726
nor volumes of heavier hydrocarbons including oil.
- At deep burial depth: thermogenic methane.
Some of the generated hydrocarbons can be stored in
coal under high in-situ pressure and temperature.
However, especially the generation of methane
greatly exceeds the storage capacity of coal. The bulk
of the methane therefore migrates into structural
traps with reservoir rocks, or it escapes into the atmosphere.
Coal beds contain a mixture of gases in which methane
makes up 80-99% (Flores I 998a). One metric ton of coal
holds <1 up to ~ 19 m 3 gas (heat combustion about 85009300 cal/m 3 ). Apart from a minimum amount of "free gas"
present in fractures, this gas is sorbed on micropore surfaces of the coal. Some of the gas migrates into adjacent
sandstones. The other components of coal-bed gas include
carbon dioxide, nitrogen, hydrogen sulfide, and sulfur dioxide. Coal-bed methane has been and still is a hazard in
underground coal mining, but during the past two decades
efforts have been made to use this gas as an energy source
(see, e.g., Flores 1998b; Lyons 1998).
14.4.4 Example of a Coal Basin (NW Germany)
The Carboniferous in northwestern Germany, extending from the Rhenish Massif with the Ruhr district to the North Sea (Fig. 14.13), is one ofthe best
explored coal basins. It demonstrates various stages
of coalification, the influence of burial depth and
later exhumation, and the release of natural gas.
The coal accumulated in a foredeep of the
Variscan orogen and was subsequently deeply buried
and moderately folded, particularly in proximal position close to the evolving mountain range. In conjunction with the exploration of gas fields, the top of
the coal-bearing strata was drilled at numerous sites.
Vitrinite reflectance data (% Rm), apatite fission
track analysis, Rock-Eval pyrolysis, and model calculations for the degassing of Ruhr coal (Fig. 14.14)
were used to unravel the his tory of this coal basin.
The cross section in Fig. 14.13 shows several characteristic features and is interpreted as folIows:
- The relatively high-rank coalification of the folded
Upper Carboniferous (Namurian and Westphalian) is
found at the present land surface in the south close to
the Variscan Rhenish Massif. Coalification is less
advanced farther north (e.g. in the Münster basin and
along the co ast of the North Sea) where the coalbearing strata are still deeply buried.
- The peak of coalification was reached during maximal burial depth (and temperature) under late Carboniferous sediments (5800 m in the south and
around 3000 m farther north), i.e. prior to the main
phase of orogenesis. The less intense coalification in
Chapter 14 Hydrocarbons and Coal
the presently buried northern part of the section is
explained by lower synsedimentary subsidence.
- At the end of the Carboniferous and during the
early Permian, uplift and subsequent deep erosion
exhumed the coal seams in the south. In the north,
the exhumed coal-bearing strata again subsided and
were buried under younger deposits (Fig. 14.13). In
regions of young (Tertiary and Quaternary) subsidence, coalification may still be in operation.
- The temperature gradient is generally relatively low
in the region, but higher in the north than in the
south.
- According to a model calculation based on the results of experimental heating of coal, the history of
degassing of the initial, highly volatile bituminous
coal may have proceeded in a way similar to that indicated in (Fig. 14.14b). During the first 16 Ma of
heating, only H 2 0, CO 2 , and CO were released. Then
methane, hydrocarbons, and some nitrogen were produced. This process continued until about 40 Ma after the onset of heating. Over all, the coal has lost
more than 30% of its weight and evolved from a
highly volatile coal into C-rich anthracite.
This calculation is made with the assumption that the initial
shallow-burial coal subsided during a time period of 20 Ma
(Fig. 14.14a) which may in fact be too long. During burial,
the temperature exerted on the peat may have risen from 20
to 140°C, which corresponds to a heating rate of 6°ClMa
or about 1.6 x 1O·8°C/day. These results are based on work
ofTeichmüller et al. (1984), Tissot and Weite (1984, based
on Jüntgen and Klein 1975) and later somewaht modified
by Büker et al. (1995).
- With the onset of uplift and exhumation, the temperature of the coal seams decreased. This cooling
had no further significant effect on the coal and its
degassing, because the coal was already highly mature prior to this development
- Most gas fields occur where the vitrinite
reflectance varies between 1 and 2% Rm. Reservoir
rocks are sands of the upper Carboniferous, the
Permian (Rotlit:gend), and Lower Triassic (Bunter
sandstone). They are sealed by evaporites of Permian
and Triassic age.
- The striking maximum in the coal rank of the
Ibbenbüren mine district (4-5% Rm, Fig. 14.13b)
results from an Upper Cretaceous magmatic intrusion, causing a very high temperature gradient. Local
strong uplift and exhumation brought the coal-bearing strata to or close to the present land surface.
The general principles outlined for this example can
also be applied to other coal deposits of various
burial and uplift histories.
nor volumes of heavier hydrocarbons including oil.
