Directory UMM :Data Elmu:jurnal:O:Organic Geochemistry:Vol32.Issue1.2001:

Organic Geochemistry 32 (2001) 163±180
www.elsevier.nl/locate/orggeochem

Composition and origin of coalbed gases in the Upper
Silesian and Lublin basins, Poland
Maciej J. Kotarba *
University of Mining and Metallurgy, Al. Mickiewicza 30, 30-059 Krakow, Poland
Received 3 November 1998; accepted 5 September 2000
(returned to author for revision 1 July 1999)

Abstract
Coalbed gases in the Upper Silesian Coal Basin (USCB) are highly variable in molecular and stable isotope composition. Geochemical indices and stable isotope ratios for coalbed gases from USCB vary within the following ranges:
CH4/(C2H6+C3H8) hydrocarbon index from 122 to more than 10,000; CDMI carbon dioxide-methane index {CDMI=
[CO2/(CO2+CH4)] 100 (%)} from 0.0 to 21.0%; d13C(CH4) from ÿ79.9 to ÿ44.5%; dD(CH4) from ÿ202 to ÿ153%;
d13C(C2H6) from-24.6 to ÿ22.3%; d13C(C3H8) ÿ24.7 % (one sample); and d13C(CO2) from ÿ27.2 to ÿ2.8%. Only two
coalbed gases were collected from the Lublin Coal Basin (LCB). Geochemical indices and stable isotope ratios for these
samples show the following values: hydrocarbon index more than 10,000; CDMI=4.3 and 34.6%; d13C(CH4)= ÿ67.3
and ÿ52.5%; dD(CH4)= ÿ201% (one sample); and d13C(CO2)= ÿ13.7 and ÿ11.9%. Methane, higher gaseous
hydrocarbons (C2 to C5) and carbon dioxide occurring in the coalbed gases in both the USCB and the LCB were
generated during the bituminous stage of the coali®cation process and, probably, during microbial reduction of carbon
dioxide. However, depth-related isotopic fractionation which has resulted from physical (e.g. di€usion and adsorption/

desorption) processes during gas migration cannot be neglected. In both basins the coali®cation process lasted no
longer than several Ma, and was completed at the end of the Variscan orogeny (at the turn of Carboniferous and
Permian). # 2001 Elsevier Science Ltd. All rights reserved.
Keywords: Stable carbon isotopes; Stable hydrogen isotopes; Upper Silesian coalbed gases; Lublin coalbed gases; Thermogenic gases;
Microbial gases; Gas origin; Gas migration

1. Introduction
The coalbed gas reserves to depths of 1000 m in the
Upper Silesian Coal Basin (USCB) are estimated to be
about 350 billion m3, with about 150 billion m3 in active
mining concessions and about 200 billion m3 in virgin
exploration ®elds (Pilcher et al., 1991; Kotas et al., 1994).
The coalbed methane reserves in the Lublin Coal Basin
(LCB) have not been estimated as yet, because of the
lack of intensive mining activity. Coalbed gas is a
potential energy source, essentially undeveloped in a

* Tel.: +48-12-617-2431; fax: +48-12-617-2431.
E-mail address: kotarba@uci.agh.edu.pl

country that depends on coal (above 80%) for most of
its energy production. Knowledge of the origin, migration pathways, and storage conditions of natural gases
within the Upper Carboniferous coal-bearing strata of
the Upper Silesian and Lublin Coal Basins is useful in
the evaluation of gas reserves and hazards of methane
explosions. In addition, coal mining contributes to the
increasing concentration of atmospheric methane which
is a potent greenhouse gas (e.g. Clayton et al., 1995b;
Clayton, 1998). Coals and carbonaceous shales of the
USCB and the LCB as a habitat of coalbed gases were
studied by Kotarba and Clayton (in press) and Clayton
et al. (1995a).
Until recently, attention in the USCB has been paid
mainly to methane as a cause of explosions in mines and

0146-6380/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.
PII: S0146-6380(00)00134-0

164

M.J. Kotarba / Organic Geochemistry 32 (2001) 163±180

an important potential energy source (e.g. Patteisky,
1926; Poborski, 1960; Rosenman, 1968). The ®rst
attempt to explain the origin of methane in the USCB
by means of isotopes was undertaken by Kotarba (1979,
1980) and Rice and Kotarba (1993). The origin of coalbed gases from the LCB has not been studied until now.
This paper discusses the constrains of coalbed gas
origin (methane, higher gaseous hydrocarbons and carbon dioxide) in the USCB and LCB. The work is based
on the results of recent analyses of molecular composition and stable carbon isotopic composition of methane,
ethane, propane and carbon dioxide, on stable hydrogen
isotope composition of methane, on geochemical studies

by Kotarba (1979) in the context of geological setting,
and on the results of geochemical studies of associated
coals (Kotarba, 1979; Clayton et al., 1995a; Kotarba
and Clayton, 2000).

2. Geological setting
The USCB, one of the major coal basins in the world,

formed as a foredeep of the Moravo-Silesian fold zone
(Figs. 1 and 2). It is a deep molasse basin of polygenetic
origin: the lower part of Upper Carboniferous coalbearing lithostratigraphic sequence (Namurian A) was

Fig. 1. Geological sketch-map of Poland showing locations of bituminous (hard) coal basins. From Kotas and Porzycki (1984). 1±8
extent of Carboniferous strata: 1. formations of unde®ned facies; 2. ¯ysch formations; 3. ¯ysch formations partly covered by continental strata; 4. predominantly carbonate formations; 5. marine-paralic and paralic coal-bearing formations; 6. continental formations; 7. continental formations locally coal-bearing; 8. outcrops of coal-bearing formations; 9. margin of the East European Platform;
10. Lednice Line (LL) and Peri-Carpathian Lineament (PCL); 11. 1800 m Carboniferous overburden contour; 12. Paleozoic top surface contour (below sea level); USCB, Upper Silesian Coal Basin; LCB, Lublin Coal Basin; L-VCB, Lviv-Volhynian Coal Basin;
LSCB, Lower Silesian Coal Basin; CFB, Cracow Fold Belt.

