724
Limnic coals normally have low sulfur contents because meteoric waters are poor in sulfate providing
the sulfur required for the formation of iron sulfides.
Coals deposited in backswamp areas of marine deltas
with switching lobes are characterized by coal seams
of limited extent with intermediate sulfur content;
coals accumulated in supratidal marshes of coastal
regions frequently display a wide lateral extent and
high sulfur contents.
Summaries on coal-forming environments are given, for
example, by Teichmüller (1973), Galloway and Hobday
(1983, 1996), Lyons and Alpern (1989), DiesseI (1992),
Cobb and Cecil (1993), Phillips and Bustin (1996), Bohacs
and Suter (1997) and in many special articles (e.g. Liu
Guanghua 1990; Dreesem et al. 1997; Liu G et al. 1998).
Coalification is described by Teichmüller and Teichmüller
(1979), Tissot and Weite (1984), Alpern (1987), and in
some of the earlier references. Regional studies are published in special volumes or journals, e.g. in Rahmani and
Flores (1984) and in the International Journal ofCoal Geology.
14.4.2 Preservation of Peat
In both limnic and paralic environments, the plant
debris has to be protected from rapid microbial decomposition under oxic conditions. This is accomplished by a water level standing just at or above the
surface of the deposited plant material. To maintain
such conditions for longer time periods, the
depositional area has to subside and/or the water table has to rise. Only that part of the plant accumulation can be preserved which is kept under the protection of a water cover or water-saturated sediment.
Some of the peat may still be subjected to aerobic
decomposition by bacteria and fungi, while deeperseated parts undergo changes by anaerobic bacteria.
The decomposition of peat under restricted oxygenation generates a low pR and antiseptic compounds
that facilitate peat preservation. In this biochemical
phase of early diagenesis, some biogenie methane is
produced.
14.4.3 Coalification and Composition of Coal
After burial, geochemical pro ces ses take over to convert the peat into brown coal and, later, into bitumiFig. 14.13. a Map showing location of section from
the Ruhr coal distriet (Rhenish Massi!), NW Germany, via the Ibbenbüren coal mine to the North Sea.
b Cross section indicated in a displaying degree of
coalification at the top of the Carboniferous. The
approximate position of isolines for mean vitrinite
reflectance, % Rm, is added in parts of the sec ti on.
Chapter 14 Hydrocarbons and Coal
nous hard coal and anthracite (cf. Fig. 14.4). These
processes are summarized under the term coalijication. Special investigations, such as the determinations of the moisture and volatile matter contents,
carbon and hydrogen contents, the reflectance of
vitrinite (see below), and the calorific value, allow
distinguishing between several stages of coalification. These stages are referred to as rank levels
which indicate the maturity of coal.
Peat and coal contain various organic components
(macerals) which, similar to the components of the
hydrocarbon source rocks, are affected by the
coalification process in different ways. The three
main groups of macerals are (Tissot and WeIte 1984;
Robert 1985):
(1) Huminite and vitrinite. Remains of woody and
humic substances which are the major components of
brown coals and bituminous hard coals.
(2) Liptinite or exinite. These are remains of lipidrich plant relics (resins, waxes, spores, cutic1es, algae).
(3) Inertinite. Oxidized or reworked older carbonrich particles.
Over all, the organic components of coal resemble
kerogen type III described earlier for hydrocarbon
source rocks. Their potential for generating hydrocarbons is similar to that of this kerogen type.
Prograding coalification leads to the following
physical and chemical changes:
- Decrease in water content and porosity, increase in
density.
- Condensation of organic molecules, polymerization, aromatization, and loss of functional groups.
The net result of these processes is a relative enrichment 01 carbon with increasing burial depth and temperature (or coal rank). These changes are accompanied first by substantial mechanical compaction and
the expulsion oJ pore water. Then, carbon dioxide,
gaseous and liquid hydrocarbons are generated and
released as follows (cf. Fig. 14.4):
- At shallow burial depth: carbon dioxide and
bio genie methane.
- At medium burial depth (range of medium volatile
bituminous coal): large amounts ofmethane and miNote influence of post-orogenie uplift in the south,
young subsidence in the north, and magmatic intrusion in the Ibbenbüren area. For further explanation
see text. c Relationship between burial depth, mean
vitrinite reflectance, coal rank, and generation of gas.
