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GEOLOGICA CARPATHICA
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, AUGUST 2012, 63, 4, 307—318 doi: 10.2478/v10096-012-0024-4
Origin of natural gases in the Paleozoic-Mesozoic basement
of the Polish Carpathian Foredeep
MACIEJ J. KOTARBA
AGH University of Science and Technology, Faculty of Geology, Geophysics and Environmental Protection, Al. Mickiewicza 30,
30-059 Kraków, Poland; kotarba@agh.edu.pl
(Manuscript received November 22, 2011; accepted in revised form March 13, 2012)
Abstract: Hydrocarbon gases from Upper Devonian and Lower Carboniferous reservoirs in the Paleozoic basement of
the Polish Carpathian Foredeep were generated mainly during low-temperature thermogenic processes (“oil window”).
They contain only insignificant amounts of microbial methane and ethane. These gaseous hydrocarbons were generated
from Lower Carboniferous and/or Middle Jurassic mixed Type III/II kerogen and from Ordovician—Silurian Type II
kerogen, respectively. Methane, ethane and carbon dioxide of natural gas from the Middle Devonian reservoir contain a
significant microbial component whereas their small thermogenic component is most probably genetically related to
Ordovician—Silurian Type II kerogen. The gaseous hydrocarbons from the Upper Jurassic and the Upper Cretaceous
reservoirs of the Mesozoic basement were generated both by microbial carbon dioxide reduction and thermogenic
processes. The presence of microbial methane generated by carbon dioxide reduction suggests that in some deposits the
traps had already been formed and sealed during the migration of microbial methane, presumably in the immature
source rock environment. The traps were successively supplied with thermogenic methane and higher hydrocarbons
generated at successively higher maturation stages of kerogen. The higher hydrocarbons of the majority of deposits
were generated from mixed Type III/II kerogen deposited in the Middle Jurassic, Lower Carboniferous and/or Devo-
nian strata. Type II or mixed Type II/III kerogen could be the source for hydrocarbons in both the Tarnów and Brzezówka
deposits. In the Cenomanian sandstone reservoir of the Brzezowiec deposit and one Upper Jurassic carbonate block of
the Lubaczów deposit microbial methane prevails. It migrated from the autochthonous Miocene strata.
Key words: Paleozoic-Mesozoic basement, Polish Carpathian Foredeep, isotope geochemistry, microbial hydrocarbon
gases, thermogenic hydrocarbon gases, carbon dioxide origin, nitrogen origin.
Introduction
This paper presents the results of molecular analyses, stable
carbon isotope analyses of methane, ethane, propane, butanes,
pentanes and carbon dioxide, stable hydrogen isotope analy-
ses of methane, and stable nitrogen isotope analyses of gas-
eous nitrogen of natural gases accumulated within the
Paleozoic-Mesozoic basement of the Polish Carpathian Fore-
deep between Kraków and the Polish-Ukrainian state border
(Fig. 1). These results are related to the geological setting and
the geochemical characterization of dispersed organic matter
hosted in the autochthonous Miocene strata (Kotarba et al.
1998, 2005) as well as in the Middle Jurassic (Kosakowski et
al. 2012a,b), Lower Carboniferous, Devonian and Ordovician-
Silurian strata (Kotarba et al. 2011; Więcław et al. 2011,
2012) of the Carpathian Foredeep. Interpretation of these data
is aimed at explaining the conditions of generation, migration
and accumulation of natural gases within these strata.
Previous molecular and isotopic studies of natural gases
accumulated within the autochthonous Miocene strata of the
Polish and Ukrainian Carpathian Foredeep revealed that the
methane, the dominating component of these gases, was gen-
erated by microbial processes (Głogoczowski 1976; Shabo &
Mamchur 1984; Kotarba et al. 1987, 2005; Kotarba 1992,
1998, 2011; Jawor & Kotarba 1993; Kotarba & Jawor 1993;
Kotarba & Koltun 2006). In contrast, natural gases accumu-
lated within the Paleozoic-Mesozoic basement of the Polish
Carpathian Foredeep represent various genetic types: (i) mi-
crobial, (ii) low-temperature, thermogenic gases associated
with oil and condensate, and (iii) high-temperature, thermo-
genic, non-associated gases. Typical microbial gases, which
have migrated from the autochthonous Miocene strata, are ac-
cumulated in the Upper Jurassic carbonate reservoir of one
block of the Lubaczów field and in the Cenomanian sandstone
reservoir of the Brzezowiec field (Jawor & Kotarba 1991,
1993; Kotarba & Jawor 1993; Kotarba & Koltun 2006).
Geological setting and petroleum occurrence
The Polish Carpathian Foredeep consists of three structural
complexes: (i) Precambrian-Paleozoic-Mesozoic basement,
(ii) folded Zgłobice and Stebnik units, and (iii) autochthonous
Miocene strata. The Carpathian Foredeep is divided into two
basins: outer and inner (Oszczypko 1997). The eastern part
of the outer basin, between Kraków and Przemyśl (Fig. 1) is
filled with Badenian and Lower Sarmatian, mostly sandy-
clayey sediments of total thickness up to 4500 m. The auto-
chthonous Miocene sediments of the outer basin were
affected by Alpine orogenic movements and rest almost
horizontally upon the Precambrian-Paleozoic-Mesozoic
basement (Oszczypko 1997; Oszczypko et al. 2006).
The Paleozoic-Mesozoic basement comprises two subunits:
(i) Paleozoic basement, which includes Caledonian and
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Variscan orogens, and (ii) Mesozoic basement (the so-called
Carpathian Foreland platform) built of Triassic, Middle—Up-
per Jurassic and Cretaceous strata. The Paleozoic-Mesozoic
basement has highly diversified lithology, which reflects
variable depositional environments, large stratigraphic and
angular unconformities, and erosional morphology of the up-
per surface (Jawor 1970; Karnkowski 1999; Buła & Habryn
2011; Krajewski et al. 2011).
The Precambrian-Paleozoic-Mesozoic basement of the Car-
pathian Foredeep includes four main structural stages associ-
ated with strong and diversified diastrophic episodes: (i) Upper
Proterozoic stage (Assynthian orogenic phase), (ii) Cambrian-
Silurian stage (Caledonian orogeny), (iii) Devonian-Carbonifer-
ous stage (Variscan orogeny), and (iv) Zechstein-Mesozoic
stage (Laramide orogenic phase) (Karnkowski 1999). Out-
lines of tectonics and lithostratigraphic sequence of the
Paleozoic/Mesozoic basement in the study area were de-
scribed by Buła & Habryn (2011) and Krajewski et al.
