11.6 Summary
In subsiding sedimentary basins, mechanical compaction caused by the increase of effective vertical
stresses is the dominating compaction process and
has the greatest influence on the sediment’s porosity
and hydro-mechanical properties down to depths of
2–2.5 km. At greater depths where temperatures are
higher (>70
C), it is mainly the chemical compaction
that contributes to the volume change and to the
hydro-mechanical properties due to the effects of dissolution, precipitation and cementation. Carbonate
rocks, though, may undergo chemical compaction at
shallower depths.
The magnitude and distribution of stresses in sedimentary basins are important in relation to petroleum
exploration, production and reservoir management.
Knowledge of in situ stress is also important in connection with drilling, particularly during deviation and
horizontal drilling. The magnitude and orientation of
stresses affect the propagation and interpretation of
seismic signals, particularly the S-waves, through sedimentary rocks.
The total vertical stress (σ v ) of a rock sequence is
carried partly by transmission of stress in the solid
grain framework (effective stress σ
0
v ) and partly by
the pressure in the fluid phase (porewater or
petroleum).
Determination of effective stresses depends on reliable estimates of the fluid pore pressure which often is in
excess of hydrostatic (overpressure). There exist semiempirical relationships for the ratio between horizontal
and vertical effective stresses, but reliable estimates of
horizontal stresses depend on field measurements like
hydraulic fracturing tests. It is common practice in the
petroleum industry to use a simplified procedure (leakoff tests) to determine the magnitude and orientation of
the minimum horizontal stress.
The virgin (in situ) distribution of stresses in sedimentary basins is the result of both mechanical and
chemical compaction, usually over geological time.
Changes in stresses during petroleum production from
a reservoir are much more short term and are mainly
mechanical. In a carbonate reservoir the chemical processes may be so fast that also chemical compaction
may become significant at that time scale.
The effective stresses in sand may cause grain-tograin contact stresses which are so large that compression (compaction) may occur due to crushing and
fracturing of grains. This is more pronounced in
coarse-grained rather than fine-grained sands and
may account for a significant component of the porosity reduction. After the grain crushing and permanent
collapse deformations have occurred, the grain size is
reduced and the reservoir regains stiffness. With
increasing depth such grain crushing is less likely
due to increased cementation of the grain structure
caused by chemical processes. In most sandstone
reservoirs quartz cementation starting at 2–2.5 km
(70–80
C) will stabilise the grain framework and prevent further mechanical compaction. Sandstone
reservoirs with a critical content of quartz cement
(>2–3%) will therefore experience very little compaction even if the effective stress is increased during
production. Similar compaction by grain breakage
may also occur for high effective stresses in a reservoir
where the framework of the sedimentary rock is very
porous with little cement (e.g. the chalk in the Ekofisk
reservoir, North Sea). Here this led to very large reservoir compaction and subsequent seafloor subsidence
(c. 10 m).
Smectitic clays are characterised by very high V p =V s
ratios when compared with other clays (Fig. 11.9c).
This implies that in a sequence of mudstones, smectitic
clays will stand out with very different characteristics
compared with kaolinite and probably also illite-rich
sequences, which have much lower velocities and
V p =V s ratios. At temperatures above 70–80
C smectite
will no longer be stable and mudstones will be more
influenced by chemical compaction and cementation. In
cold basins, however, mechanical compaction can be
dominant down to 4–5 km burial depth. Every mudstone has a unique compaction curve which will depend
on a number of factors such as mineralogy, grain size,
pore fluids, pore pressure, pore aspect ratio, etc.
Well log data from well sorted Jurassic sandstone
like the Etive Fm in the North Sea basin show that
natural compaction curves agree well with experimental mechanical compaction in the laboratory (Fig 4.16).
The physical properties of silt and clay mixtures
depend strongly on both the clay mineralogy and the
content of silt and sand. (Manzar et al. 2010). This
includes porosity and velocity and also anisotrophy as
measured in the laboratory. The properties of
mudstones are also influenced by carbonate and silica
cement.