(After Teichmüller et al. 1984)
Limnic coals normally have low sulfur contents because meteoric waters are poor in sulfate providing
the sulfur required for the formation of iron sulfides.
Coals deposited in backswamp areas of marine deltas
with switching lobes are characterized by coal seams
of limited extent with intermediate sulfur content;
coals accumulated in supratidal marshes of coastal
regions frequently display a wide lateral extent and
high sulfur contents.
Summaries on coal-forming environments are given, for
example, by Teichmüller (1973), Galloway and Hobday
(1983, 1996), Lyons and Alpern (1989), DiesseI (1992),
Cobb and Cecil (1993), Phillips and Bustin (1996), Bohacs
and Suter (1997) and in many special articles (e.g. Liu
Guanghua 1990; Dreesem et al. 1997; Liu G et al. 1998).
Coalification is described by Teichmüller and Teichmüller
(1979), Tissot and Weite (1984), Alpern (1987), and in
some of the earlier references. Regional studies are published in special volumes or journals, e.g. in Rahmani and
Flores (1984) and in the International Journal ofCoal Geology.
14.4.2 Preservation of Peat
In both limnic and paralic environments, the plant
debris has to be protected from rapid microbial decomposition under oxic conditions. This is accomplished by a water level standing just at or above the
surface of the deposited plant material. To maintain
such conditions for longer time periods, the
depositional area has to subside and/or the water table has to rise. Only that part of the plant accumulation can be preserved which is kept under the protection of a water cover or water-saturated sediment.
Some of the peat may still be subjected to aerobic
decomposition by bacteria and fungi, while deeperseated parts undergo changes by anaerobic bacteria.
The decomposition of peat under restricted oxygenation generates a low pR and antiseptic compounds
that facilitate peat preservation. In this biochemical
phase of early diagenesis, some biogenie methane is
produced.
14.4.3 Coalification and Composition of Coal
After burial, geochemical pro ces ses take over to convert the peat into brown coal and, later, into bitumiFig. 14.13. a Map showing location of section from
the Ruhr coal distriet (Rhenish Massi!), NW Germany, via the Ibbenbüren coal mine to the North Sea.
b Cross section indicated in a displaying degree of
coalification at the top of the Carboniferous. The
approximate position of isolines for mean vitrinite
reflectance, % Rm, is added in parts of the sec ti on.
Chapter 14 Hydrocarbons and Coal
nous hard coal and anthracite (cf. Fig. 14.4). These
processes are summarized under the term coalijication. Special investigations, such as the determinations of the moisture and volatile matter contents,
carbon and hydrogen contents, the reflectance of
vitrinite (see below), and the calorific value, allow
distinguishing between several stages of coalification. These stages are referred to as rank levels
which indicate the maturity of coal.
Peat and coal contain various organic components
(macerals) which, similar to the components of the
hydrocarbon source rocks, are affected by the
coalification process in different ways. The three
main groups of macerals are (Tissot and WeIte 1984;
Robert 1985):
(1) Huminite and vitrinite. Remains of woody and
humic substances which are the major components of
brown coals and bituminous hard coals.
(2) Liptinite or exinite. These are remains of lipidrich plant relics (resins, waxes, spores, cutic1es, algae).
(3) Inertinite. Oxidized or reworked older carbonrich particles.
Over all, the organic components of coal resemble
kerogen type III described earlier for hydrocarbon
source rocks. Their potential for generating hydrocarbons is similar to that of this kerogen type.
Prograding coalification leads to the following
physical and chemical changes:
- Decrease in water content and porosity, increase in
density.
- Condensation of organic molecules, polymerization, aromatization, and loss of functional groups.
The net result of these processes is a relative enrichment 01 carbon with increasing burial depth and temperature (or coal rank). These changes are accompanied first by substantial mechanical compaction and
the expulsion oJ pore water. Then, carbon dioxide,
gaseous and liquid hydrocarbons are generated and
released as follows (cf. Fig. 14.4):
- At shallow burial depth: carbon dioxide and
bio genie methane.
- At medium burial depth (range of medium volatile
bituminous coal): large amounts ofmethane and miNote influence of post-orogenie uplift in the south,
young subsidence in the north, and magmatic intrusion in the Ibbenbüren area. For further explanation
see text. c Relationship between burial depth, mean
vitrinite reflectance, coal rank, and generation of gas.
(After Teichmüller et al. 1984)