of considerably greater value than those we find in
metamorphic and eruptive rocks. The most important
are oil, gas and coal deposits, but a very large amount
of the world’s ore deposits is also found in sedimentary rocks and many types of ore have been formed
through sedimentary processes. Limestone, clay, sand
and gravel are also important raw materials which
require sedimentological expertise.
The petroleum industry employs a very high percentage of the world’s professional geologists. This
industry has a particular need for research, and also
has the financial capacity to invest in it. Because oil
and gas are found largely in sedimentary rocks, exploration for and recovery of hydrocarbons is based to a
large extent on sedimentology. Much of what we now
know about the world’s sedimentary basins and their
regional geology is derived from seismic profiles
which have been shot in connection with oil exploration and drilling for oil and gas. The oil industry has
also helped to stimulate pure sedimentological
research, and significant contributions to research in
this area are published by the research laboratories of
the oil companies. Research based on economic
interests is also useful from a purely scientific point
of view, because it often focuses on particular
questions which may be quite fundamental.
Petroleum geology requires close teamwork
between reservoir engineers and geologists, to establish in great detail the geometry and distribution of
porosity and permeability in reservoir rocks. We also
need very much to know more about the physical and
chemical properties of reservoir rocks, for reasons
which are discussed at the end of the book. Geophysical methods provide most of the information used in
petroleum exploration and production and many petroleum geologists rarely examine real rocks in cores and
cuttings. It is however important to know something
about the textural and mineralogical composition of
the sedimentary sequences. The geophysical data
rarely provide unique solutions when inverting seismic and log data to rock properties. Facies models
provide ideas about how different lithologies may be
distributed in a sedimentary basin. With few wells far
between it is difficult to make a realistic representation
in 3D. The diagenetic processes which change rock
properties after deposition are equally important. Sedimentary facies and the primary sediment composition
represent however the starting material for burial diagenetic reactions determining the rock properties.
The depositional environment determines to a large
extent the shape and extension of potential reservoir
rocks such as sandstones and limestones. Sequence
stratigraphy of the sequences filling a sedimentary
basin often provides the key to predict the distribution
of source rocks, migration pathways and traps. The
internal properties of reservoir rocks such as porosity,
density and velocity depend however very much on
the mineralogical and textural composition at the time
of deposition.
Sandstones may vary greatly with respect to grain
size, feldspar and clay content and compact very differently. The source of clastic minerals (provenance)
is an important part of basin analyses which will help
to predict rock properties at greater burial depth.
Mudstones and shales should also be analysed with
respect to grain size (silt content) and clay mineralogy
because different types of clay and mud are important
parts of sedimentary sequences, particularly as source
rocks and cap rocks.
Limestone rocks are to a very large extent composed of material precipitated biologically and the
types of marine organisms forming reefs and other
carbonate reservoir rocks reflect to a large extent the
biological evolution through time.
Also in the clastic sediments like sandstones and
shales biologically precipitated carbonate and also
silica are important components controlling rock
properties. Quartz-rich sandstones may have very little
porosity because of carbonate cement and calcareous
mudstones and shales may be very tight and brittle.
This also influences the seismic data recorded in sedimentary basins.
Sedimentology and facies analyses represent an
important basis for the different petroleum related
disciplines discussed in the following chapters in this
book.
2.1
Description of Sedimentary Rocks
2.1.1 Sediment Composition. Mineralogy
and Other Components
Clastic sediments consist of grains and fine particles
which may be composed of single minerals, rock
fragments and also some amorphous compounds
such as organic matter, biogenic silica and volcanic
glass. The sediment composition is a function of the
2 Introduction to Sedimentology
33
metamorphic and eruptive rocks. The most important
are oil, gas and coal deposits, but a very large amount
of the world’s ore deposits is also found in sedimentary rocks and many types of ore have been formed
through sedimentary processes. Limestone, clay, sand
and gravel are also important raw materials which
require sedimentological expertise.
The petroleum industry employs a very high percentage of the world’s professional geologists. This
industry has a particular need for research, and also
has the financial capacity to invest in it. Because oil
and gas are found largely in sedimentary rocks, exploration for and recovery of hydrocarbons is based to a
large extent on sedimentology. Much of what we now
know about the world’s sedimentary basins and their
regional geology is derived from seismic profiles
which have been shot in connection with oil exploration and drilling for oil and gas. The oil industry has
also helped to stimulate pure sedimentological
research, and significant contributions to research in
this area are published by the research laboratories of
the oil companies. Research based on economic
interests is also useful from a purely scientific point
of view, because it often focuses on particular
questions which may be quite fundamental.
Petroleum geology requires close teamwork
between reservoir engineers and geologists, to establish in great detail the geometry and distribution of
porosity and permeability in reservoir rocks. We also
need very much to know more about the physical and
chemical properties of reservoir rocks, for reasons
which are discussed at the end of the book. Geophysical methods provide most of the information used in
petroleum exploration and production and many petroleum geologists rarely examine real rocks in cores and
cuttings. It is however important to know something
about the textural and mineralogical composition of
the sedimentary sequences. The geophysical data
rarely provide unique solutions when inverting seismic and log data to rock properties. Facies models
provide ideas about how different lithologies may be
distributed in a sedimentary basin. With few wells far
between it is difficult to make a realistic representation
in 3D. The diagenetic processes which change rock
properties after deposition are equally important. Sedimentary facies and the primary sediment composition
represent however the starting material for burial diagenetic reactions determining the rock properties.
The depositional environment determines to a large
extent the shape and extension of potential reservoir
rocks such as sandstones and limestones. Sequence
stratigraphy of the sequences filling a sedimentary
basin often provides the key to predict the distribution
of source rocks, migration pathways and traps. The
internal properties of reservoir rocks such as porosity,
density and velocity depend however very much on
the mineralogical and textural composition at the time
of deposition.
Sandstones may vary greatly with respect to grain
size, feldspar and clay content and compact very differently. The source of clastic minerals (provenance)
is an important part of basin analyses which will help
to predict rock properties at greater burial depth.
Mudstones and shales should also be analysed with
respect to grain size (silt content) and clay mineralogy
because different types of clay and mud are important
parts of sedimentary sequences, particularly as source
rocks and cap rocks.
Limestone rocks are to a very large extent composed of material precipitated biologically and the
types of marine organisms forming reefs and other
carbonate reservoir rocks reflect to a large extent the
biological evolution through time.
Also in the clastic sediments like sandstones and
shales biologically precipitated carbonate and also
silica are important components controlling rock
properties. Quartz-rich sandstones may have very little
porosity because of carbonate cement and calcareous
mudstones and shales may be very tight and brittle.
This also influences the seismic data recorded in sedimentary basins.
Sedimentology and facies analyses represent an
important basis for the different petroleum related
disciplines discussed in the following chapters in this
book.
2.1
Description of Sedimentary Rocks
2.1.1 Sediment Composition. Mineralogy
and Other Components
Clastic sediments consist of grains and fine particles
which may be composed of single minerals, rock
fragments and also some amorphous compounds
such as organic matter, biogenic silica and volcanic
glass. The sediment composition is a function of the
2 Introduction to Sedimentology
33