M.J. Kotarba / Organic Geochemistry 32 (2001) 163±180

165

Fig. 2. Geological sketch-map of the Upper Silesian Coal Basin (Permian, Mesozoic and Cenozoic subcrops) showing locations of the
gas and rock sampling sites. Geology from Kotas (1994). 1±4, Upper Carboniferous coal-bearing series: 1. Cracow sandstone mudstone (Westphalian C-D); 2. mudstone (Westphalian A-B); 3. Upper Silesian sandstone (Namurian B-C); 4. Paralic (Namurian A); 5.
faults and/or overthrusts; 6. main overthrusts: MR, Micha•kowice-Rybnik, OB, Or•owa-Boguszowice; 7. northern boundary of marine
Miocene strata; 8. locations of gas and coal sampling sites; 9. locations of tested wells for methane content (cf. Fig. 12).

deposited in a paralic environment, and the upper part
(Namurian B to Wesphalian D) is of continental origin.

Formation of the Upper Silesian Variscan orogen proceeded in several orogenic phases. Uplifting and the
main fold structures were formed during Asturian and
Leonian orogenic phases at the turn of Carboniferous
and Permian (Kotas and Porzycki, 1984; Kotas, 1994).
After the Variscan uplift, the Upper Carboniferous
coal-bearing formations were exposed over most of the
basin, and subjected to erosion and denudation. During
the Alpine movements the Upper Silesian Variscan
orogen behaved as a consolidated basement for the
Alpides. Fig. 3 presents burial history curves of the
sedimentary strata in two locations in the southern
USCB, in the vicinity of the ``Silesia'' mine and the former ``Morcinek'' mine, where erosion depth was about
200 and about 800 m, respectively (Kosakowski et al.,
1995). In the southern part of the USCB, the Miocene
(Karpatian-Badenian) marine, clayey-sandstone sediments were deposited (Figs. 2 and 3). In the northern
part of the USCB the Permian-Jurassic strata were laid
down (recent thickness less than 200 m). In the central
part of the USCB the Upper Carboniferous strata are
covered only by Quaternary sediments.

The LCB is an epi-platform, molasse basin, developed
as a pericratonic depression within the East-European
Platform (Kotas and Porzycki, 1984). Its Upper Carboniferous coal-bearing lithostratigraphic sequence is of
polygenetic origin: the lower part (Upper Visean and
Namurian A) is marine-paralic, the middle part (Namurian B and C and Wesphalian A) is paralic, and the upper
part (Wesphalian B to D) is continental (limnic) (e.g.
Porzycki, 1990). After the Variscan uplift the Upper
Carboniferous coal-bearing formations were also exposed
and subjected to erosion and denudation. In studied area
of the LCB the Upper Carboniferous strata are covered
by Middle and Upper Jurassic carbonates.
The coali®cation process in the both basins was completed at the end of Variscan orogeny and was not
rejuvenated later.

3. Experimental
3.1. Sampling procedure
Gas samples were collected only from the virgin parts
of the coal deposits, mostly in the years 1992±1994 from

166

M.J. Kotarba / Organic Geochemistry 32 (2001) 163±180

Fig. 3. Burial history curves of the sedimentary strata in two locations of the Upper Silesian coal basin in the vicinity of (a) ``Silesia''
mine and (b) former ``Morcinek'' mine modi®ed from Kosakowski et al. (1995). Locations cf. Fig. 2.

the same mines and at nearby locations where associated coals and carbonaceous shales were taken for
geochemical and petrographic studies (Clayton et al.,
1995a; Kotarba and Clayton, 2000). Samples marked 75
and 76 (Table 1, Fig. 2) were collected in the years 1975±
1976 (Kotarba, 1979). A special sampling procedure was
applied for the collection of ``free'' and ``adsorbed''
gases from the coal seams. In 35 fresh faces in the USCB
and two drift faces in the LCB, 4±6-m-long holes were
drilled near the roof of the coal seams. In the USCB, 33
``free'' gas samples were collected in 15 mines, from an
almost full Upper Carboniferous sequence from
Namurian A (group of seams 900) to Westphalian C
(group of seams 200). In the LCB, two ``free'' gas samples
were collected from the only operating mine, the ``Bogdanka'' mine, where single No. 382 seam (Westphalian

B) is currently excavated. Details about locations of gas
sampling sites are presented in Tables 1 and 2, and Figs.
1 and 2.
After the installation of a special probe, ``free'' gas
samples were collected from the deepest 0.5 m interval
of each borehole and transferred into glass vessels ®lled
with saturated NaCl solution. Samples were taken not
longer than 2 min after the completion of a borehole. In
six boreholes ``free'' gas samples were also collected after
20 min (Table 2). The isotopic di€erence (
13C) between
samples taken after 2 and 20 min varies between 0.6 and
ÿ1.0% (Table 2). Therefore, it can be assumed that after
2 min from borehole completion gas equilibrium conditions more or less were established, and isotopic composition did not depend on the time of sampling. In three
sampling sites both the roof and bottom of the coal seam

167

M.J. Kotarba / Organic Geochemistry 32 (2001) 163±180
Table 1

Information on gas and coal sample sites
Sample code

Mine

Seam no.

Agea

Depth below
surface (m)

Depth below
U.Carb. top (m)

Upper Silesian coal basin
A-5
B-1/76
B-2/76
B-5

B-7
H-1
J-50/76
J-1/76
J-5
Kr-10
Mi-1
Mk-3
Mk-5
Ml-1/76
Mo-1
Mo-2
Mo-13
NM-1
Pa-1
P-3/75
P-1/76
P-2/76
P-3/76
P-4/76

Pn-1
S-2/76
S-10
S-11
We-1
Z-53/75
Z-55/75
Z-4

Anna
Brzeszcze
Brzeszcze
Brzeszcze
Brzeszcze
Halemba
JastrzcecÎbie
JastrzcecÎbie
JastrzcecÎbie
KrupinÂski
Miechowice
Morcinek
Morcinek
Marcel
Moszczenica
Moszczenica
Moszczenica
Niwka-Mod.
Paskov
PnioÂwek
PnioÂwek
PnioÂwek
PnioÂwek
PnioÂwek
PnioÂwek
Silesia
Silesia
Silesia
Weso•a
Zo®oÂwka
Zo®oÂwka
Zo®oÂwka

718
364
334
510
356
506
417
418
502/1
348
509
406/2
404/2
507
506/3
510/1
605
510
906 eq.
356/1
360/1
356/1
356/1
3571
363
214/1-2
308
214/1
501
407/2-3
407/2-3
404/4

