Chapter 3
Sedimentary Geochemistry
How Sediments are Produced
Knut Bjørlykke
The composition and physical properties of sedimentary rocks are to a large extent controlled by chemical
processes during weathering, transport and also during
burial (diagenesis). We can not avoid studying chemical processes if we want to understand the physical
properties of sedimentary rocks. Sediment transport
and distribution of sedimentary facies is strongly
influenced by the sediment composition such as the
content of sand/clay ratio and the clay mineralogy.
The primary composition is the starting point for the
diagenetic processes during burial.
We will now consider some simple chemical and
mineralogical concepts that are relevant to sedimentological processes.
Clastic sediments are derived from source rocks
that have been disintegrated by erosion and
weathering. The source rock may be igneous, metamorphic or sedimentary. The compositions of clastic
sediments are therefore the product of the rock types
within the drainage basin (provenance), of climate and
relief. The dissolved portion flows out into the sea or
lakes, where it is precipitated as biological or chemical
sediments. Weathering and abrasion of the grains
continues during transport and sediments may be
deposited and eroded several times before they are
finally stored in a sedimentary basin.
After deposition sediments are also being subjected
to mineral dissolution and precipitation of new
minerals as a part of the diagenetic processes. For the
most part we are concerned with reactions between
minerals and water at relatively low temperatures. At
temperatures above 200–250
C these processes are
referred to as metamorphism which is principally similar in that unstable minerals dissolve and minerals
which are thermodynamically more stable at certain
temperatures and pressures precipitate.
At low temperatures, however, unstable minerals
and also amorphous phases may be preserved for a
long time and there may be many metastable phases.
Many of the reactions associated with the dissolution and precipitation of minerals proceed so slowly
that only after an extremely long period can they
achieve a degree of equilibrium.
Reactions will always be controlled by thermodynamics and will be driven towards more stable phases.
The kinetic reaction rate is controlled by temperature
and distance from equilibrium. The rate is zero at
equilibrium.
Silicate reactions are very slow at low temperature
and this makes it very difficult to study them in the
laboratory.
Biological processes often accompany the purely
chemical processes, adding to the complexity. Bacteria have been found to play an important role in both
the weathering and precipitation of minerals. Their
chief contribution is to increase reaction rates, particularly during weathering.
In this chapter we shall examine the processes
between water and sediments from a simple
physical-chemical viewpoint. A detailed treatment of
sediment geochemistry is however beyond the scope
of this book.
Water (H 2 O) consists of one oxygen atom linked to
two hydrogen atoms, with the H-O-H bonds forming
an angle of 105
(Fig. 3.1). The distance between the
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_3, # Springer-Verlag Berlin Heidelberg 2015
91
Sedimentary Geochemistry
How Sediments are Produced
Knut Bjørlykke
The composition and physical properties of sedimentary rocks are to a large extent controlled by chemical
processes during weathering, transport and also during
burial (diagenesis). We can not avoid studying chemical processes if we want to understand the physical
properties of sedimentary rocks. Sediment transport
and distribution of sedimentary facies is strongly
influenced by the sediment composition such as the
content of sand/clay ratio and the clay mineralogy.
The primary composition is the starting point for the
diagenetic processes during burial.
We will now consider some simple chemical and
mineralogical concepts that are relevant to sedimentological processes.
Clastic sediments are derived from source rocks
that have been disintegrated by erosion and
weathering. The source rock may be igneous, metamorphic or sedimentary. The compositions of clastic
sediments are therefore the product of the rock types
within the drainage basin (provenance), of climate and
relief. The dissolved portion flows out into the sea or
lakes, where it is precipitated as biological or chemical
sediments. Weathering and abrasion of the grains
continues during transport and sediments may be
deposited and eroded several times before they are
finally stored in a sedimentary basin.
After deposition sediments are also being subjected
to mineral dissolution and precipitation of new
minerals as a part of the diagenetic processes. For the
most part we are concerned with reactions between
minerals and water at relatively low temperatures. At
temperatures above 200–250
C these processes are
referred to as metamorphism which is principally similar in that unstable minerals dissolve and minerals
which are thermodynamically more stable at certain
temperatures and pressures precipitate.
At low temperatures, however, unstable minerals
and also amorphous phases may be preserved for a
long time and there may be many metastable phases.
Many of the reactions associated with the dissolution and precipitation of minerals proceed so slowly
that only after an extremely long period can they
achieve a degree of equilibrium.
Reactions will always be controlled by thermodynamics and will be driven towards more stable phases.
The kinetic reaction rate is controlled by temperature
and distance from equilibrium. The rate is zero at
equilibrium.
Silicate reactions are very slow at low temperature
and this makes it very difficult to study them in the
laboratory.
Biological processes often accompany the purely
chemical processes, adding to the complexity. Bacteria have been found to play an important role in both
the weathering and precipitation of minerals. Their
chief contribution is to increase reaction rates, particularly during weathering.
In this chapter we shall examine the processes
between water and sediments from a simple
physical-chemical viewpoint. A detailed treatment of
sediment geochemistry is however beyond the scope
of this book.
Water (H 2 O) consists of one oxygen atom linked to
two hydrogen atoms, with the H-O-H bonds forming
an angle of 105
(Fig. 3.1). The distance between the
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_3, # Springer-Verlag Berlin Heidelberg 2015
91