Biodegradation and migrational fractionation of oils from
the Eastern Carpathians, Poland
Irena Matyasik
a,*, Anna Steczko
a, R. Paul Philp
b aGeology and Geochemistry Department, Oil and Gas Institute, 31-503 Cracow, ul Lubicz 25a, PolandbSchool of Geology and Geophysics, University of OklahomaÐNorman, OK 73019, USA
Abstract
Nineteen oil samples from Silesian Unit of the eastern Carpathian Overthrust have been characterised geochemically in order to determine the causes of compositional dierences among them and elucidating the processes responsible for their dierences. Some of analysed crude oils have undergone post-emplacement alteration in the reservoir such as biodegradation and evaporative fractionation. This explains much of the chemical and physical properties variability across individual ®elds from one tectonic unit. Geochemical correlation based on biomarker distributions showed a close relationship between all oils (included biodegraded oils). However, data based on the whole oil GC analysis of selected oils suggest that the process of evaporative fractionation may change the composition of lower molecular weight hydrocarbons of the oils in this region. This paper outlines the probable mechanisms for oil mixing in the region
and describes how this can lead to observable lateral dierences in the composition of oils. # 2000 Published by
Elsevier Science Ltd.
Keywords:Biodegradation; Biomarkers; Carpathian Overthrust; Evaporative fractionation; Lower weight hydrocarbons
1. Introduction
The results described in this study will centre on the geochemical characterisation of oils from the Outer Carpathian region of Poland which has been a major petroleum province for over one hundred years. The Outer Carpathians consist of several major tectonic units, involving Cretaceous and Paleogene ¯ysch. From north to south, namely from the external to the more internal zones, the Polish Outer Carpathian ¯ysch com-prises the following units: Stebnik, Skole, Sub-Silesian, Silesian, Dukla, Magura. These units exhibit strong lateral facies and thickness changes which were induced by dierentiation of the basin ¯oor into cordilleras during successive compressional episodes, culminating in the Laramide inversion episode. In the Polish part of the Carpathian, the main oil ®elds were discovered in both the Paleogene and Cretaceous ¯ysch. Moreover, they also contain the source rocks, especially in the early
Oligocene Menilite Shales and in the Albo-Aptian Spas of the Lower Cretaceous as well in the Upper Cretaceous Lower Istebna Beds. The Menilite Formation contains the best potential source-rocks (ten Haven et al., 1993; Bessereau et al., 1996; Koster et al., 1998a,b). Several early Cretaceous shales also represent potential, though less proli®c, source-rocks. The Spas shales are restricted to the Skole Unit. In the Skole basin, maturation levels of both the Spas and the Menilite Beds show evidence of depth-related evolution and reach values corresponding to the beginning of the ``oil-window'' at depths of about 4000 m (Koltun et al., 1995). In the Silesian basin, Early Cretaceous strata were located within the oil-window and Menilite shales might have entered the ``oil-window'' in the deepest parts of the basin (Bessereau et al., 1996).
Previous studies on characterisation of oils from this region have shown that the oils can be divided into two main families (ten Haven et al., 1993). The ®rst oil family, represented by only one sample, has an isotopically light
signature, abundant C29steranes, and lacks, in contrast
to the second family, characteristic biomarkers such as oleanane. The second oil family comprises all other oil samples from the foredeep, as well allochthon, between
0146-6380/00/$ - see front matter#2000 Published by Elsevier Science Ltd. P I I : S 0 1 4 6 - 6 3 8 0 ( 0 0 ) 0 0 1 0 3 - 0
www.elsevier.nl/locate/orggeochem
* Corresponding author.
which no signi®cant dierences could be observed. This family has an isotopically heavier signature and is char-acterised by the presence of oleanane, 28,30-dinor-hopane,
and in some oils C25isoprenoid alkane (HBI) and several
additional biomarkers characteristic of terrigenous input. Based on sulphur contents and relative abundance of these biomarkers, the second family was divided into four subfamilies.
In this study, we describe the geochemical analysis that were completed on samples from Silesian Unit-Potok Fold area, in order to reveal similarities and distinctions among oils from dierent tectonic blocks. Particular attention was paid to post-generative alteration processes on crude oil composition.