N-A
W-A
W-A
N-B
W-A
N-B
N-C
N-C
N-C
W-A
N-B
W-A
W-A
N-B
N-B
N-B
N-A
N-B
N-A
W-A
W-A
W-A
W-A
W-A
W-A
W-C
W-B
W-C
N-B
W-A
W-A
W-A

749
583
540
611
558
920
486
479
560
570
811
862
819
589
503
457
465
722
756
572
559
574
542
569
672
323
637
518
670
475
497
611

689
475
478
584
528
915
68
84
130
470
613
126
88
513
418
171
292
716
140
267
113
239
125
150
89
160
178
154
667
44
81
59

Lublin coal basin
Bo-1
Bo-2

Bogdanka
Bogdanka

382
382

W-B
W-B

882
864

169
174

a
Age: N-A, Namurian A; N-B, Namurian B; N-C, Namurian C; W-A, Westphalian A; W-B, Westphalian B; W-C, Westphalian C;
U. Carb., Upper Carboniferous. Abbreviations: eq., equivalent; Mod., ModrzejoÂw.

were sampled for gases (Table 2). Only at one sampling
site the isotopic di€erence (
13C) in methane between
roof and bottom samples taken after 2 min slightly
exceeds 1% (P-2/76 sample, Table 2). Hence, it can be
assumed that methane is isotopically homogenous
within the whole thickness of the seam.
The ``adsorbed'' gas was sampled with a di€erent
procedure. Immediately after completion of a borehole,
cuttings (1.0±2.0 mm fraction) from the deepest 0.5 m
interval were collected into a 2-dm3 hermetic stainless
steel container containing metal balls. Time span
between completion of borehole and closure of the container did not exceed 2 min. After about 24 h, in the

laboratory, the container was evacuated and samples
ground to 10,000
>10,000
>10,000
1,570
2,670
114
>10,000
>10,000
>10,000
1,880
422
>10,000
4,910
>10,000
>10,000
>10,000
6,560
1,190
122
>10,000
>10,000
>10,000
>10,000
>10,000
>10,000
>10,000
>10,000
9,520
970
>10,000
2,690
>10,000
>10,000
9,840

1.0
0.3
0.3
0.3
1.8
1.1
3.3
0.0
0.0
0.4
1.6
21.0
4.1
3.5
0.7
2.5
0.4
0.2
2.0
0.9
0.6
0.0
0.0
1.4
1.3
0.0
0.0
1.2
0.0
0.7
0.3
2.1
0.2
0.0
0.6

ÿ50.7
ÿ47.1
ÿ47.3
ÿ56.2
ÿ48.2
ÿ48.9
ÿ45.7
ÿ78.7
ÿ74.5
ÿ72.8
ÿ52.6
ÿ61.6
ÿ69.4
ÿ69.9
ÿ77.1
ÿ74.0
ÿ70.5
ÿ69.9
ÿ54.2
ÿ44.5
ÿ71.9
ÿ69.8
ÿ70.3
ÿ69.2
ÿ70.6
ÿ69.0
ÿ71.4
ÿ67.9
ÿ71.1
ÿ69.9
ÿ69.3
ÿ47.8
ÿ79.8
ÿ79.9
ÿ67.2

ÿ202
n.a.
n.a.
n.a.
ÿ179
ÿ153
ÿ189
n.a.
n.a.
ÿ179
ÿ190
ÿ159
ÿ157
ÿ158
n.a.
ÿ161
ÿ171
ÿ184
ÿ196
ÿ193
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
ÿ178
n.a.
ÿ170
ÿ179
ÿ196
n.a.
n.a.
ÿ170

ÿ2.8
n.a.
n.a.
n.a.
ÿ4.6
ÿ6.0
ÿ15.1
n.a.
n.a.
n.a.
ÿ8.1
ÿ27.2
n.a.
ÿ13.1
n.a.
ÿ13.2
n.a.
n.a.
ÿ12.0
ÿ17.1
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
ÿ14.0
n.a.
n.a.
n.a.
ÿ14.4
n.a.
n.a.
n.a.

n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
ÿ24.6
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
ÿ22.3
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.

47.9

43.6
42.9
40.6


13C
(C2H6±CH4)f

21.1

44.5
34.4
56.8
60.8

42.2
27.4

22.2

M.J. Kotarba / Organic Geochemistry 32 (2001) 163±180

Upper Silesian coal basin
A-5
95.0
0.074
B-1/76r
95.8
±
B-1/76b
95.9
±
B-2/76
94.8
±
B-5
97.4
0.062
B-7
96.2
0.036
H-1a
93.9
0.65
J-50/75
94.6
±
J-1/76
98.0
±
J-5
97.8
±
Kr-10
95.8
0.051
Mi-1
62.9
0.14
Mk-3
75.0
±
Mk-5
81.5
0.016
Ml-1/76
97.3
±
Mo-1
91.3
±
Mo-2
97.0
±
Mo-13
91.8
0.014
NM-1
90.2
0.076
Pa-1
95.3
0.73
P-3/75
92.0
±
P-1/76r
98.3
±
P1/76/b
98.2
±
P-2/76r
96.5
±
P-2/76b
96.6
±
P-3/76
98.2
±
P-4/76
97.9
±
Pn-1
94.1
±
S-2/76
95.2
0.007
S-10
89.4
0.092
S-11
96.7
tr.
We-1
96.9
0.036
Z-53/75
97.4
±
Z-55/75
99.0
±
Z-4
98.4
0.01

Gas indices

53.9

33.4

169

(continued on next page)

170

13C
(C2H6±CH4)f

M.J. Kotarba / Organic Geochemistry 32 (2001) 163±180

n.a.
n.a.
ÿ13.7
ÿ11.9
n.a.
ÿ201
b

a

H-1 sample: d13C(C3H8)= ÿ24.7(%).
C4-C5=C4H10 +C5H12.
c
CHC=CH4/(C2H6+C3H8).
d
CDMI=[CO2/(CO2+CH4)] 100 (%).
e

13C(CO2-CH4)=d13C (CO2)- d13C(CH4) (%).
f

13C(C2H6-CH4)=d13C(C2H6)- d13C(CH4) (%).
Abbreviations: tr., traces; n.a., not analysed; r., roof; b., bottom;±, concentration below limit of detection.