(2011). The generalized lithostratigraphic column of the
Paleozoic-Mesozoic basement was presented in a number of
publications, including Karnkowski (1999), Kotarba &
Koltun (2006: Fig. 15) and Kotarba et al. (2011: Fig. 2).
The major petroleum reservoir rocks in the Paleozoic-
Mesozoic basement are the following lithostratigraphic units
(Karnkowski 1999): (i) Middle and Upper Devonian carbon-
ates (Lachowice deposit, Trzebownisko-Krasne and Zalesie
accumulations), (ii) Lower Carboniferous carbonates (Nosówka
deposit), (iii) Triassic sandstones (Niwiska deposit), (iv) Malm
limestones (Tarnów, Lubaczów, Korzeniów, Partynia-
Podborze, Dąbrowa Tarnowska and Smęgorzów deposits),
(v) Cenomanian sandstones alone (Brzezowiec, Grobla, Łąkta
and Rylowa deposits), (vi) Upper Cretaceous (Turonian-
Fig. 1. Sketch map showing the major tectonic units of the Polish Carpathian region with the gas sampling locations. Names and codes of
sampled wells are listed in Table 1. OCF – outer part of the Carpathian Foredeep; ZG – Zgłobice Unit; ST – Stebnik (Sambir) Unit;
OC – the Outer (Flysch) Carpathians; PKB – Pieniny Klippen Belt.
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Senonian) sandstones locally intercalated with Upper Creta-
ceous marls, Malm limestones and Cenomanian sandstones
(Jastrząbka Stara and Swarzów deposits).
The traps in oil and gas deposits of the Paleozoic/Mesozoic
basement can be sealed either by the Miocene strata alone
(e.g. Dąbrowa Tarnowska, Smęgorzów deposits), or by the
Miocene cover and the Upper Cretaceous marls (e.g. Grobla,
Rylowa, Wierzchosławice deposits). Moreover, the flysch
strata of the Outer Carpathians provide an additional seal
(e.g. Leszczyna-Łąkta) (Kotarba & Jawor 1993).
Methodology
Gas sampling procedure
Twenty-two natural gas samples were collected in the study
area from the following producing wells drilled into Upper
and Middle Devonian, Lower Carboniferous, Upper Jurassic
and Upper Cretaceous reservoirs of the Paleozoic-Mesozoic
basement (Table 1; Fig. 1). Free gases were collected directly
at the producing wellheads into metal containers (volume
~ 1000 cm
3
) and gases dissolved in oils were taken from sepa-
rators to glass containers (volume ~ 500 cm
3
) (Table 1). For
interpretation, the results of molecular and isotopic analyses
U. – Upper; M. – Middle; L. – Lower; Cenom. – Cenomanian (Upper Cretaceous); Ss – sandstones; Cong. – conglomerates; Carb. – Car-
bonates; Sh. – shales (mudstones & claystones); # – after Jawor & Kotarba (1993); ^ – after Kotarba (1998); * – after Kotarba & Jawor
(1993); ** – after Kotarba et al. (2004); *** – after Kotarba & Nagao (2008).
Table 1: Information on gas sample sites in the Paleozoic-Mesozoic basement.
Well
Field
Sample
Lithology of reservoir
Age of reservoir
Depth (m)
Brzezowiec-3
#
Brzezowiec
Bc-3
#
Ss Cenom.
1346–1352
Grobla-40
Grobla-Pławowice
Ga-40
Ss Cenom.
639–650
Grobla-109
Grobla-Pławowice
Ga-109
Ss Cenom.
712–721
Grobla-49
Grobla-Pławowice
Ga-49
Ss Cenom.
750–754
Grobla-89
Grobla-Pławowice
Ga-89
Carb. & Cong.
Turonian & Cenom.
664–702
Jastrząbka Stara-6
Jastrząbka Stara
JS-6
Marls-Carb.-Ss
Turonian & Cenom.
1295
Jastrząbka Stara-15
Jastrząbka Stara
JS-15
Marls-Carb.-Ss
Turonian & Cenom.
1290
Leszczyna-24*
Łąkta
Ln-24*
Ss Cenom.
2268–2282
Rylowa-3* Rylowa
Ry-3*
Ss Cenom.
1035–1038
Ż
ukowice-21
Ż
ukowice
Zu-21
Ss
Cenom.
1320–1322
Brzezówka-12 Brzezówka
Bw-12 Carb. U.Jurassic
1958–1985
Brzezówka-23 Brzezówka
Bw-23 Carb. U.Jurassic
1930–1953
Brzezówka-24*** Brzezówka
Bw-24*** Carb.
U.Jurassic
1892–1913
Brzezówka-25 Brzezówka
Bw-25 Carb. U.Jurassic
1902–1922
Dąbrowa Tarnowska-11*
Dąbrowa Tarnowska
DT-11*
Carb.
U.Jurassic
688-698
Góra Ropczycka-1K
Czarna Sędziszowska GR-1K Carb.-Marls
U.Jurassic
2040–2054
Góra Ropczycka-2
Czarna Sędziszowska GR-2
Carb.
U.Jurassic
2049–2057
Grobla-36 Grobla
Ga-36
Carb.
U.Jurassic
766–781
Lubaczów-22^ Lubaczów
Lb-22^
Carb. U.Jurassic
1020–1045
Łapanów-1
Łapanów
Lp-1
Carb. U.Jurassic
1772–1776
Łąkta-27
Łąkta
Lk-27
Carb. U.Jurassic
2258–2268
Smęgorzów-3*
Smęgorzów Sg-3*
Carb.
U.Jurassic
473–470
Tarnów-5 Tarnów
Ta-5
Carb.
U.Jurassic
1650–1662
Tarnów-17***
Tarnów
Ta-17***
Carb.
U.Jurassic
1644–1670
Tarnów-23 Tarnów
Ta-23
Carb.
