Chapter 14
Source Rocks and Petroleum Geochemistry
Knut Bjørlykke
As discussed in Chap. 1, petroleum is generated from
organic matter which accumulates in sedimentary
basins. Only a small fraction of the organic matter
produced in the photic zone in the ocean becomes
trapped in sediments (Fig. 14.1). Most of the organic
matter is formed by photosynthesis producing algae
(bacteria) and higher organisms which feed on algae.
There there may also be a supply of organic matter
from land. Most of the organic matter is oxidised in the
water column or on the seafloor and the nutrients are
released into the water and become available for new
organic production near the surface during upwelling.
Most source rocks are black shales like the Upper
Jurassic Kimmeridge Clay and its equivalents in the
North Sea basin (Fig. 14.2). Formation of rich source
rocks requires that the organic matter is not diluted too
much by deposition of clastic material or by precipitation of carbonates.
The organic matter is transformed into kerogen
which consists of very large and complex molecules.
We do not usually apply the term kerogen to fresh
organic material, but to material which is somewhat
dehydrated after burial to about 100 m or more. Kerogen is formed gradually within the upper few hundred
metres of the sediment column after deposition from
precursor products like humus, and humic and fulvic
acids. The organic matter may be derived from marine
organisms, mostly algae, or from plants derived from
land.
The transformation of amino acids, carbohydrates,
humic acids and other compounds into kerogen is
achieved by the removal of functional groups such as
acid groups, aldehydes and ketones. This involves a
loss of oxygen from the organic material, also of
nitrogen, water and CO 2 .
Kerogen therefore has higher H/C, and lower O/C,
ratios than the initial compounds (Fig. 14.3).
Kerogen may also include organic particles of morphologically recognisable biological origin such as
vitrinite (derived from woody tissues and liptinite
materials, e.g. algae spores, cuticles, etc.). Because
of its resistance to strong oxidising acids kerogen can
be recovered from sedimentary rocks by dissolving
most of the rock away with HCl or HF.
It is also possible to separate kerogen by a density
method, using heavy liquids, because kerogen is lighter than minerals. The resulting concentrate of kerogen
can be studied microscopically using transmitted and
reflected normal light, to identify the biological origin
and the degree of thermal alteration. These phases of
altered organic material are called macerals. Algal
material has a dull appearance, while wood and material from higher plants is called vitrinite. Vitrinite
becomes increasingly shiny when exposed to higher
temperatures and by measuring the amount of
reflected light in the microscope we obtain an expression for the degree of thermal alteration (Vitrinite
index).
It is also useful to use ultraviolet light microscopy,
since certain components, i.e. liptinites, display characteristic fluorescence colours. Infra-red spectroscopy
(IR) or nuclear magnetic resonance (NMR) spectroscopy can be used to investigate the chemical composition and structure of kerogen.
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_14, # Springer-Verlag Berlin Heidelberg 2015
361
Source Rocks and Petroleum Geochemistry
Knut Bjørlykke
As discussed in Chap. 1, petroleum is generated from
organic matter which accumulates in sedimentary
basins. Only a small fraction of the organic matter
produced in the photic zone in the ocean becomes
trapped in sediments (Fig. 14.1). Most of the organic
matter is formed by photosynthesis producing algae
(bacteria) and higher organisms which feed on algae.
There there may also be a supply of organic matter
from land. Most of the organic matter is oxidised in the
water column or on the seafloor and the nutrients are
released into the water and become available for new
organic production near the surface during upwelling.
Most source rocks are black shales like the Upper
Jurassic Kimmeridge Clay and its equivalents in the
North Sea basin (Fig. 14.2). Formation of rich source
rocks requires that the organic matter is not diluted too
much by deposition of clastic material or by precipitation of carbonates.
The organic matter is transformed into kerogen
which consists of very large and complex molecules.
We do not usually apply the term kerogen to fresh
organic material, but to material which is somewhat
dehydrated after burial to about 100 m or more. Kerogen is formed gradually within the upper few hundred
metres of the sediment column after deposition from
precursor products like humus, and humic and fulvic
acids. The organic matter may be derived from marine
organisms, mostly algae, or from plants derived from
land.
The transformation of amino acids, carbohydrates,
humic acids and other compounds into kerogen is
achieved by the removal of functional groups such as
acid groups, aldehydes and ketones. This involves a
loss of oxygen from the organic material, also of
nitrogen, water and CO 2 .
Kerogen therefore has higher H/C, and lower O/C,
ratios than the initial compounds (Fig. 14.3).
Kerogen may also include organic particles of morphologically recognisable biological origin such as
vitrinite (derived from woody tissues and liptinite
materials, e.g. algae spores, cuticles, etc.). Because
of its resistance to strong oxidising acids kerogen can
be recovered from sedimentary rocks by dissolving
most of the rock away with HCl or HF.
It is also possible to separate kerogen by a density
method, using heavy liquids, because kerogen is lighter than minerals. The resulting concentrate of kerogen
can be studied microscopically using transmitted and
reflected normal light, to identify the biological origin
and the degree of thermal alteration. These phases of
altered organic material are called macerals. Algal
material has a dull appearance, while wood and material from higher plants is called vitrinite. Vitrinite
becomes increasingly shiny when exposed to higher
temperatures and by measuring the amount of
reflected light in the microscope we obtain an expression for the degree of thermal alteration (Vitrinite
index).
It is also useful to use ultraviolet light microscopy,
since certain components, i.e. liptinites, display characteristic fluorescence colours. Infra-red spectroscopy
(IR) or nuclear magnetic resonance (NMR) spectroscopy can be used to investigate the chemical composition and structure of kerogen.
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_14, # Springer-Verlag Berlin Heidelberg 2015
361