
reduction in porosity.
Sandstones may behave as if they were
overconsolidated due to cementation (chemical compaction) and will only compact following the stressstrain curve for overconsolidated rocks. We must distinguish between overconsolidation due to previously
higher effective stresses and “pseudo overconsolidation” caused by cementation and chemical compaction. This because the highest effective stress must
be estimated from the burial curve and the pore pressure while the chemical compaction may be rather
insensitive to changes in stress.
Chemical compaction involving the dissolution and
precipitation of quartz is controlled mainly by temperature because the rate of quartz cementation seems to
312
K. Bjørlykke et al.
of reservoir rocks the stain rates may become high.
In sedimentary basins down to depths of 1.5–2 km
mechanical compaction caused by the increase of
effective vertical stresses is the dominating compaction process and has the greatest influence on the
sediment’s hydro-mechanical properties. However, at
greater depths where temperatures are higher
(>70–80
C), it is mainly chemical compaction that
contributes to the volume change and to the hydromechanical properties through the effects of dissolution, precipitation and cementation.
11.5.1 Sands and Sandstones
The effective stress of the overburden is transmitted
through a framework of load-bearing grains. These
grain-to-grain contact stresses may become very
much higher than the average effective stress. Not all
grains will be subjected to high stresses because they
may be shielded by other grains within the loadbearing grain framework. The stress from the overburden will thus be concentrated on these other grains,
which then may fracture. The small areas of grain
contact make the stress at these contacts very high,
even at moderate burial depth.
Natural sand grains of quartz and feldspar are
mostly blocky rather than spherical and have an irregular surface. This means that the area of contact is
likely to be very small even for the larger grains,
resulting in much higher contact stresses than for
smaller grains. If sand grains had been perfectly spherical the contact would be controlled by the elasticity,
and the stress would then be independent of the grain
size.
Experimental compaction of well sorted sand
reveals that coarse-grained sand aggregates are
subjected to significant grain fracturing and compaction at 20–30 MPa effective stresses while fine-grained
sand does not fracture, and compacts much less at the
same stress levels (Fig. 11.6) (Chuhan et al. 2002,
2003, Bjørlykke et al. 2004).
Well sorted sand (e.g. beach sand) with an intitial
porosity of 40–45% may compact mechanically to
30–38% porosity, depending on the stress level and
the grain size. Poorly sorted sand and sand with high
mud contents will compact much more at even lower
stresses.
With increasing compaction due to grain rearrangement or breakage, and cementation, the rock becomes
less porous, less compressible and stronger. This process increases both the number and area of grain
contacts.
Loose uncemented sands subjected to shear deformation may develop thin deformation bands. Such
shear bands may be composed of densely packed
grains where smaller silt-sized grains have been
packed between larger grains during the shearing. If
there is little clay the shear strength of the shear band
will exceed the shear strength of the matrix and the
shear deformation will shift laterally to an area where
there has been no strain (deformation). This is called
strain-hardening and results in a network of shear
bands which have only been subjected to small offsets.
Large displacements recorded on seismics may in
reality consist of a broad zone of deformation bands.
Shear deformation in sand may also result in grain
crushing. Experimental deformation of sand suggests,
however, that the effective stress normal to the shear
band must be at least 10 MPa for coarse sand to be
crushed. In the case of normal faults the horizontal
stress must have been 10 MPa and the vertical stress
20–25% higher, which corresponds to burial depths of
1–1.5 km.
Precipitation of quartz or other cements increases
the stiffness of sand and reduces its compressibility,
transforming loose sand into indurated sandstones.
Only relatively small amounts of quartz cement, probably only 2–4%, are required to effectively stop the
mechanical compaction that is due to rearrangement of
grains. As a result the velocity, and particularly the
shear velocity, will increase sharply for a modest
reduction in porosity.
Sandstones may behave as if they were
overconsolidated due to cementation (chemical compaction) and will only compact following the stressstrain curve for overconsolidated rocks. We must distinguish between overconsolidation due to previously
higher effective stresses and “pseudo overconsolidation” caused by cementation and chemical compaction. This because the highest effective stress must
be estimated from the burial curve and the pore pressure while the chemical compaction may be rather
insensitive to changes in stress.
Chemical compaction involving the dissolution and
precipitation of quartz is controlled mainly by temperature because the rate of quartz cementation seems to
312
K. Bjørlykke et al.