U.Jurassic
1660–1670
Korzeniów-15 Korzeniów
Ke-15 Carb. U.Jurassic
1340–1375
Wierzchosławice-5*
Łętowice-Wierzchosławice
Wi-5*
Carb.-Marls U.Jurassic
1450–1486
Zagórzyce-6 Zagórzyce
Ze-6
Carb.-Marls
U.Jurassic
2817–2871
Ż
ukowice-11
Ż
ukowice Zu-11
Carb.-Marls
U.Jurassic
1292–1295
Zalesie-8 Zalesie
Zl-8
Carb.
U.Devonian
2765–2800
Trzebownisko-3 Krasne
To-3 Carb.
M.Devonian
2003–2065
Nosówka-1**
Nosówka
Na-1**
Ss-Sh.
L.Carboniferous
3465–3540
of ten natural gases from the Paleozoic-Mesozoic basement
published by Jawor & Kotarba (1991, 1993), Kotarba & Jawor
(1993), Kotarba (1998), Kotarba et al. (2004) and Kotarba &
Nagao (2008) were also used (Table 1). Information on the lo-
cations of sampling sites is given in Table 1 and in Fig. 1.
Analytical procedures
The molecular composition of collected natural gases (CH
4
,
C
2
H
6
, C
3
H
8
, iC
4
H
10
, nC
4
H
10
, C
5
H
12
, C
6
H
14
, C
7
H
16
, unsaturated
hydrocarbons, CO
2
, O
2
, H
2
, N
2
, He, Ar) were analysed with the
Agilent 7890A and Chrom-5 gas chromatographs (GC). The
Agilent GC is equipped with a three-valve system using three
1/8 inch packed columns (3 ft Hayesep Q 80/100 mesh, 6 ft
Hayesep Q 80/100 mesh and 10 ft molecular sieve 13X 45/60
mesh) and a GS-Alumina capillary column (50 m 0.53 mm).
The system consists of two independent channels. The channel
using the FID for the detailed hydrocarbon analysis is a simple
gas sampling valve injecting the sample into the GS-Alumina
column. The second channel using packed columns is for deter-
mination of methane, ethane and non-hydrocarbon gases. The
GC oven is programmed: initial temperature 60 °C held for
1 min, then increase to 90 °C at rate of 10 °C/min, next increase
to 190 °C at rate of 20 °C/min and finally held for 5 min. Front
detector (TCD) is operated with a temperature of 150 °C and
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back detector (FID) – with a temperature of 250 °C. Helium is
used as a carrier gas with flow through the TCD channel of
28 ml/min and through the FID channel of 7 ml/min. The con-
centration is determined on Chrom-5 GC equipped with TCD
and 1/8 in. 2.5 m-long packed column with mixture of 4A and
5A molecular sieves (2 : 1 v/v) using argon as the carrier gas at a
constant temperature of 25 °C. The Ar concentration is deter-
mined on the same GC at the same temperature program as he-
lium using a 2.5 m-long packed column filled with molecular
sieve 13X and hydrogen as carrier gas.
Stable carbon, hydrogen, and nitrogen isotope analyses
were carried out with Finnigan Delta Plus and Micromass
VG Optima mass spectrometers. The stable carbon and hydro-
gen isotope data are presented in the -notation relative to
the V-PDB and V-SMOW standards (Coplen 1995), respec-
tively. Analytical precision is estimated to be ± 0.2 ‰ and
± 3 ‰, respectively. The result of stable nitrogen isotope anal-
ysis is presented in the -notation relative to the air nitrogen
standard. Analytical precision is estimated to be ± 0.4 ‰.
For stable carbon isotope analyses methane, ethane,
propane, butanes, pentanes and carbon dioxide were separated
chromatographically. The gases were combusted over hot
copper oxide (850 °C) and the carbon dioxide produced was
transferred on-line to a mass spectrometer. For the stable hy-
drogen isotope analyses, water resulting from the combustion
of methane was reduced to gaseous hydrogen using metallic
zinc (Florkowski 1985). Gaseous nitrogen was separated chro-
matographically for stable nitrogen isotope analysis and was
transferred to the mass spectrometer with the on-line system.
Results and discussion
Natural gases from Middle and Upper Devonian, and Lower
Carboniferous reservoirs of the Paleozoic basement
The analysed gases of three samples collected from the
Middle and Upper Devonian, and the Lower Carboniferous
reservoirs of the Paleozoic basement of the Polish Car-
pathian Foredeep (Table 1) vary in their molecular and isoto-
pic compositions. The molecular and isotopic compositions,
and hydrocarbon (C
HC
) [C
HC
= CH
4
/(C
2
H
6
+C
3
H
8
)], carbon di-
oxide methane (CDMI) {CDMI = [CO
2
/(CO
2
+CH
4
)] 100 (%)}
and iC
4
H
10
/nC
4
H
10
gas indices of the analysed gases (3 sam-
ples) are reported in Tables 2 and 3.
For classification of the analysed hydrocarbon gases, the dia-
gnostic diagrams (Figs. 2— 4) were applied after Whiticar et al.
(1986), Schoell (1988), Whiticar (1994) and Berner & Faber
(1996). An important implication for the interpretation is that a
linear relationship of stable carbon isotopes of methane, ethane,
propane, butanes and pentanes versus their reciprocal carbon
number (Fig. 5) as assumed, for example, by Chung et al.
(1988) and Rooney et al. (1995) is not a sufficient indicator of
natural gas generated from a single source. Zou et al. (2007) and
Kotarba et al. (2009) suggested that in this type of plot a “dogleg”
trend, characterized by relatively
13
C-depleted methane and
13
C-enriched propane compared to ethane, is indicative of natu-
ral gas that was not generated from a single source rock (multi-
ple source) or that has undergone post-generation alteration
Fig. 2. Hydrocarbon index (C
HC
) versus
13
C(CH
4
) for natural gas-
es accumulated in Paleozoic-Mesozoic reservoirs of the basement
of the Polish Carpathian Foredeep. Compositional fields after
Whiticar (1994).
Fig. 3.
13
C(CH
4
) versus D(CH
4
) for natural gases accumulated in
Paleozoic-Mesozoic reservoirs of the basement of the Polish
Carpathian Foredeep. Compositional fields after Whiticar et al. (1986).