ÿ52.5
ÿ67.3
34.6
4.3
>10,000
>10,000
±
±
1.90
0.44
0.09
0.06
76.2
18.7
7.40
3.90
±
±
±
±
±
±
Lublin coal basin
Bo-1
14.4
Bo-2
76.9

d13C
(CO2)
dD
(CH4)
d13C
(CH4)
N2
CO2
C4±C5b
C3H8
C2H6
CH4
Sample code

Molecular composition (vol.%)

Table 3 (continued)

Gas indices

Ar

He

H2

CHCc

CDMId

Stable isotopes (%)

d13C
(C2H6)


13C
(CO2±CH4)e

4.1. Stable carbon and hydrogen isotopes of methane
The last two decades have seen a growing interest in
studies on the origin of natural gases based on stable
carbon and hydrogen analyses of methane (e.g. Stahl,
1977; Schoell, 1983; Faber, 1987; Whiticar; 1994; Berner
and Faber, 1996). Recent developments in organic geochemistry point to uncertainties in the interpretations of
the stable isotope data for gases associated with coals
(Smith et al., 1982, 1985, 1992, Schoell, 1983; Kotarba,
1988, 1990; Rice, 1993; Whiticar, 1996). These uncertainties are connected with the di€erent mechanisms of
gas generation from di€erent types of macerals/kerogens of humic organic matter, either accumulated in
coal-seams or dispersed, and from fractionation of
coalbed gases during secondary, physical and chemical
processes (such as sorption, di€usion and oxidation)
operating during migration and/or mixing.
Stable carbon and hydrogen isotope studies of methane from gases accompanying bituminous coals and
anthracites in coal basins of Germany, China, the former Soviet Union, The Netherlands, Australia and
Poland revealed high variability of both d13C(CH4) and
dD(CH4) values from ÿ80 to ÿ12%, and from ÿ333 to
ÿ117%, respectively (Bokhoven and Theeuwen, 1966;
TeichmuÈller et al., 1968; Colombo et al., 1970; Alekseev
and Lebedev, 1977; Kotarba, 1979, 1980, 1988, 1990;
Kravtsov and Voytov, 1980; Smith et al., 1982, 1985;
1992; Dai et al., 1987; Voytov, 1988; Rice, 1993; Scott et
al., 1994; Smith and Pallasser, 1996). Such high isotopic
variations may result from various primary (generation)
and secondary (migration) processes.
The d13C(CH4) and dD(CH4) values for the coalbed
gases from USCB vary from ÿ79.9 to ÿ44.5% and from
ÿ202 to ÿ153%, respectively. Such a high variability of
both molecular and isotopic composition (Figs. 4±6) may

Fig. 4. Genetic characterization of coalbed gases from USCB
and LCB using d13C(CH4) versus CH4/(C2H6+C3H8). Diagnostic ®elds from Whiticar (1990). USCB, Upper Silesian Coal
Basin; LCB, Lublin Coal Basin.

M.J. Kotarba / Organic Geochemistry 32 (2001) 163±180

171

Fig. 5. Genetic characterization of coalbed gases from USCB
and LCB using d13C(CH4) versus dD(CH4). Diagnostic ®elds
from Whiticar et al. (1986). USCB, Upper Silesian Coal Basin;
LCB, Lublin Coal Basin.
Fig. 7. Genetic characterization of coalbed gases from USCB
using d13C(CH4) versus d13C(C2H6). Position of the vitrinite
re¯ectance curve for type III kerogen after Berner and Faber
(1996).

Fig. 6. Stable carbon isotope composition of methane for
USCB and LCB coalbed gases versus vitrinite re¯ectance (Ro)
of associated coals. Position of the genetic curve after Berner
and Faber (1996). USCB, Upper Silesian Coal Basin; LCB,
Lublin Coal Basin.

re¯ect multiple origins of these gases (thermogenic,
microbial, and mixed), and/or physical and physicochemical processes operating during gas migration.
Discrepancy between measured d13C(CH4) Ð vitrinite
re¯ectance relationship and maturity curve (Fig. 6)
indicates that the methane does not seem to be genetically connected with the associated coals. In Fig. 6, and
also in Figs. 7 and 8, the calculation of the positions of
maturity curves on the vitrinite re¯ectance scale was
made (after Berner and Faber, 1996) for the mean d13Cvalue of studied coals (ÿ23.9%; Table 4).
The d13C(CH4) and dD(CH4) values for the coalbed
gases from LCB are ÿ67.3 and ÿ52.5%, and ÿ201%,
respectively. Based on diagrams of genetic classi®cations
(Figs. 4 and 5) the methane was generated via microbial
CO2-reduction.

Fig. 8. Genetic characterization of coalbed gases from USCB
using d13C(C2H6) versus d13C(C3H8). Position of the vitrinite
re¯ectance curve for type III kerogen after Berner and Faber
(1996).

4.2. Stable carbon isotopes of ethane and propane
Ethane as well as the other higher gaseous hydrocarbons can be generated during the coali®cation process from macerals of the exinite group and also from
the hydrogen-rich vitrinite macerals (e.g. Kotarba, 1988;
Rice, 1993; Clayton, 1998). As compared with the coexisting methane, ethane generated during the same
process of coali®cation is enriched in 13C (Faber, 1987;
Berner and Faber, 1996). Stable carbon isotope analyses
of ethane and propane indicate the maturity level of the

172

Table 4
Results of petrographic, vitrinite re¯ectance, volatile matter and stable carbon isotope analyses of coals (pillar samples)
Sample code

Petrographic composition (%)
Vitrinite MGb

Lublin coal basin
Bo-1a
47.9
Bo-2a
39.6
a
b

Volatile Matterdaf
(wt.%)

d13C
(%)

34.6
n.a.
n.a.
35.6
33.6
31.0
n.a.
24.4
24.2
35.4
37.0
30.0
34.0
n.a.
22.9
22.6
22.5
38.3
19.0
31.8
32.3
32.5
31.3
n.a.
30.1
n.a.
38.0
39.9
36.0
n.a.
n.a.
27.7