- At deep burial depth: thermogenic methane.
Some of the generated hydrocarbons can be stored in
coal under high in-situ pressure and temperature.
However, especially the generation of methane
greatly exceeds the storage capacity of coal. The bulk
of the methane therefore migrates into structural
traps with reservoir rocks, or it escapes into the atmosphere.
Coal beds contain a mixture of gases in which methane
makes up 80-99% (Flores I 998a). One metric ton of coal
holds <1 up to ~ 19 m 3 gas (heat combustion about 85009300 cal/m 3 ). Apart from a minimum amount of "free gas"
present in fractures, this gas is sorbed on micropore surfaces of the coal. Some of the gas migrates into adjacent
sandstones. The other components of coal-bed gas include
carbon dioxide, nitrogen, hydrogen sulfide, and sulfur dioxide. Coal-bed methane has been and still is a hazard in
underground coal mining, but during the past two decades
efforts have been made to use this gas as an energy source
(see, e.g., Flores 1998b; Lyons 1998).
14.4.4 Example of a Coal Basin (NW Germany)
The Carboniferous in northwestern Germany, extending from the Rhenish Massif with the Ruhr district to the North Sea (Fig. 14.13), is one ofthe best
explored coal basins. It demonstrates various stages
of coalification, the influence of burial depth and
later exhumation, and the release of natural gas.
The coal accumulated in a foredeep of the
Variscan orogen and was subsequently deeply buried
and moderately folded, particularly in proximal position close to the evolving mountain range. In conjunction with the exploration of gas fields, the top of
the coal-bearing strata was drilled at numerous sites.
Vitrinite reflectance data (% Rm), apatite fission
track analysis, Rock-Eval pyrolysis, and model calculations for the degassing of Ruhr coal (Fig. 14.14)
were used to unravel the his tory of this coal basin.
The cross section in Fig. 14.13 shows several characteristic features and is interpreted as folIows:
- The relatively high-rank coalification of the folded
Upper Carboniferous (Namurian and Westphalian) is
found at the present land surface in the south close to
the Variscan Rhenish Massif. Coalification is less
advanced farther north (e.g. in the Münster basin and
along the co ast of the North Sea) where the coalbearing strata are still deeply buried.
- The peak of coalification was reached during maximal burial depth (and temperature) under late Carboniferous sediments (5800 m in the south and
around 3000 m farther north), i.e. prior to the main
phase of orogenesis. The less intense coalification in
Chapter 14 Hydrocarbons and Coal
the presently buried northern part of the section is
explained by lower synsedimentary subsidence.
- At the end of the Carboniferous and during the
early Permian, uplift and subsequent deep erosion
exhumed the coal seams in the south. In the north,
the exhumed coal-bearing strata again subsided and
were buried under younger deposits (Fig. 14.13). In
regions of young (Tertiary and Quaternary) subsidence, coalification may still be in operation.
- The temperature gradient is generally relatively low
in the region, but higher in the north than in the
south.
- According to a model calculation based on the results of experimental heating of coal, the history of
degassing of the initial, highly volatile bituminous
coal may have proceeded in a way similar to that indicated in (Fig. 14.14b). During the first 16 Ma of
heating, only H 2 0, CO 2 , and CO were released. Then
methane, hydrocarbons, and some nitrogen were produced. This process continued until about 40 Ma after the onset of heating. Over all, the coal has lost
more than 30% of its weight and evolved from a
highly volatile coal into C-rich anthracite.
This calculation is made with the assumption that the initial
shallow-burial coal subsided during a time period of 20 Ma
(Fig. 14.14a) which may in fact be too long. During burial,
the temperature exerted on the peat may have risen from 20
to 140°C, which corresponds to a heating rate of 6°ClMa
or about 1.6 x 1O·8°C/day. These results are based on work
ofTeichmüller et al. (1984), Tissot and Weite (1984, based
on Jüntgen and Klein 1975) and later somewaht modified
by Büker et al. (1995).
- With the onset of uplift and exhumation, the temperature of the coal seams decreased. This cooling
had no further significant effect on the coal and its
degassing, because the coal was already highly mature prior to this development
- Most gas fields occur where the vitrinite
reflectance varies between 1 and 2% Rm. Reservoir
rocks are sands of the upper Carboniferous, the
Permian (Rotlit:gend), and Lower Triassic (Bunter
sandstone). They are sealed by evaporites of Permian
and Triassic age.
- The striking maximum in the coal rank of the
Ibbenbüren mine district (4-5% Rm, Fig. 14.13b)
results from an Upper Cretaceous magmatic intrusion, causing a very high temperature gradient. Local
strong uplift and exhumation brought the coal-bearing strata to or close to the present land surface.
The general principles outlined for this example can
also be applied to other coal deposits of various
burial and uplift histories.