(e.g. secondary gas cracking, microbial oxidation, thermochem-
ical sulphate reduction). Moreover, the degree of
13
C depletion
of methane in relation to ethane can be applied to evaluate the
mixing proportion of microbial methane and thermogenic gases
(Kotarba & Lewan 2004; Kotarba et al. 2009).
The results of stable carbon and hydrogen isotope composi-
tions of methane from natural gas accumulations in the Upper
Devonian (Zl-8 sample) and the Lower Carboniferous (Na-1
sample) reservoirs (Figs. 2, 3) indicate that this gas was generat-
ed mainly during the low-temperature, thermogenic process.
However, methane and ethane from these localities contain in-
significant amounts of microbial components (Figs. 3, 4A).
Ethane is generated in small quantities during microbial pro-
cesses, for example, in the proportion of one molecule of ethane
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Molecular composition (vol. %)
Sample
code
N
2
CO
2
He Ar H
2
CH
4
C
2
H
6
C
3
H
8
iC
4
H
10
nC
4
H
10
iC
5
H
12
nC
5
H
12
C
6
H
14
Bc-3
#
0.89
0.04
0.01
0.000
0.00
98.6
0.32
0.06
0.05
0.02
0.04
n.a.
n.a.
Ga-40 14.3 0.63 0.02 0.03 0.05 77.7
3.32 1.86 0.68 0.80
0.37
0.11
Ga-109
13.1 0.36 0.03 0.04 0.81 76.7
4.40 3.21 0.60 0.41 0.012 0.05 0.34
Ga-49
8.49
0.76
0.000
0.02
0.05
69.1
7.40
5.50
3.14
3.41
1.51
0.49
Ga-89
4.14
0.07
0.002
0.02
0.06
68.7
9.32
9.09
2.64
3.62
0.88
0.93
0.47
JS-6
14.2
0.08
0.09
0.02
0.000
80.6
2.65
1.41
0.28
0.38
0.20
0.25
JS-15
14.7
0.00
0.00
0.03
0.000
76.6
4.15
2.77
0.60
0.67
0.20
0.16
0.13
Ln-24*
4.60
0.26
0.02
0.01
0.000
91.2
2.15
0.77
0.34
0.40
0.35
n.a.
Ry-3*
37.4
0.09
0.10
0.000
0.000
59.4
1.63
0.76
0.13
0.27
0.20
0.15
Zu-21
31.2
0.11
tr.
0.16
1.10
64.7
2.26
0.38
0.03
0.05
0.02
0.000
n.a.
Bw-12
3.14
0.16
0.009
0.005
0.001
89.0
5.10
1.69
0.30
0.35
0.12
0.101
0.06
Bw-23
2.86
0.12
0.02
0.006
0.008
78.1
8.16
4.42
1.79
2.63
1.48
0.44
Bw-24***
4.07
0.08
0.02
0.008
0.002
80.0
7.37
3.56
1.30
1.91
1.19
0.47
Bw-25
0.93
0.11
0.004
0.005
0.005
70.2
15.0
6.82
2.22
2.76
1.52
0.48
DT-11*
4.85
0.03
0.03
0.000
0.000
93.1
1.39
0.28
0.15
0.05
0.10
0.000 0.000
GR-1K
11.2
0.75
0.08
0.012
0.000
84.3
2.64
0.76
0.09
0.10
0.02
0.017
0.02
GR-2
7.6
0.37
0.13
0.000
0.16
87.3
3.1
0.94
0.12
0.15
0.03
0.017
0.07
Ga-36
8.7
0.19
0.02
0.04
0.003
74.5
7.3
5.10
1.27
1.63
0.40
0.45
0.41
Lb-22^
4.79
0.33
0.07
0.000
0.09
92.4
0.75
0.47
0.29
0.41
0.17
Lp-1
5.3
0.06
0.01
0.02
0.009
91.9
1.0
0.49
0.46
0.19
0.30
0.08
0.21
Lk-27
1.24
0.27
0.12
0.005
0.000
89.4
4.06
2.64
0.64
0.88
0.27
0.27
0.18
Sg-3*
6.00
0.02
0.000
0.000
0.000
91.8
1.32
0.48
0.33
0.00
0.05
0.02
Ta-5
21.4
4.94
0.09
0.03
0.000
72.0
0.98
0.33
0.06
0.09
0.04
0.04
0.05
Ta-17***
22.4 4.59 0.09 0.04 tr. 71.3
0.97 0.34 0.06 0.09 0.04 0.05 0.06
Ta-23 22.4 4.54 0.09 0.03 0.03 71.4
0.98 0.34 0.06 0.09 0.04 0.04 0.05
Ke-15
9.70
0.14
0.01
0.01
0.000
86.1
2.91
0.73
0.10
0.13
0.08
0.03
Wi-5* 28.6 4.95 0.06 0.11 0.03 63.6
1.88 0.60 0.12 0.18
0.10
0.05
Ze-6
4.36
0.13
0.000
0.000
0.000
86.8
5.21
1.94
0.36
0.57
0.23
0.18
0.20
Zu-11
4.66
0.04
tr.
0.01
tr.
92.5
1.60
0.70
0.18
0.16
0.16
0.03
Zl-8
8.60
0.07
0.01
0.000
0.007
85.4
2.98
1.00
0.34
0.37
0.33
n.a.
To-3
5.11
0.28
0.05
0.007
0.007
92.9
0.97
0.31
0.14
0.06
0.07
0.02
0.04
Na-1**
8.40
0.76
0.00
0.01
0.016
84.6
3.26
2.06
0.18
0.49
0.17
0.06
Table 2: Molecular composition of natural gases from the Paleozoic-Mesozoic basement.
# – after Jawor & Kotarba (1993); * – after Kotarba & Jawor (1993); ** – after Kotarba et al. (2004); *** – after Kotarba & Nagao (2008); tr. – traces;
n.a. – not analysed.
Fig. 4.
13
C(C
2
H
6
) versus (A)
13
C(CH
4
) and (B)
13
C(C
3
H
8
) for natural gases accumulated in Paleozoic-Mesozoic reservoirs of the base-
ment of the Polish Carpathian Foredeep. Position of vitrinite reflectance curves for Type II and III kerogens after Berner & Faber (1996).