ÿ24.3
n.a.
n.a.
ÿ23.7
ÿ23.7
ÿ24.0
n.a.
n.a.
ÿ24.1
ÿ24.0
ÿ24.0
ÿ23.9
ÿ23.6
n.a.
ÿ23.7
ÿ23.4
ÿ23.8
ÿ24.5
ÿ24.0
n.a.
n.a.
n.a.
n.a.
n.a.
ÿ24.2
n.a.
ÿ23.6
ÿ24.3
ÿ23.9
n.a.
n.a.
ÿ23.7

37.5
36.8

ÿ23.4
ÿ23.3

Inertinite MG

Clays

Carbonates

Pyrite

SiO2

5.3

30.2

4.5

0.2

0.2

29.1
10.8
17.4

26.6
35.1
36.9

3.1
±
0.4

0.3
0.2
±

±
±
±

8.8
7.8
15.6
2.9
6.3

40.4
28.5
25.2
16.9
18.2

0.3
0.3
0.7
1.1
3.9

1.1
1.1
0.7
0.9
3.2

±
±
±
±
±

10.2
10.1
0.8
16.6
0.0

27.3
37.1
36.9
43.7
30.7

±
0.3
1.0
0.5
±

1.5
0.3
2.7
0.5
1.0

±
±
±
±
±

5.6

30.5

±

1.2

0.2

11.5
7.9
17.3

12.7
25.1
49.6

0.5
3.1
3.1

3.2
1.4
0.2

±
2.2
±

2.7

31.5

6.3
Not analysed
Not analysed
2.7
4.2
5.4
Not analysed
Not analysed
1.2
7.7
1.6
3.2
2.3
Not analysed
7.8
6.5
3.9
2.8
3.9
Not analysed
Not analysed
Not analysed
Not analysed
Not analysed
11.7
Not analysed
3.1
5.4
0.2
Not analysed
Not analysed
3.0

0.3

0.3

±

0.82
0.90
0.83
0.80
0.83
0.98
1.23
1.22
1.17
0.90
0.85
1.07
0.95
1.01
1.20
1.16
1.22
0.60
1.50
1.01
0.99
1.03
0.98
0.97
1.02
0.63
0.70
0.59
0.69
1.16
1.17
1.09

12.1
12.9

26.5
40.0

10.7
4.6

0.6
0.5

2.2
2.4

±
±

0.67
0.71

Data from Clayton et al. (1995a).
Abbreviations: MG, maceral group; daf, dry-and-ash-free; tr., traces; n.a., not analysed.

M.J. Kotarba / Organic Geochemistry 32 (2001) 163±180

Upper Silesian coal basin
A-5a
53.3
B-1/76
B-2/76
B-5a
38.2
B-7a
49.7
H-1a
39.9
J-50/75
J-1/76
J-5a
48.2
54.6
Kr-10a
Mi-1a
56.2
75.0
Mk-3a
Mk-5a
65.1
Ml-1/76
Mo-1a
53.2
Mo-2a
45.7
Mo-13a
54.7
NM-1a
35.9
Pa-1a
64.4
P-3/75
P-1/76
P-2/76
P-3/76
P-4/76
Pn-1a
50.8
S-2/76
S-10a
69.0
54.9
S-11a
We-1a
29.6
Z-53/75
Z-55/75
Z-4a
62.2

Exinite MG

Mean Vitrinite Re¯ectance
(%)

M.J. Kotarba / Organic Geochemistry 32 (2001) 163±180

source organic matter (e.g. Faber, 1987; Berner et al.,
1992; Berner and Faber, 1996). Circumstantial evidence
suggests (Claypool, 1999) that isotopically light ethane
(d13C of ÿ70 to ÿ50%) is formed in a process similar to
microbial methanogenesis.
The d13C(C2H6) values for the analysed gases from
the USCB vary from ÿ24.6 to ÿ22.3% and the
d13C(C3H8) value is ÿ24.7% (one sample) (Table 3;
Figs. 7 and 8). Both ethane and higher gaseous hydrocarbons from the analysed gases were generated during
the bituminous coal stage of the coali®cation process. A
signi®cant isotopic shift from the vitrinite re¯ectance
curves (Fig. 7) suggests the contribution of microbial
methane, and/or the in¯uence of physical processes
(di€usion and sorption). Small isotopic inversion
between stable carbon isotope compositions of ethane
and propane in the H-1 sample (Table 3, Fig. 8) may
suggest that thermogenic generation of hydrocarbons
was a multi-phase (at least two) process. Also, lack of
distinct correlation between the CHC hydrocarbon index
and the content of exinites (Fig. 9) suggests that ethane
and higher gaseous hydrocarbons were formed during
the thermogenic coali®cation process of associated
coals. However, methane, which was also generated
during this process, contains the microbial component
and/or the high-temperature thermogenic component
which migrated from deeper-seated coal seams.
4.3. Stable carbon isotopes of carbon dioxide
During the coali®cation process the intensity of carbon
dioxide generation decreases. On the basis of theoretical

Fig. 9. CHC hydrocarbon index from USCB and LCB coalbed
gases versus the content of exinite maceral group of coals in
which the gases have accumulated. E. exinite maceral group; V.
vitrinite maceral group; I. inertinite maceral group. USCB,
Upper Silesian Coal Basin; LCB, Lublin Coal Basin.

173

considerations (Galimov, 1985; Kotarba, 1988), it is
expected that generated carbon dioxide should be about
5±10% isotopically heavier in comparison with precursors (bulk plants, tropical grasses, humic coals and
type III kerogen). Hence, the d13C values of thermogenic carbon dioxide generated from humic organic
matter vary between ÿ25 and ÿ5% (Fig. 10). A similar
range of d13C(CO2) values (ÿ25 to ÿ10%) was given by
Schoell (1983). Gutsalo and Plotnikov (1981) claimed
that thermogenic CO2 has d13C values between ÿ30 and
ÿ16% although they did not provide the criteria for
generation of this carbon dioxide. The d13C values for
carbon dioxide associated with microbial methane vary
from about ÿ40 to about +20% (Whiticar et al., 1986)
(Fig. 10). The d13C values for endogenic carbon dioxide
are close to the mean value for elemental carbon in the
upper mantle, and varied from ÿ5 to ÿ9% (about ÿ7%
in average) (e.g. Fleet et al., 1998; Jenden et al., 1993).
In general, there is signi®cant overlap in the carbon
dioxide genetic ®elds (Fig. 10).
The d13C values of the analysed carbon dioxide from
the USCB vary from ÿ27.2 to ÿ2.8% (Table 3 and Fig.
10). The d13C values of analysed carbon dioxide from
the LCB are ÿ13.7 and ÿ11.9% (Table 3 and Fig. 10).
These d13C(CO2) values indicate that the carbon dioxide
accumulated within the Upper Carboniferous coalbearing strata of both the LSCB and LCB was generated during microbial methanogenesis and/or the bituminous stage of coali®cation. In the LCB, magmatic
events have not been observed, and in the USCB they
occur only sporadically. The maximum concentrations

Fig. 10. d13C(CO2) versus CDMI for coalbed gases from USCB
and LCB. Brackets show ranges of d13C values for CO2 originating from various sources modi®ed from Jenden et al. (1993).
USCB, Upper Silesian Coal Basin; LCB, Lublin Coal Basin.