The effect of time on mechanical compaction will
always be difficult to evaluate in the laboratory but
11 Introduction to Geomechanics: Stress and Strain in Sedimentary Basins
317
In subsiding sedimentary basins, mechanical compaction caused by the increase of effective vertical
stresses is the dominating compaction process and
has the greatest influence on the sediment’s porosity
and hydro-mechanical properties down to depths of
2–2.5 km. At greater depths where temperatures are
higher (>70
C), it is mainly the chemical compaction
that contributes to the volume change and to the
hydro-mechanical properties due to the effects of dissolution, precipitation and cementation. Carbonate
rocks, though, may undergo chemical compaction at
shallower depths.
The magnitude and distribution of stresses in sedimentary basins are important in relation to petroleum
exploration, production and reservoir management.
Knowledge of in situ stress is also important in connection with drilling, particularly during deviation and
horizontal drilling. The magnitude and orientation of
stresses affect the propagation and interpretation of
seismic signals, particularly the S-waves, through sedimentary rocks.
The total vertical stress (σ v ) of a rock sequence is
carried partly by transmission of stress in the solid
grain framework (effective stress σ
0
v ) and partly by
the pressure in the fluid phase (porewater or
petroleum).
Determination of effective stresses depends on reliable estimates of the fluid pore pressure which often is in
excess of hydrostatic (overpressure). There exist semiempirical relationships for the ratio between horizontal
and vertical effective stresses, but reliable estimates of
horizontal stresses depend on field measurements like
hydraulic fracturing tests. It is common practice in the
petroleum industry to use a simplified procedure (leakoff tests) to determine the magnitude and orientation of
the minimum horizontal stress.
The virgin (in situ) distribution of stresses in sedimentary basins is the result of both mechanical and
chemical compaction, usually over geological time.
Changes in stresses during petroleum production from
a reservoir are much more short term and are mainly
mechanical. In a carbonate reservoir the chemical processes may be so fast that also chemical compaction
may become significant at that time scale.
The effective stresses in sand may cause grain-tograin contact stresses which are so large that compression (compaction) may occur due to crushing and
fracturing of grains. This is more pronounced in
coarse-grained rather than fine-grained sands and
may account for a significant component of the porosity reduction. After the grain crushing and permanent
collapse deformations have occurred, the grain size is
reduced and the reservoir regains stiffness. With
increasing depth such grain crushing is less likely
due to increased cementation of the grain structure
caused by chemical processes. In most sandstone
reservoirs quartz cementation starting at 2–2.5 km
(70–80
C) will stabilise the grain framework and prevent further mechanical compaction. Sandstone
reservoirs with a critical content of quartz cement
(>2–3%) will therefore experience very little compaction even if the effective stress is increased during
production. Similar compaction by grain breakage
may also occur for high effective stresses in a reservoir
where the framework of the sedimentary rock is very
porous with little cement (e.g. the chalk in the Ekofisk
reservoir, North Sea). Here this led to very large reservoir compaction and subsequent seafloor subsidence
(c. 10 m).
Smectitic clays are characterised by very high V p =V s
ratios when compared with other clays (Fig. 11.9c).
This implies that in a sequence of mudstones, smectitic
clays will stand out with very different characteristics
compared with kaolinite and probably also illite-rich
sequences, which have much lower velocities and
V p =V s ratios. At temperatures above 70–80
C smectite
will no longer be stable and mudstones will be more
influenced by chemical compaction and cementation. In
cold basins, however, mechanical compaction can be
dominant down to 4–5 km burial depth. Every mudstone has a unique compaction curve which will depend
on a number of factors such as mineralogy, grain size,
pore fluids, pore pressure, pore aspect ratio, etc.
Well log data from well sorted Jurassic sandstone
like the Etive Fm in the North Sea basin show that
natural compaction curves agree well with experimental mechanical compaction in the laboratory (Fig 4.16).
The physical properties of silt and clay mixtures
depend strongly on both the clay mineralogy and the
content of silt and sand. (Manzar et al. 2010). This
includes porosity and velocity and also anisotrophy as
measured in the laboratory. The properties of
mudstones are also influenced by carbonate and silica
cement.
The effect of time on mechanical compaction will
always be difficult to evaluate in the laboratory but
11 Introduction to Geomechanics: Stress and Strain in Sedimentary Basins
317