Curves were shifted based on average
13
C = —29.8 ‰ for Ordovician and Silurian Type II kerogen (Więcław et al. 2011, 2012), average
13
C values = —24.9 ‰ for Lower Carboniferous (clastic), and average
13
C values = —24.8 ‰ for Middle Jurassic Type III kerogen (Kotarba
et al. 2003; Więcław et al. 2011; Kosakowski et al. 2012a,b).
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per one thousand molecules of methane (Oremland et al. 1986).
Microbial ethane enriched in
12
C (—61.2 to —52.5 ‰) has been
reported in producing microbial gas accumulations (Lillis 2007)
and microbial propane in some deep marine sediments
(Hinrichs et al. 2006). Stable carbon isotope compositions of
ethane, propane, butanes and pentanes (Figs. 4, 5) suggest that
hydrocarbons from the Nosówka deposit (Na-1 sample) were
generated from Ordovician-Silurian Type II kerogen (Fig. 5I) of
maturity about 1.0 % on the vitrinite reflectance scale (Fig. 4)
whereas gases from the Zalesie deposit (Zl-8 sample) were most
probably produced from the Lower Carboniferous or Middle
Jurassic, mixed ype III/II kerogen (Fig. 5F) of maturity about
1.1 % on the vitrinite reflectance scale (Fig. 4). Oil from the
Nosówka deposit was generated from the same source of organ-
ic matter (Więcław 2011). Methane and ethane contained in the
natural gas from the Middle Devonian reservoir of the
Trzebownisko deposit (To-3 sample) reveals a significant mi-
crobial component (Figs. 2, 3, 4A, 5F, 6D). The small ther-
mogenic component is most probably genetically related to the
Ordovician-Silurian Type II kerogen (Fig. 5F). In all analysed
gases the microbial component is genetically related to carbon
dioxide reduction (Fig. 3), which occurs mainly in the marine
environment (Whiticar et al. 1986; Rice 1992).
The hydrogen concentrations in the analysed gases vary
from 0.007 to 0.016 vol. % (Table 2). Natural hydrogen is
Table 3: Isotopic composition and gas ratios of natural gases from the Paleozoic-Mesozoic basement.
Stable isotopes (‰)
Ratios
13
C
D
13
C
13
C
13
C
13
C
13
C
13
C
13
C
13
C
C
HC
CDMI iCH
4
Sample
code
(CH
4
) (CH
4
) (C
2
H
6
) (C
3
H
8
) (iC
4
H
10
) (nC
4
H
10
) (iC
5
H
12
) (nC
5
H
12
) (CO
2
) (N
2
)
nCH
4
Bc-3
#
–67.3
–191
–53.0
–29.8
n.a. n.a. n.a. n.a.
n.a.
n.a.
259
0.04
2.50
Ga-40 –38.9
–159
–28.3
–26.4
n.a. n.a. n.a. n.a.
n.a.
n.a.
15
0.81
0.84
Ga-109
–38.9
–157 –27.9
–26.6 –27.1 –26.0 n.a. n.a. –17.8
1.4
10 0.47
1.46
Ga-49
–42.1 –173 –27.6 –26.3 –27.2 –26.4 n.a. n.a. –16.6 1.2
5 1.09 0.92
Ga-89
–39.4 –144 –28.3 –27.1 –27.0 –26.7 n.a. n.a. n.a. n.a.
4 0.10 0.73
JS-6 –57.0
–167
–26.9
–25.7
–26.7
–24.7
–25.6
–23.8
–16.2
1.7
20
0.10
0.74
JS-15 –57.4
–172
–24.8
–26.0
–23.7
–25.4
–23.5
n.a.
–27.2
1.7
11
0.00
0.90
Ln-24* –40.8
–146
–26.6
–24.5 n.a. n.a. n.a. n.a. n.a.
n.a.
31
0.28
0.85
Ry-3* –37.2
–141
–28.4
–26.2
n.a. n.a. n.a. n.a.
n.a.
n.a.
25
0.15
0.47
Zu-21 –50.4
–151
–25.6
–25.6
n.a. n.a. n.a. n.a.
n.a.
n.a.
25
0.17
0.60
Bw-12 –45.8
–173
–26.9
–25.8
–26.9 –25.6 n.a. n.a. –8.2
0.5
13
0.17
0.87
Bw-23 –45.6
n.a. n.a. n.a. n.a. n.a. n.a. n.a.
n.a.
n.a.
6
0.15
0.68
Bw-24*** –43.4 –173 –25.7 –24.8 –26.1 –24.6 n.a. n.a. –8.6 0.7
7 0.10 0.68
Bw-25 –45.7
n.a. n.a. n.a. n.a. n.a. n.a. n.a.
n.a.
n.a.
3
0.16
0.80
DT-11* –57.0
–181
–28.8
–23.6 n.a. n.a. n.a. n.a. n.a.
n.a.
56 0.03
3.00
GR-1K –45.3
–196
–27.2
–26.3
–26.3 –25.9 n.a. n.a. –5.4
0.2
25
0.88
0.89
GR-2 –45.6
–188
–27.5
–26.4
–26.8
–26.8
–26.6
–26.9
–10.9
0.4
22
0.42
0.76
Ga-36
–43.1 –187 –27.5 –26.7 –27.6 –26.5 –26.9 –26.2 –15.5 1.4
6 0.26 0.78
Lb-22^ –66.6
–201
–38.8
–29.0 n.a. n.a. n.a. n.a. n.a.
n.a.
75
0.36
n.c.
Lp-1
–55.8 –177 –27.8 –25.8 –30.0 –25.5 –28.9 –26.0 –9.2 1.5
62 0.06 2.40
Lk-27
–41.4 –148 –26.8 –25.5 –26.9 –25.3
n.a.
n.a. –10.4 n.a. 13 0.30 0.73
Sg-3* –58.5
–182
–28.4
–22.3
n.a. n.a. n.a. n.a.
n.a.
n.a.
51
0.02
0.00
Ta-5 –36.1
–188
–27.9
–27.1
–27.4
–27.3
n.a.
n.a.
–5.8
2.0
55
6.42
0.71
Ta-17*** –36.0 –144 –28.4 –26.0 –27.5 –26.7 n.a.
n.a. –6.0 2.0 54 6.05 0.70
Ta-23
–36.2 –152 –28.4 –27.2 –28.6 –27.3
n.a.
n.a. –6.1 n.a. 54 5.98 0.71
Ke-15 –44.5
–167
–26.6
–25.9
n.a. n.a. n.a. n.a.
n.a.
n.a.
