Chapter 11
Introduction to Geomechanics: Stress and Strain in
Sedimentary Basins
Knut Bjørlykke, Kaare Høeg and Nazmul Haque Mondol
At shallow depths in sedimentary basins there are soft
clays and loose silts and sands, unless there are
carbonates or carbonate cemented layers. At greater
depths they are transformed by diagenetic processes to
hard claystones, shales, siltstones and sandstones. Sedimentary rocks continuously undergo physical and
chemical changes as a function of burial depth, temperature and time, and important hydro-mechanical
parameters change during burial, erosion and uplift.
An understanding of these processes is important
in order to predict the magnitude and distribution
of sediment properties and stresses in the basin. The
in situ stress condition affects the rock response
(strain) to changes in the stress field due to drilling
and petroleum production.
Soil and rock mechanics (geomechanics) have
mainly been developed to solve engineering problems
in relation to landslides and surface and underground
construction. These are usually at very shallow depths
compared to that of a petroleum reservoir. We will
here focus on some aspects of geomechanics of particular relevance for the petroleum geologist.
Porosity loss (volumetric compaction) with time
due to increased effective stress is referred to in the
engineering literature as consolidation, while compaction at constant effective stress is usually called
secondary compression or creep. Compaction in
deep sedimentary basins has occurred over geologic
time scales at very low strain rates, and at higher
temperatures than shallow sediment compaction.
Therefore, in addition to mechanical compaction,
there are important effects of mineral grain dissolution, precipitation and cementation. This process is
called chemical compaction. Here the rate of compaction is controlled by the rate of chemical reactions
involving dissolution and precipitation of minerals.
Compaction determines the porosity, density and permeability of the sediments which are essential input
for basin modelling; petroleum reservoir quality is
dependent on the porosity and permeability. The processes of mechanical and chemical sediment compaction (diagenetic processes) determine the physical
properties and are also important for understanding
seismic velocity records and seismic attributes in sedimentary basins.
11.1 Subsurface Fluid Pressure and
Effective Stress Condition
A distinction should be made between total stress,
effective stress and fluid pore pressure. This is not
always done in technical reports and publications
related to petroleum geology.
11.1.1 Total and Effective Stress
In general, stress (σ) is defined as force per unit area.
The overburden weight of the sediment including the
weight of the fluid in the pore space produces a
K. Bjørlykke (*) K. Høeg
Department of Geosciences, University of Oslo, Oslo, Norway
e-mail: knut.bjorlykke@geo.uio.no; Kaare.Hoeg@geo.uio.no
N.H. Mondol
Department of Geosciences, University of Oslo, Oslo, Norway
Norwegian Geotechnical Institute (NGI), Oslo, Norway
e-mail: nazmul.haque@geo.uio.no
K. Bjørlykke (ed.), Petroleum Geoscience: From Sedimentary Environments to Rock Physics,
DOI 10.1007/978-3-642-34132-8_11, # Springer-Verlag Berlin Heidelberg 2015
301
Introduction to Geomechanics: Stress and Strain in
Sedimentary Basins
Knut Bjørlykke, Kaare Høeg and Nazmul Haque Mondol
At shallow depths in sedimentary basins there are soft
clays and loose silts and sands, unless there are
carbonates or carbonate cemented layers. At greater
depths they are transformed by diagenetic processes to
hard claystones, shales, siltstones and sandstones. Sedimentary rocks continuously undergo physical and
chemical changes as a function of burial depth, temperature and time, and important hydro-mechanical
parameters change during burial, erosion and uplift.
An understanding of these processes is important
in order to predict the magnitude and distribution
of sediment properties and stresses in the basin. The
in situ stress condition affects the rock response
(strain) to changes in the stress field due to drilling
and petroleum production.
Soil and rock mechanics (geomechanics) have
mainly been developed to solve engineering problems
in relation to landslides and surface and underground
construction. These are usually at very shallow depths
compared to that of a petroleum reservoir. We will
here focus on some aspects of geomechanics of particular relevance for the petroleum geologist.
Porosity loss (volumetric compaction) with time
due to increased effective stress is referred to in the
engineering literature as consolidation, while compaction at constant effective stress is usually called
secondary compression or creep. Compaction in
deep sedimentary basins has occurred over geologic
time scales at very low strain rates, and at higher
temperatures than shallow sediment compaction.
Therefore, in addition to mechanical compaction,
there are important effects of mineral grain dissolution, precipitation and cementation. This process is
called chemical compaction. Here the rate of compaction is controlled by the rate of chemical reactions
involving dissolution and precipitation of minerals.
Compaction determines the porosity, density and permeability of the sediments which are essential input
for basin modelling; petroleum reservoir quality is
dependent on the porosity and permeability. The processes of mechanical and chemical sediment compaction (diagenetic processes) determine the physical
properties and are also important for understanding
seismic velocity records and seismic attributes in sedimentary basins.
11.1 Subsurface Fluid Pressure and
Effective Stress Condition
A distinction should be made between total stress,
effective stress and fluid pore pressure. This is not
always done in technical reports and publications
related to petroleum geology.
11.1.1 Total and Effective Stress
In general, stress (σ) is defined as force per unit area.
The overburden weight of the sediment including the
weight of the fluid in the pore space produces a
K. Bjørlykke (*) K. Høeg
Department of Geosciences, University of Oslo, Oslo, Norway
e-mail: knut.bjorlykke@geo.uio.no; Kaare.Hoeg@geo.uio.no
N.H. Mondol
Department of Geosciences, University of Oslo, Oslo, Norway
Norwegian Geotechnical Institute (NGI), Oslo, Norway
e-mail: nazmul.haque@geo.uio.no
K. Bjørlykke (ed.), Petroleum Geoscience: From Sedimentary Environments to Rock Physics,
DOI 10.1007/978-3-642-34132-8_11, # Springer-Verlag Berlin Heidelberg 2015
301