174

M.J. Kotarba / Organic Geochemistry 32 (2001) 163±180

of CO2 in coalbed gases reach 16.7% (Table 3; Fig. 10).
Thus, the in¯ux of endogenic carbon dioxide to the
Upper Carboniferous coal-bearing strata in both basins
seems to be rather doubtful.
4.4. Microbial gases
A relatively new concept (Kotarba, 1988, 1990; Smith
et al., 1992; Rice, 1993; Scott et al., 1994; Smith and
Pallasser, 1996) is the recognition of secondary, microbial methane and carbon dioxide generation processes
within coal basins. Microbial methane can be produced
during fermentation and/or reduction of carbon dioxide
(e.g. Zhang and Chen, 1985; Whiticar et al., 1986).
Although the reactions mentioned above are known
from both the marine and fresh-water sediments, fermentation predominates in fresh-water environments,
mainly within young, shallow sediments, whereas carbon dioxide reduction prevails in the marine, sulphatefree zone, mainly within deeper, older sediments (Whiticar et al., 1986; Rice, 1992). In microbial methane
connected with fermentation, d13C values vary from
ÿ65 to ÿ50% and dD values from ÿ400 to ÿ250%. For
CO2-reduction d13C values vary from ÿ110 to ÿ65%
and dD values from ÿ250 to ÿ170% (Whiticar et al.,
1986; Rice, 1992). Details of the microbial generation of
gases were described by Zhang and Chen (1985), Whiticar et al. (1986), Rice (1992) and Sugimoto and Wada
(1995).
Microbial gas produced during biochemical coali®cation (peat and lignite stages) cannot be retained in large
volumes within the coal structure because of the insigni®cant gravitational compaction of the clayey-muddy,
low-permeability cover and the intense natural degassing processes. Moreover, during peati®cation, the
microstructure of coal was not suciently developed for
accumulation of gases within the structure of coaly
matter (e.g. Kotarba, 1988).
Diagrams of genetic classi®cations (Figs. 4 and 5)
indicate that the methane was formed mainly by microbial CO2-reduction. The isotopic di€erence
13C(CO2±
CH4) is about 60% for the microbial gases (Claypool
and Kaplan, 1974). Hence, coalbed gases from Mo-1,
Mk-5 and Pn-1 samples are mostly genetically linked to
microbial methanogenesis (Table 3). In the southern
part of the USCB, where Upper Carboniferous coalbearing strata are sealed by Miocene clayey-sandstone
.
overburden, paleometeoric brines occur (Rozkowski
.
and RudzinÂska-Zapas nik, 1983; Rozkowski, 1994; Pluta
and Zuber, 1995). Methanogens probably tolerate such
very saline brines (Gerling et al., 1995). In this southern
region, 31 gas samples collected from Upper Carboniferous coal-bearing strata contain methane at a level of
over 2 dm3/kg coaldaf (Kotarba et al., 1995). Isotope
.
and hydrochemical investigations (Rozkowski and
RudzinÂska-Zapas nik, 1983; Zuber and Grabczak, 1985)

reveal that saline waters occurring within Carboniferous
strata of the LCB are also of paleometeoric origin (older
than Pleistocene). They occur in a hydrogeologic environment isolated from present-day in®ltration waters
.
(Rozkowski and RudzinÂska-Zapas nik, 1983). Thus, the
process of recent microbial methane generation from
nutrients supplied from the surface into the Upper Carboniferous coal-bearing strata of the southern part of
the USCB and LCB is considered to be impossible. In
the USCB increased amounts of isotopically lighter
methane at the top of the Upper Carboniferous
sequence, beneath the low-permeable Miocene (Karpatian-Badenian) sediments (Figs. 11 and 12), may result
from microbial reactions. Hence, microbial CO2-reduction (Fig. 5) in the USCB had to proceed after deposition of a sealing Miocene cover, i.e. between Karpatian
and recent time. In the LCB the process has to take
place before the Pleistocene. However, the question
arises whether the amounts of nutrients were sucient
for generation of such amounts of isotopically light
methane (Figs. 11 and 12).
In the central part of the USCB, where Upper Carboniferous coal-bearing strata are covered only by
Quaternary sediments, and in the northern part of the
basin where permeable Permian-Jurassic strata are preserved (thickness less than 200 m), the recent in¯ow of
.
fresh, meteoric waters is possible (Rozkowski and RudzinÂska-Zapas nik, 1983; Pluta and Zuber, 1995). In this
region, Upper Carboniferous coal-bearing strata have
insigni®cant amounts of gases at depths less than 600
meters. Four gas samples were collected here at depths

Fig. 11. Stable carbon composition of methane of USCB coalbed gases versus depth (a) below surface and (b) below top of
Upper Carboniferous strata and base of Miocene caprock.