54
0.16
0.77
Wi-5* –36.4
–142
–28.3
–27.0
n.a. n.a. n.a. n.a.
n.a.
n.a.
26
7.22
0.67
Ze-6 –38.0
–153
–26.4
–25.0
n.a. n.a.
n.a.
n.a.
n.a.
n.a.
12
0.15
0.64
Zu-11 –61.3
–183
–31.4
–28.2
n.a. n.a. n.a. n.a.
n.a.
n.a.
40
0.04
1.13
Zl-8 –48.6
–171
–28.7
–27.6
–28.2
–27.1
n.a.
n.a.
n.a.
n.a.
21
0.08
0.92
To-3 –61.9
–188
–32.4
–29.1
–30.7
–29.1
n.a.
n.a.
–14.8
–2.1
73
0.30
2.31
Na-1** –46.4
–160
–34.1
–33.9 n.a. n.a. n.a. n.a. n.a.
n.a.
16 0.89
0.37
# — after Jawor & Kotarba (1993); ^ — after Kotarba (1998); * — after Kotarba & Jawor (1993); ** — after Kotarba et al. (2004); *** — after
Kotarba & Nagao (2008); C
HC
= CH
4
/(C
2
H
6
+C
3
H
8
); CDMI = [CO
2
/(CO
2
+CH
4
)]100 (%); n.a. — not analysed.
generated by various biogenic and abiogenic processes: mi-
crobial fermentation of sedimentary organic matter, microbial
carbon dioxide reduction, thermal decomposition of sedimen-
tary organic matter, hydrolysis, water radiolysis (dissociation
of water molecules bombarded by alpha particles) and natural
nuclear reactions (Zobell 1947; Zinger 1962; Hawkes 1972;
Whiticar et al. 1986; Dubessy et al. 1988; Savary & Pagel
1997). Hydrogen is a very reactive and mobile gas, hence, its
retention in petroleum traps and in sedimentary rocks is rather
ephemeral. Thus, its presence in natural gases indicates that it
is either recently generated in secondary reactions within the
reservoir and/or in the adjacent source beds, or it is ascending
from deep-seated sources (Hunt 1996). Corrosion should be
mentioned as a potential H
2
source. Depending on the well
conditions and casing it may be even the most important pro-
cess. Analysis of deuterium content in the analysed gases can
perhaps be used to better constrain its origin.
The carbon dioxide concentrations and the values of the car-
bon dioxide-methane index (CDMI) in the analysed natural
gases hosted in the Middle and Upper Devonian, and in the
Lower Carboniferous reservoirs vary from 0.07 to 0.76 vol. %
and from 0.08 to 0.89, respectively (Tables 2, 3). The
13
C(CO
2
) value in the To-3 sample is —8.2 ‰ (Table 3). The
13
C(CH
4
) versus
13
C(CO
2
) indicate that this carbon dioxide
was generated by microbial processes (Fig. 7).
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Fig. 5. (A, B and C) Stable carbon isotope composition of methane, ethane, propane, butanes and pentanes (D, E and F), stable carbon isotope
composition of methane, ethane, propane and butanes and (G, H and I) stable carbon isotope composition of methane, ethane and propane versus
the reciprocal of their carbon number for natural gases accumulated in the Paleozoic-Mesozoic strata of (A, D and F) Kraków-Brzesko zone, (B, E
and H) Brzesko-Dębica zone, and (C, F and I) Dębica-state border zone (for information on locations see Fig. 1). Structure of the graph (G, H
and I) for methane, ethane and propane after Rooney et al. (1995). Average values of
13
C = —29.8 ‰ for Ordovician and Silurian kerogen
(Więcław et al. 2011, 2012),
13
C = —24.9 ‰ for Lower Carboniferous (clastic) kerogen (Więcław et al. 2011),
13
C = —24.8 ‰ for Middle Juras-
sic kerogen (Kotarba et al. 2003; Kosakowski et al. 2012a,b), and
13
C values = —24.6 ‰ for Miocene kerogen (Kotarba et al. 1998, 2005).
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Nitrogen is produced in large quantities during both the
microbial processes and the thermogenic transformation of
organic matter (Kotarba 1988; Krooss et al. 1995). For in-
stance, coalification of 1 kg of humic coal is associated with
a change volatile matter (VM
daf
) yield from initially 40 down
to 4 % generates about 3.5 dm
3
of N
2
(Kotarba 1988).
Sapropelic organic matter is richer in nitrogen, therefore,
more molecular nitrogen can be produced from it than from
the humic matter (Maksimov et al. 1982). The process of
molecular nitrogen generation from organic matter was also
documented by pyrolytic experiments (Gerling et al. 1997).
The
15
N-values of molecular nitrogen from natural gases
change from —15 to 18 ‰ (Gerling et al. 1997). This isotopic
fractionation results from both, the primary genetic factors
and the secondary processes taking place during gas migra-
tion through the gas-rock and gas-reservoir fluids interfaces
Fig. 6. A – Hydrocarbon index, B – carbon dioxide-methane index, C –
13
C(CO
2
), D –
13
C(CH
4
), E –
13
C(C
2
H
6
) and F –
13
C(C
3
H
8
)
versus depth of natural gas accumulations in the basement of the Polish Carpathian Foredeep.
(Stahl 1977; Gerling et al. 1997; Zhu et al. 2000; Ballentine
& Sherwood Lollar 2002; Krooss et al. 2005). Nitrogen con-
centrations in the analysed natural gases vary from 5.11 to
8.60 vol. % and
15
N(N
2
) in the To-3 sample is —2.1 ‰
(Tables 2, 3). The position of the To-3 sample in Fig. 8 can
suggests that nitrogen was generated by both the microbial
processes and the thermal transformation of organic matter.
However, I cannot exclude that at least part of the nitrogen
found in the analysed samples might have originated from
the atmosphere during sedimentation.
Natural gases from the Upper Jurassic and Upper
Cretaceous reservoirs of the Mesozoic basement
The analysed gases (29 samples) collected from both the
Upper Jurassic and the Upper Cretaceous reservoirs of the
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Mesozoic basement of the Polish Carpathian Foredeep (Ta-
ble 1) also vary in their molecular and isotopic compositions.
