Chapter 13
Compaction of Sedimentary Rocks: Shales, Sandstones
and Carbonates
Knut Bjørlykke
The physical properties of sedimentary rocks change
continuously during burial as a response to increasing
stress and temperature; they also change to a certain
extent during uplift and cooling. There is an overall
drive towards lower porosity with depth, which
increases the sediment density and sonic/seismic
velocity.
Increased effective stress from the overburden or
from tectonic stress will always cause some mechanical sediment compaction (strain), expressed by the
compressibility and the bulk modulus (see Chap. 11)
which can be measured in the laboratory. During
mechanical compaction the solids, mainly minerals,
remain nearly constant so that the reduction in bulk
volume is equal to the porosity loss. Sediments may
also contain solid amorphous phases which are chemically unstable (e.g. opal A) The transformation of
solid kerogen to fluids (petroleum) and dehydration
of minerals involves however some changes in the
volume of solids.
Chemically, the mineral assemblage will be driven
towards higher thermodynamic stability (Lower Gibbs
Free Energy) (Fig. 13.1b). These reactions involve the
dissolution of minerals or mineral assemblages that
are unstable, and precipitation of minerals that are
thermodynamically more stable with respect to the
composition of the porewater and the temperature.
Higher temperatures will favour minerals with lower
water content, for example by dissolving smectite and
kaolinite and precipitating illite (see Chap. 4). The
rates of these reactions are controlled by the kinetic
parameters such as the activation energy and thereby
the temperature.
The main lithologies in sedimentary basins are
shales, sandstones and carbonates, and they respond
very differently to increased stress and temperature
during burial.
This is important both for basin modelling and in
seismic data interpretation.
There are no precise definitions for mud, mudrock
and shale. The term mud is used to describe fine-grained
sediment with a relatively high content of clay-sized
particles, chiefly clay minerals. Carbonate mud will be
discussed separately, under carbonate compaction.
The compaction (porosity loss) as a function of
burial depth varies greatly because each primary
lithology has a different compaction curve. While
porosity may increase with depth through an interval
due to changes in lithology, for each individual lithology the porosity will nearly always be reduced with
depth (Fig. 13.1a).
13.1 Compaction of Mudrocks
and Shales
Mudrocks and shales are often treated as one lithology
in connection with basin analyses, seismic interpretation and well log analyses, but in reality they span a
wide range of properties determined by the diversity
of mineral composition and grain-size distribution.
Furthermore, the composition of mudstones and shales
changes during progressive burial due to diagenesis,
which includes both mechanical and chemical
compaction.
K. Bjørlykke (*)
Department of Geosciences, University of Oslo, Oslo, Norway
e-mail: knut.bjorlykke@geo.uio.no
K. Bjørlykke (ed.), Petroleum Geoscience: From Sedimentary Environments to Rock Physics,
DOI 10.1007/978-3-642-34132-8_13, # Springer-Verlag Berlin Heidelberg 2015
351
Compaction of Sedimentary Rocks: Shales, Sandstones
and Carbonates
Knut Bjørlykke
The physical properties of sedimentary rocks change
continuously during burial as a response to increasing
stress and temperature; they also change to a certain
extent during uplift and cooling. There is an overall
drive towards lower porosity with depth, which
increases the sediment density and sonic/seismic
velocity.
Increased effective stress from the overburden or
from tectonic stress will always cause some mechanical sediment compaction (strain), expressed by the
compressibility and the bulk modulus (see Chap. 11)
which can be measured in the laboratory. During
mechanical compaction the solids, mainly minerals,
remain nearly constant so that the reduction in bulk
volume is equal to the porosity loss. Sediments may
also contain solid amorphous phases which are chemically unstable (e.g. opal A) The transformation of
solid kerogen to fluids (petroleum) and dehydration
of minerals involves however some changes in the
volume of solids.
Chemically, the mineral assemblage will be driven
towards higher thermodynamic stability (Lower Gibbs
Free Energy) (Fig. 13.1b). These reactions involve the
dissolution of minerals or mineral assemblages that
are unstable, and precipitation of minerals that are
thermodynamically more stable with respect to the
composition of the porewater and the temperature.
Higher temperatures will favour minerals with lower
water content, for example by dissolving smectite and
kaolinite and precipitating illite (see Chap. 4). The
rates of these reactions are controlled by the kinetic
parameters such as the activation energy and thereby
the temperature.
The main lithologies in sedimentary basins are
shales, sandstones and carbonates, and they respond
very differently to increased stress and temperature
during burial.
This is important both for basin modelling and in
seismic data interpretation.
There are no precise definitions for mud, mudrock
and shale. The term mud is used to describe fine-grained
sediment with a relatively high content of clay-sized
particles, chiefly clay minerals. Carbonate mud will be
discussed separately, under carbonate compaction.
The compaction (porosity loss) as a function of
burial depth varies greatly because each primary
lithology has a different compaction curve. While
porosity may increase with depth through an interval
due to changes in lithology, for each individual lithology the porosity will nearly always be reduced with
depth (Fig. 13.1a).
13.1 Compaction of Mudrocks
and Shales
Mudrocks and shales are often treated as one lithology
in connection with basin analyses, seismic interpretation and well log analyses, but in reality they span a
wide range of properties determined by the diversity
of mineral composition and grain-size distribution.
Furthermore, the composition of mudstones and shales
changes during progressive burial due to diagenesis,
which includes both mechanical and chemical
compaction.
K. Bjørlykke (*)
Department of Geosciences, University of Oslo, Oslo, Norway
e-mail: knut.bjorlykke@geo.uio.no
K. Bjørlykke (ed.), Petroleum Geoscience: From Sedimentary Environments to Rock Physics,
DOI 10.1007/978-3-642-34132-8_13, # Springer-Verlag Berlin Heidelberg 2015
351