2. Samples
Nineteen oil samples were collected from separate tectonic blocks belonging to the Silesian Unit of the eastern Carpathian Overthrust. Ten samples were col-lected in the central area Jaszczew and Potok-Turaszowka
Fields (Moderowka-6; Gaz-11; Maksymilan-2,-3;
Witold-1,-2,-3; Potok-19; Ewa-7,-13), ®ve samples were taken from western Roztoki-Sobniow Fields (Polmin-7,-9; Sobniow-27A,-2(Polmin-7,-9; Roztoki-37) and only four from eastern Kroscienko and Trzesniow Fields (Poznan-11; Mac Allan-3; Kronen-41 and Trzesniow-10) (Fig. 1a and b). In the western area, small amounts of gas is associated with the oil. All the oil- and gas-®elds in the Carpathians are found in the structural traps, usually anticlinal folds, which are frequently bounded by faults, thrust and decollement planes (Karnkowski and Ozim-kowski, 1999). The detailed Upper Cretaceous±Tertiary (Oligocene) lithostratigraphic column for Moderowka-6 well is shown in Fig. 2, considered to be lithologically representative of the Potok Fold area. The Lower Istebna beds may be suggested as a potential source rocks for oils accumulated in upper horizons of this area. For the characterisation of the oils a multi-parameter geochemical approach was applied including determination of bulk and physical properties of the oils (gravity, sulphur content, viscosity), as well as whole oil GC (18 samples, excluding oil from Mac Allan well), GC of saturated hydrocarbon fractions and GC±MS of saturated hydrocarbons.
3. Experimental procedures
Whole oil gas chromatography was performed on a Rtx-1 fused silica capillary column (105 m length, 0.32 mm
i.d., 0.50mm ®lm thickness) installed in a Fisons GC-800.
The conditions of the GC temperature programme were:
isothermal at 35oC for 10 min, programmed at 3C/min
to 240C then at 1C/min to 300C and held at the ®nal
temperature for 20 min. Approximately 1ml of crude oil
was injected without prior preparation and carbon dis-ulphide was used as a solvent.
The asphaltenes were removed from the crude oils by
precipitation fromn-hexane. The deasphaltened oils were
fractionated by silica/alumina column chromatography into saturate, aromatic and polar fractions by elution with
n-hexane, toluene/n-hexane (3:1, v/v) dichloromethane/
methanol (1:1, v/v) respectively. The fractions were ana-lysed by GC and combined gas chromatography-mass spectrometry on a Fisons quadrupole mass spectrometer equipped with DB5 fused silica capillary column (60
m0.32 mm0.25mm) with helium as carrier gas. The
GC temperature program was initially held at 60C for
2 min, then increased at 4C/min to a ®nal temperature
of 310C, which was held for 15 min. Samples were
analysed in the full scan mode, scanning from 50 to 550 amu.
4. Results and discussion
Bulk geochemical data for the oils are given in Table 1. The gravity of oils range from 793.6 for unaltered oils
to 902.6 g/cm3for oils which have undergone alteration
in reservoir. Sulphur contents of the oils range from 0.02 to 0.34% and are proportional to oil gravity. These sulphur content variations may be related to alteration processes such as biodegradation. Oils recognised as having experienced biodegradation are found in reservoirs shallower than 750 m in Ciezkowice Sands, from the central and the eastern area of Potok Fold. The Ciez-kowice Sands possess meteoric water input trough faults to carry dissolved oxygen and micro-organisms into the reservoirs resulting in biodegradation (Bessereau at al., 1996). Bacteria introduced into an oil pool with oxygen-rich meteoric waters, apparently utilise this dissolved oxygen and metabolise preferentially certain types of hydrocarbons. Under anaerobic condition, the oxygen supply of bacteria is probably derived from dissolved sulphate ions. The dierent eects of biodegradation on the molecular composition of crude oils are relatively well known (Volkman et al., 1984; Peters and Moldowan, 1993; van Aarssen, 1999). Our studies have indicated that the more extensively biodegraded oils (Po-11, MA-3, K-41 ) are found in Kroscienko ®eld and among them the oil from Po-11 is the most altered; absence of the
C15+n-alkanes and isoalkanes. Asphaltene contents are
low and range from 0.4 to 1.7%. Maltene fractions are dominated by saturated compound especially in Sobniow-Roztoki ®eld with Sat/Aro ratios in the range 3.3±5.8
andHC/NSO in the range 8.3±20.0 in this reservoir.
These ratios seem to have depth dependence (Fig. 3) excluding two samples from the Roztoki-Sobniow ®eld (R-37, P-9). It is due to deeper sedimentary burial of the Istebna beds in Silesian basin during the generation time.
GC studies indicated that all non-biodegraded oils
contain n-alkanes, mainly in the C13±C32 range, with
dominant C17,C19 members. Pr/Ph ratios for these oils
are in the 1.89±3.84 range, ratios of Pr/nC17 and Ph/
nC18are 0.71±4.67 and 0.26±1.51, respectively (Table 2).