The stable carbon and hydrogen isotope compositions of
methane from natural gas accumulated within the Upper Ju-
rassic and the Upper Cretaceous reservoirs of the Mesozoic
basement (Figs. 2, 3) indicate that this gas was generated by
both, microbial carbon dioxide reduction and thermogenic
Fig. 8.
15
N(N
2
) versus N
2
concentration of natural gases accumu-
lated in Paleozoic-Mesozoic reservoirs of the basement of the Polish
Carpathian Foredeep. Direction of increasing source rock maturity
after Gerling et al. (1997).
Fig. 7.
13
C(CH
4
) versus
13
C(CO
2
) for natural gases accumulated
in Paleozoic-Mesozoic reservoirs of the basement of the Polish
Carpathian Foredeep. Compositional fields modified from Gutsalo
& Plotnikov (1981) and Kotarba (1988).
processes. In the Brzezowiec deposit (sample Bc-3) and in
one block of the Lubaczów deposit (sample Lu-22) microbial
methane prevails (Figs. 2—6). It was generated by carbon di-
oxide reduction (Fig. 3). This microbial methane migrated
from the autochthonous Miocene strata to the Cenomanian
sandstone and the Upper Jurassic carbonate reservoirs (Jawor
& Kotarba 1993; Kotarba & Koltun 2006). Significant micro-
bial gas components occur in natural gases from the Swarzów
(Sw-3), Dąbrowa Tarnowska (DT-11), Żukowice (Zu-11) and
Łapanów (La-1) deposits (Figs. 2, 3, 4A, 5, 6). The presence
of microbial methane generated by microbial carbon dioxide
reduction (Fig. 3) suggests that in these deposits the traps
had already been formed and sealed during the migration of
microbial methane. The traps have been successively sup-
plied with thermogenic methane and higher hydrocarbons
generated from Type III/II kerogen (Fig. 5) at successively
higher maturation stages (Fig. 4). Microbial methane was
also generated by the same process within the autochthonous
Miocene strata (Kotarba 2011). Therefore, under favourable
geological conditions partial migration of Miocene-sourced
microbial methane into the basement cannot be excluded
(Kotarba & Jawor 1993).
The remaining deposits (Brzezówka, Czarna Sędziszowska-
Góra Ropczycka, Grobla, Korzeniów, Łąkta-Leszczyna,
Rylowa, Tarnów, Wierzchosławice and Zagórzyce) are dom-
inated by thermogenic methane (Figs. 2—6). Comparison of
stable carbon isotope values of propane, butanes and pen-
tanes with those for different types of kerogen (Fig. 5) indi-
cates the presence of mixed, Type III/II kerogen with a broad
maturity range (from 1.1 to 2.2 % in the vitrinite reflectance
scale) in the Middle Jurassic (Kosakowski et al. 2012a,b)
and/or the Lower Carboniferous strata (Więcław et al. 2011),
as the source of these thermogenic gases (Fig. 4). Type II or
mixed Type II/III kerogen could be the source of hydrocar-
bons in the Tarnów (Fig. 5E) and the Brzezówka (Fig. 5F)
deposits. A kerogen of similar type in the Middle and Upper
Devonian strata could be at least partly the source of the hy-
drocarbons in the Grobla and Rylowa deposits (Kotarba et
al. 2011; Więcław et al. 2011). A “dogleg” trend of the isoto-
pic curves in Fig. 5 suggests that thermogenic gases were
generated in at least two phases: first, the low-temperature,
thermogenic gases associated with oil and condensate were
produced, followed by the second, high-temperature, ther-
mogenic, non-associated gases.
The carbon dioxide concentrations and the values of the car-
bon dioxide-methane index (CDMI) in the analysed natural
gases vary from 0.02 to 4.95 vol. % and from from 0.02 to
7.22 vol. %, respectively (Tables 2, 3). The
13
C(CO
2
) values
range from —17.8 to —5.4 ‰ (Table 3). The
13
C(CH
4
) versus
13
C(CO
2
) plot (Fig. 7) indicates that carbon dioxide was gen-
erated both in microbial and thermogenic processes. Microbial
carbon dioxide almost exclusively occurs in the natural gases
from the Jastrząbka Stara and the Łapanów deposits. On the
contrary, thermogenic carbon dioxide dominates in the
Brzezówka, Czarna Sędziszowska-Góra Ropczycka, Grobla,
Łąkta-Leszczyna and Tarnów deposits (Fig. 7). The vertical
distribution of carbon dioxide-methane index (CDMI) and the
13
C(CO
2
) values are presented in Fig. 6B and C. Such varia-
tions in concentration and stable isotope composition of car-
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bon dioxide with depth indicate the influence of secondary
processes, mainly CO
2
dissolution in water during migration.
The N
2
concentrations in the natural gases from the Meso-
zoic basement vary from 0.89 to 37.4 vol. % and
15
N(N
2
)
values range from 0.2 to 2.0 ‰ (Tables 2, 3). Correlation be-
tween
15
N(N
2
) and N
2
concentration (Fig. 8) could suggests
that nitrogen was generated by both, microbial processes and
thermal transformation of organic matter. However, it cannot
be excluded that atmospheric gases would have been intro-
duced into the source rocks or into the pathways of gas mi-
gration to the surface, as for example, in the natural gases
accumulated in the Upper Jurassic reservoir of the Tarnów
deposit (Kotarba & Nagao 2008). The notably high H
2
(1.10 vol. %) and N
2
(31.2 vol. %, Table 2) contents in the
Zu-21 sample are attributed to secondary recovery methods
applied in the Żukowice field.