M.J. Kotarba / Organic Geochemistry 32 (2001) 163±180

175

Fig. 12. Methane content versus depth in the coal seams of the selected wells in the Upper Silesian coal basin after Kotarba et al.
(1995). Locations of wells cf. Fig. 2. U.C.c-b.s.- Upper Carboniferous coal-bearing strata.

between 670 and 920 m (H-1, Mi-1, NM-1 and We-1,
see Table 1 and Fig. 2). Methane-producing and other
gas-producing microorganisms were frequently present
in coals and associated waters. Simultaneously, these
waters also carried nutrients for the microorganisms
(alcohols, organic acids, phenols, etc.). Within the Upper
Carboniferous coal-bearing strata of both the USCB
and LCB, hydrogeological conditions do not appear to
be favourable for recent generation of microbial methane from nutrients supplied from the surface. The
abandoned mine workings provide favourable conditions for anaerobic decomposition of cellulose and generation of both methane and carbon dioxide. Hence,
this type of microbial methane can occur in local zones
of the central and northern parts of the USCB only near
old, abandoned mine workings where timber linings
were used, e.g. near Mi-1 gas collecting site in ``Miechowice'' mine (Tables 1 and 3). In other (H-1, NM-1
and We-1) gas collecting sites of the central and northern parts of the USCB, the thermogenic component
dominates.
4.5. Thermogenic gases
Recent knowledge on generation mechanisms of gas
from coals and type III kerogen during the thermogenic
process was summarised by Rice (1993), Berner and
Faber (1996), Whiticar (1996) and Clayton (1998). For
instance, d13C-values of methane generated during this
process increase from ÿ36 to ÿ22%, depending on
maturity of source organic matter and on generation
models. The relationship between d13C values of methane

and vitrinite re¯ectance is given by several formulae
(e.g. Stahl and Carey, 1975; Schoell, 1983; Faber, 1987;
Galimov, 1988; Berner et al., 1992; Berner and Faber,
1996). Thermogenic methane generated during coali®cation processes in western Germany coal basins reveals
a gradual increase of d13C(CH4) values from ÿ31% at
vitrinite re¯ectance Ro= 0.6% to ÿ22% at Ro=2.5%
(Stahl, 1977; Schoell, 1983). A separate mechanism of
gas generation during coali®cation process within the
seams has been proposed by Smith et al. (1982, 1985).
Isotopically light methane was initially interpreted to be
produced by thermogenic decomposition of higher nalkanes which formed at the initial coali®cation stage of
bituminous coals and were subsequently trapped within
the microporous structure of the coals. However, this
isotopically light methane was later considered by Smith
et al. (1992) and Smith and Pallasser (1996) to be of
microbial origin.
Methane, ethane, other gaseous hydrocarbons (C3±
C5) and carbon dioxide occurring in the coalbed gases in
both the USCB and LCB were generated during the
bituminous stage of the coali®cation process. In both basins
this process lasted no longer than several Ma, being completed at the end of Variscan orogeny, during the Asturian
and Leonian orogenic phases (at the turn of Carboniferous
and Permian). The volume of gases generated in this
process exceeds by a few times the sorption capacity of
coals (Kotarba, 1979, 1988; Kotarba and Lewan, 1998).
Thus, most of these gases escaped from the source coals.
Signi®cant depletion in 13C isotope in methane generated from coal seams can be explained by the reactions occurring during coali®cation (e.g. Friedrich and

176

M.J. Kotarba / Organic Geochemistry 32 (2001) 163±180

JuÈntgen, 1972, 1973; Smith et al., 1985) or by microbial
generation (Kotarba, 1988, 1990; Smith et al., 1992;
Rice, 1993; Scott et al., 1994). However, the deviations
of isotope compositions of both the USCB and LCB
coalbed gases from the genetic ®elds and curves on
classi®cation diagrams (Figs. 4±8) can also be interpreted as due to sorption and di€usion processes.
4.6. Migration and mixing processes
Secondary isotope fractionation is known to occur in
the gas-rock systems during migration from source to
reservoir rocks. A signi®cant role is attributed to di€usion (May et al., 1968; Bondar, 1987; Pernaton et al.,
1996; Prinzhofer and Pernaton, 1997; Prinzhofer et al.,
1999). Theoretical calculations indicate that, as a result
of di€usion in the homogeneous rock complex, methane
can be either enriched (up to 5%) or depleted (up to
100%) in 13C (Bondar, 1987). It is suggested that the
multiple sorption-desorption processes taking place
during migration of methane through microporous coal
structure may cause strong isotope fractionation.
Methane released early during desorption can be depleted in 13C isotope as much as 30% in comparison to the
total sorbed methane (Wingerning and JuÈntgen, 1977).
Similar depletion of methane in the heavy hydrogen
isotope is also observed during sorption-desorption and
di€usion processes (Smith et al., 1985; Pernaton et al.,
1996). In the last few years both theoretical and experimental research has been undertaken in France on second-order e€ects (di€usion, adsorption) causing postgenetic isotope fractionation in gases (Pernaton et al.,
1996; Prinzhofer and Pernaton, 1997; Caja et al., 1999;
Prinzhofer et al., 1999).
``Free'' methane in coal-bed gases from the USCB is
depleted in 13C, in comparison with ``adsorbed'' methane. Isotopic di€erences (
13C) between ``free'' and
``adsorbed'' methane vary from 3.2 to 10.3% (average
6.8%) and is independent of maturity of associated
coals (Table 4). A similar isotopic shift between ``free''
and ``adsorbed'' methane (average
13C about 5%) in
German basins was ascribed to selective desorption of
12
CH4 from coals (Colombo et al., 1970).
The uplift resulting from both the Asturian and Leonian orogenic phases led to the formation of the USCB
and development of a fault system. After the uplift, the
Upper Carboniferous coal-bearing formations were
exposed over most of the basin and subjected to erosion
and denudation. Later on, natural degassing of coals
took place by both convection (e€usion) and di€usion.
In the central and northern parts of the Basin this process lasted from 290 Ma to recent. In the southern
region where low-permeable Miocene (Karpatian-Badenian) was laid down, the process occurred between 290
and up to about 15 Ma before present. During this period, large volumes of thermogenic coalbed gases gener-

