Composition and origin of coalbed gases in the Upper Silesian and Lublin basins, Poland

Organic Geochemistry 32 (2001) 163±180

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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: [email protected]

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

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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

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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

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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 <2 mm over 2 h. Grinding was performed in the same container in a shaker. After grinding, the samples were degassed with a vacuum apparatus (Borowski, 1975), and desorbed gas was collected in glass vessels. The terms ``free'' and ``adsorbed'' gases are used conventionally. The ``free'' gas includes both the volatiles ®lling the pores and cracks (fractures/cleats) within the coal structure and some gas desorbed from the coal during drilling and sampling. It is well-known that a signi®cant part of the gas is sorbed within the ultra-®ne structure of the coals (e.g. Kotarba, 1988; Yee et al., 1993). The ``adsorbed'' gas does not represent the whole

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Table 2 Stable carbon isotope composition of methane from the Upper Silesian basin related to sampling location within coal seam, time of sampling and physiochemical type of accumulation Sample code

Thickness (m)

Sampling location within coal seam

d13C(CH4) (%) ``Free'' gases Time of sample collecting after drilling

B-1/76

2.6

B-2/76 J-1/76 P-3/75 P-1/76

1.2 1.7 1.2 1.8

P-2/76

1.2

P-3/76

1.2

a b c d e

Roof Bottom Roof Roof Bottom Roof Bottom Roof Bottom Roof

2 min

20 min

ÿ47.1 ÿ47.3 n.a. ÿ74.5 ÿ71.9 ÿ69.8 ÿ70.3 ÿ69.2 ÿ70.6 ÿ69.0

ÿ48.1 ÿ47.9 ÿ56.2 n.a. n.a. ÿ71.2 ÿ69.7 n.a. ÿ70.2 ÿ69.0

``Adsorbed'' gases

ÿ36.8 n.a. ÿ49.9 ÿ71.3 ÿ65.0 ÿ62.8 n.a. ÿ63.2 ÿ64.9 ÿ60.2


13C (r2-b2)a (%)


13C (r20-b20)b (%)


13C (2-20)c (%)


13C (fr2-ads)d (%)

ÿ0.2

0.2

ÿ1.0 ÿ0.6

10.3

ÿ0.5

0.5

ÿ1.4 0.6

ÿ1.4

0.4 0.0

6.7e 3.2 6.9 7.0 6.0 5.7 8.8


13C (r2-b2)=d13C (roof, after 2 min) Ð d13C (bottom, after 2 min).
13C (r20-b20)=d13C (roof, after 20 min) Ð d13C (bottom, after 20 min).
13C (2-20)=d13C (after 2 min) Ð d13C (after 20 min).
13C (fr2-ads)=d13C (``free'' gases, after 2 min) Ð d13C (``adsorbed'' gases).
13C (fr20-ads)=d13C (``free'' gases, after 20 min) Ð d13C (``adsorbed'' gases).

molecules adsorbed on coal under natural conditions, as some portion of the gas is expelled from samples during sampling and pumping out of air prior to grinding. 3.2. Analytical procedure The molecular composition of natural gases was analysed in a set of columns on Hewlett Packard 5990, Chrom-5 and Chrom-4 gas chromatographs with ¯ame ionization and thermal conductivity detectors. Stable isotope analyses were performed using Finnigan Delta, Micromass MM 602C, Varian MAT 230, and MI-1201 mass spectrometers. The stable carbon and hydrogen isotope data are presented in d-notation, relative to the PDB and the SMOW standards, respectively. The analytical precision is  0.2% for carbon and  3% for hydrogen. Methane, ethane, propane and carbon dioxide were separated chromatographically for stable isotope analyses. Methane, ethane and propane were, in turn, combusted over hot copper oxide at 850 C. The resulting CO2 fraction was puri®ed and directly analysed for 13C content, whereas the water resulting from the combustion of methane was reduced to gaseous hydrogen using the uranium or zinc method. Measurements of vitrinite re¯ectance (Ro) were carried out with the OPTON microphotometer at a wavelength of

546 nm, in oil. Volatile matter content (VMdaf) for the dry-and-ash-free state was analysed according to the procedure recommended by the Polish Standard (PN66/G-04516). 4. Results and discussion Geochemical indices and stable isotope ratios for ``free'' coalbed gases from the USCB vary within the following ranges (Table 3): 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 ÿ24.6 to ÿ22.3%; d13C(CO2) from ÿ27.2 to ÿ2.8%; and d13C(C3H8) is ÿ24.7% (one sample). d13C-values of methane of ``adsorbed'' gases vary from ÿ71.3 to ÿ36.8% (Table 2). Only two ``free'' coalbed gases were collected from the LCB (Table 3). Geochemical indices and stable isotope ratios for these samples show the following values: CH4/(C2H6+C3H8) hydrocarbon indices above 10,000, CDMI carbon dioxide-methane indices are 4.3 and 34.6%; d13C(CH4) values are ÿ67.3 and ÿ52.5%; dD (CH4) is ÿ201% (one sample); and d13C(CO2) values are ÿ13.7 and ÿ11.9%.

Table 3 Molecular and isotopic composition of coalbed gases Molecular composition (vol.%) Sample code

CH4

C2H6

Stable isotopes (%)

C3H8

C4±C5b

CO2

N2

Ar

He

H2

CHCc

CDMId

d13C (CH4)

dD (CH4)

d13C (CO2)

d13C (C2H6)


13C (CO2±CH4)e

± ± ± ± ± ± 0.15 ± ± ± ± 0.007 ± 0.0006 ± ± ± ± ± 0.05 ± ± ± ± ± ± ± ± 0.003 ± ± ± ± ± ±

± ± ± ± tr. ± 0.02 ± ± ± ± 0.002 ± ± ± ± ± ± ± 0.007 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.98 0.30 0.30 0.29 1.77 1.07 3.20 ± ± 0.36 1.52 16.7 3.2 3.0 0.7 2.34 0.43 0.23 1.88 0.83 0.6 ± ± 1.4 1.3 ± ± 1.13 ± 0.67 0.32 2.1 0.2 ± 0.57

3.9 3.8 3.8 4.8 0.77 2.7 2.0 5.4 2.0 1.8 2.6 20.0 21.3 15.4 1.8 6.2 2.6 8.0 7.5 3.0 7.3 1.7 1.7 1.7 1.9 1.8 2.1 4.8 4.8 9.6 2.9 0.93 2.2 1.0 1.0

0.07 0.01 ± 0.02 0.01 0.01 0.02 0.03 0.01 0.04 0.02 0.18 0.20 0.13 0.08 0.16 0.04 0.01 0.04 0.03 ± 0.01 ± 0.01 0.01 ± ± 0.04 0.03 0.08 0.03 0.03 ± ± 0.01

0.07 ± ± 0.06 tr. 0.12 0.03 ± ± ± 0.02 0.04 0.35 0.04 0.06 ± ± ± 0.35 tr. 0.07 0.02 0.07 0.20 0.18 0.03 0.01 0.01 0.01 0.14 0.01 0.02 0.21 ± ±

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± tr. ± ± ± ± ± ± ±

1,280 >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)

M.J. Kotarba / Organic Geochemistry 32 (2001) 163±180
13C (C2H6±CH4)f

170

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 39.9 H-1a 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 45.7 Mo-2a Mo-13a 54.7 35.9 NM-1a 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 thanks are directed to E.M. Galimov who has introduced me to the secrets of isotopic research as early as the 70-ties and enabled the stable carbon isotope measurements in methane for samples Nos. 75 and 76. Technical assistance and help in the sampling procedure by Mrs. Z. Stecko, Mr. A.

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