The hydrogen sulphide concentrations in the natural gases
from the Mesozoic basement in the Grobla and Tarnów depos-
its vary from 0.10 to 0.12 vol. % and from 0.04 to 0.07 vol. %
(Table 2). The origin of hydrogen sulphide is one of the most
complex problems in petroleum geochemistry. Hydrogen sul-
phide can be generated in a number of processes, namely:
(i) microbial sulphate reduction (MSR), (ii) thermochemical
sulphate reduction (TSR), (iii) thermal decomposition of or-
ganic sulphur components of oil and fossil organic matter,
(iv) reaction of elemental sulphur and fossil organic matter
(hydrocarbons) and (v) magmatic reactions (abiogenic, vol-
canic and/or plutonic processes). Stable sulphur isotope
(
34
S) compositions of hydrogen sulphide, sulphates, sul-
phides, and elemental sulphur, as related to the geological
and geothermal conditions in a given petroleum basin, may
be used to recognize the origin of hydrogen sulphide, though
not all of its generation mechanisms have been fully ex-
plained so far (e.g. Anissimov 1995; Hałas et al. 1973;
Krouse 1980; Krouse et al. 1988; Worden et al. 1995). Pre-
liminary results of stable sulphur isotope analyses of hydro-
gen sulphide from the Grobla and Tarnów deposits suggest
that this gas component was generated during thermal de-
composition of organic sulphur components of oil and fossil
organic matter (Kotarba & Hałas, unpublished data).
Conclusions
The results of molecular and stable isotopic analyses of nat-
ural gases from the Paleozoic-Mesozoic basement of the
Polish Carpathian Foredeep lead to the following conclusions:
1. Hydrocarbon gases from the Upper Devonian (Zalesie
deposit) and the Lower Carboniferous (Nosówka deposit)
reservoirs were generated mainly by a low-temperature, ther-
mogenic process (“oil window”). Methane and ethane from
these accumulations contain insignificant microbial compo-
nents. The thermogenic hydrocarbon gases in the Nosówka
deposit were generated from Ordovician-Silurian Type II
kerogen at a maturity level corresponding to around 1.0 % in
the vitrinite reflectance scale, and those from the Zalesie de-
posit were most probably produced from Lower Carbonifer-
ous and/or Middle Jurassic, mixed Type III/II kerogen with a
vitrinite reflectance around 1.1 %;
2. Methane, ethane and carbon dioxide in the Middle Devo-
nian reservoir of the Trzebownisko deposit contain a signifi-
cant microbial component. The small thermogenic component
is most probably genetically related to Ordovician-Silurian
Type II kerogen;
3. The microbial components of all analysed gases from
the Paleozoic basement are related to carbon dioxide reduc-
tion, which occurs mainly in the marine environment;
4. The nitrogen component of Paleozoic gases was gener-
ated by both, microbial and thermogenic processes but its
partial atmospheric origin cannot be excluded;
5. Hydrocarbon gases from the Upper Jurassic and the Up-
per Cretaceous reservoirs of the Mesozoic basement were
generated by both the microbial carbon dioxide reduction
and the thermogenic processes;
6. In the Cenomanian sandstone reservoir of the Brzezowiec
deposit and an Upper Jurassic carbonate block of the Lubaczów
deposit, microbial methane prevails; according to its isotopic
signature it was generated by carbon dioxide reduction. This
gas has migrated from the autochthonous Miocene strata to
the Mesozoic reservoirs;
7. Significant portions of microbial gas occur in the
Swarzów, Dąbrowa Tarnowska, Żukowice, and Łapanów de-
posits. The presence of microbial methane generated by car-
bon dioxide reduction suggests that in these deposits the
traps had already been formed and sealed during the migra-
tion of microbial methane produced presumably from imma-
ture source-rocks. The traps have been successively supplied
with thermogenic methane and higher hydrocarbons generat-
ed from Type III/II kerogen at successively higher matura-
tion stages;
8. Microbial methane was generated by the same process
within the autochthonous Miocene strata. Therefore, partial
migration of Miocene-sourced microbial methane into the
basement may have occurred under favourable geological
conditions;
9. Natural gases from the Czarna Sędziszowska-Góra
Ropczycka, Grobla, Korzeniów, Łąkta-Leszczyna, Rylowa,
Wierzchosławice and Zagórzyce deposits hosted in the
Mesozoic basement are dominated by thermogenic methane.
These gaseous hydrocarbons were generated from the mixed
Type III/II kerogen of the Middle Jurassic, Lower Carbonif-
erous and/or Devonian strata, which extends over a broad
range of maturity – from 1.1 to 2.2 % in the vitrinite reflec-
tance scale;
10. Type II or mixed Type II/III kerogen may have been
the sources of hydrocarbons in the Tarnów and the Brzezówka
deposits;
11. Thermogenic gases found in both, Upper Jurassic and
Upper Cretaceous deposits were generated in at least two
phases: first, low-temperature, thermogenic gases associated
with oil and condensate were formed, followed by, high-tem-
perature, thermogenic, non-associated gases. Thus, the traps in
these deposits were supplied during a long time span corre-
sponding to the successive hydrocarbon generation phases;
12. Carbon dioxide found in both, Upper Jurassic and Up-
per Cretaceous deposits was generated by both, microbial
and thermogenic processes. Microbial carbon dioxide is pre-
dominant in natural gases of the Jastrząbka Stara and the
317
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GEOLOGICA CARPATHICA
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GEOLOGICA CARPATHICA
GEOLOGICA CARPATHICA, 2012, 63, 4, 307—318
Łapanów deposits, whereas in the Brzezówka, Czarna Sędzi-
szowska-Góra Ropczycka, Grobla, Łąkta-Leszczyna and
Tarnów deposits thermogenic carbon dioxide prevails;
13. Nitrogen found in both the Upper Jurassic and Upper
Cretaceous deposits was generated by both, microbial pro-
cesses and thermal transformation of fossil organic matter.
However, it cannot be excluded, that atmospheric gases were
introduced into the source rocks or into the ascension path-
ways of gases to the surface, as for example, the Upper
Jurassic reservoir of the Tarnów deposit;
14. Hydrogen sulphide found in the Grobla and Tarnów
deposits was most probably generated by thermal decompo-
sition of organic sulphur components of oil and fossil organic
matter.
Acknowledgments: The research was undertaken in the
framework of the Project No. UKRAINE/193/2006 of the
Ministry of Science and Higher Education carried out at the
AGH University of Science and Technology in Kraków and
the Polish Geological Institute in Warsaw. Scientific studies
were financed in the years 2007—2010. The detailed com-
ments of Bernhard Krooss, Eckhard Faber and Kazimierz
Różański were of great assistance in the revisions of this
manuscript. Analytical work by Ms. Zofia Stecko and Mr.
Tomasz Kowalski from the AGH University of Science and
Technology in Kraków is gratefully acknowledged.
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