ated within the coal seams were released to the
atmosphere. The depth range of intensive, natural
degassing of coal beds was dependent on the reservoir
parameters (mainly permeability) of the Upper Carboniferous coals and sandstones, the lack or presence of
low-permeable overburden and the presence of fractures
and faults related to tectonic zones. Taking into
account: (i) isotopic data from the USCB, (ii) the d13C
value of indigenous (autochthonous) methane generated
from coals, which is about ÿ30% (e.g. Whiticar, 1996),
and (iii) linear regression of the depth/d13C(CH4) relationship (Fig. 11b), the depth range of the natural
degassing zone is estimated to be about 1000 m below
the top of Upper Carboniferous coal-bearing strata. The
spread of isotope values seen in Fig. 11 can be explained
by the in¯uence of local, tectonic and lithofacial disturbances of gas migration pathways. The zone of
intensive desorption is about 400 meters beneath the top
of Carboniferous sequence. Beneath this desorption
zone, indigenous thermogenic gases dominate. Results
of isotopic studies allow one also to distinguish the natural degassing zone in the Saar and Ruhr Districts in
Germany (TeichmuÈller et al., 1968; Colombo et al.,
1970) which belong to the same type of Variscan coal
basins as the USCB.
The correlation between d13C(CH4) values and the
depth of associated coals below the top of Upper Carboniferous coal-bearing strata (Fig. 11b), and the less
distinct correlation between d13C(CH4) values and the
depth of associated coals below the surface (Fig. 11a), as
well as the depth distribution of methane content (Fig.
12), all indicate the good sealing properties of the Miocene cover. Since the end of Variscan orogeny (about
290 Ma ago) to the time of deposition of the Miocene
caprock (about 15 Ma ago), the uplifted coal beds were
subjected to intensive degassing. Beneath the base of
Miocene overburden, the thermogenic gases generated
in deeper seams were accumulated.
A similar process probably took place in the LCB,
though the sealing properties of overlying Jurassic carbonates and Cretaceous chalks are not as good as those
of the Miocene clayey sediments in the USCB. The period of natural degassing of Upper Carboniferous coalbearing strata was shorter, from about 290 to 170 Ma,
when sedimentation of Middle Jurassic cover began.

5. Conclusions
The results of analyses of molecular and isotopic
compositions of coalbed gases of the Upper Silesian and
Lublin basins related to geological and hydrogeological
history of both basins reveal that:
1. In both the USCB and LCB the coalbed gases
(methane, higher gaseous hydrocarbons and carbon

M.J. Kotarba / Organic Geochemistry 32 (2001) 163±180

2.

3.

4.

5.

6.

dioxide) were generated during thermogenic and
probably microbial processes, followed by migration and mixing.
Methane, small amounts of higher gaseous
hydrocarbons (C2±C5) and carbon dioxide forming the coalbed gases in both the USCB and LCB
were generated during the bituminous stage of the
coali®cation process. In both basins this process
lasted no longer than several Ma, being completed
at the end of Variscan orogeny, during the Asturian
and Leonian orogenic phases (at the turn of
Carboniferous and Permian).
The uplifted coal beds were subjected to erosion,
denudation and intensive degassing. In the
central and northern parts of the USCB, degassing
has proceeded from the end of Variscan orogeny
(about 290 Ma ago) to recent. In the southern
and southwestern parts of the USCB the process
has started at the same time and lasted until
deposition of low-permeable Miocene cover
(about 15 Ma ago). Natural degassing of coals
took place by both convection (e€usion) and
di€usion. Large volumes of previously generated
thermogenic coalbed gases were released to the
atmosphere.
The depth range of intensive, natural degassing of
coal beds depends on: (i) the reservoir parameters
(mainly permeability) of the Upper Carboniferous
coals and sandstones, (ii) the lack or presence of
low-permeable overburden, and (iii) the presence
of fractures and faults. In the southern and
southwestern parts of the USCB the depth range
of the natural, intensive degassing zone is estimated to be about 400 metres and extends down
to about 1000 m. In central and northern parts of
the USCB where sealing sedimentary caprocks are
absent, the depth range of degassing zone is even
larger. Down to the depth of 1000 m coalbed
gases occur only locally and in minor amounts
(less than 1 dm3 CH4/kg of coaldaf). Beneath this
desorption zone, indigenous (autochthonous)
thermogenic gases dominate.
In the southern and southwestern parts of the
USCB, below the sealing Miocene cover the secondary accumulation zone of isotopically light
methane (d13C ÿ80 to ÿ60%) occurs. Probably,
this methane was generated during microbial
reduction of carbon dioxide which took place
after the Miocene. The nutrients for microbial
methanogenesis had to be supplied to the Upper
Carboniferous strata already, before the Miocene
deposition. Therefore, the conditions suitable for
such intensive microbial processes are problematic.
The process of recent generation of microbial
methane from nutrients supplied from the surface

7.

8.

9.

10.

177

into the Upper Carboniferous coal-bearing strata
of both USCB and LCB seems to be impossible
from a hydrogeological point of view. This type of
microbial methane can occur in local zones of the
central and northern parts of the USCB only near
old, abandoned mine workings where timber linings were used.
It cannot be excluded, however, that depletion of
13
C in methane resulted from di€usion and adsorption-desorption processes during migration through
microporous coal structure. Thus, the accumulation
zone beneath the Miocene cover contains the
allochthonous, thermogenic gases which migrated
from deep coal seams at a depth of 1000 m below
the top of Upper Carboniferous strata.
A similar process probably took place in the LCB.
The period of natural degassing of Upper Carboniferous coal-bearing strata was shorter, from
about 290 to 170 Ma, until deposition of the
Middle Jurassic cover began.
Small amounts of carbon dioxide accumulated
within the Upper Carboniferous coal-bearing
strata of both the LSCB and LCB were generated
during the bituminous stage of coali®cation and/
or are associated with microbial methanogenesis.
A de®nite explanation of depletion of 13C in the
coalbed methane in the USCB and LCB requires
further microbiological studies as well as theoretical considerations and empirical research on
isotope fractionation during physicochemical and
physical processes.

Acknowledgements
The research was undertaken as part of AmericanPolish geochemical studies of coals, carbonaceous shales
and associated gases ®nanced by the Joint Maria Sk•odowska-Curie II Fund (grant MEN/USGS-91-62). The
author appreciates very much the valuable comments of
J.L. Clayton and D.D. Rice from the US Geological
.
Survey, Denver, and of W. Mayer, K. RozanÂski and A.
Zuber from the University of Mining and Metallurgy,
Krakow. P. Gerling and a second anonymous reviewer
gave very constructive reviews which greatly improved
the discussion and the possible consequences of the
hypotheses discussed in the manuscript. Thanks are due
to Z. Rakowski from Ostrava for enabling the collection
of gas sample from PetrÆ kovice Beds (Namurian A,
group of seams 900) which are inaccessible in the Polish
part of the USCB. Sincere t