Pr/Ph ratios usually are dependent upon the source of
organic matter and diagenetic transformations,
although as ten Haven (1988) noted, the relationship between speci®c depositional environments and Pr/Ph ratio is not fully understood. In general, high Pr/Ph ratio are associated with oxidising depositional envir-onments or contribution of signi®cant proportions of terrestrially derived organic matter to the sedimentary environment. However, it is also suggested that pristane/ phytane ratios increases with increasing migration-frac-tionation (Curiale, 1996).
In the present study, the whole oil GC analyses clearly showed that a number of the oils from the eastern part of the Carpathians Overthrust had undergone one or more episodes of biodegradation (Fig. 4a and b). A moderate extent of the biodegradation is documented
based on absence of the C15+-n-alkanes from these oils
and by the presence of pristane and phytane (Fig. 5). Since the light ends fractions of a crude oil is very sensitive to factors inducing heterogeneities (source rocks maturity, pressure and temperature regimes, biodegradation and water washing) there is growing interest for the detec-tion of composidetec-tional variability within this fracdetec-tion among dierent oils. Selected light hydrocarbons
para-meters for the oils from the Potok Fold are reported in
Table 3. The oils have heptane values (H) ranging from
9.93 to 26.14 and isoheptane values (I) less then 1.7.
Normal oils possess heptane values between 18 and 22,
generally associated with values of F between 0.5 and
0.8, representing equivalent vitrinite re¯ectance levels of approximately 0.86±1.05% (Thompson, 1987). All of
oils discussed in this study haveFratios less then 0.59
(Figs. 6 and 7).
However, the more intriguing aspect of these analyses was the fact that many of these biodegraded oils also contained a relatively high concentration of lower
molecular weight hydrocarbons in the range of C4±C11
giving these oils the appearance of having a condensate entering the reservoir after the initial oil had been bio-degraded. It is proposed that in this particular region the major mechanism responsible for this is migration-fractionation which was initially described by Thomp-son (1988). He suggested that oil is frequently partially vaporised in the reservoir and secondly that gas bearing substantial portions of the oil in solution can be con-ducted along faults to form independent gas condensate accumulations. Residual oils formed in this fashion will show signs of internal fractionation. These are mani-fested in the conspicuous loss of light ends along with an increase in the content of light aromatic and naphtenic hydrocarbons relative to the parans in the residual oil (indices A and B in Table 3). The process of evaporative fractionation involves the intersection of an active fault
Table 1
Bulk parameters and compositions of oils from Potok Fold ®elds
Code Well Depth
P-7 Polmin-7 1304±1300 II Ciezkowice Sand. 839.6 77.6 0.20 68.4 20.8 10.2 0.6 3.3 8.3 S-27A SobnioÂw-27A 2295±2285 Istebna Beds 840.1 10.95 0.08 75.9 18.5 4.9 0.7 4.1 17.1 S-29 SobnioÂw-29 2240±2203 Istebna Beds 845.3 11.47 0.09 74.5 18.5 6.3 0.7 4.0 13.4 P-9 Polmin-9 1230±1222.6 Istebna Beds 840.2 6.94 0.09 80.8 13.9 4.8 0.5 5.8 18.2 R-37 Roztoki-37 1523±1500 Lower Istebna Beds 793.6 2.00 0.02 79.3 15.6 4.4 0.7 5.1 20.0 Mo-6 ModeroÂwka-6 1776±1753 Lower Istebna Beds 873.2 18.11 0.18 72.3 20.0 6.6 1.1 3.6 12.0 G-11 Gaz-11 1133±1123 Upper Istebna Beds 822.1 3.76 0.05 72.2 21.7 5.4 0.7 3.3 15.4 M-2 Maksymilian-2 1108±1105 Upper Istebna Beds 828.1 4.84 0.08 73.8 19.3 6.0 0.9 3.8 13.7 M-3 Maksymilian-3 1138±1077 Ciezkowice Sand. 824.9 3.57 0.06 64.3 27.2 7.6 0.9 2.4 10.9 W-1 Witold-1 742±738 II Ciezkowice Sand. 826.5 3.26 0.15 52.1 30.7 15.8 1.4 1.7 4.8 W-3 Witold-3 708±620 II Ciezkowice Sand. 824.3 3.14 0.16 53.9 27.9 17.1 1.1 1.9 4.5 W-6 Witold-6 728±720 II Ciezkowice Sand. 828.1 3.33 0.11 55.2 28.0 15.8 1.0 2.0 5.0 P-19 Potok-19 1750±1712 Upper Istebna Beds 842.5 18.69 0.05 75.5 17.4 6.7 0.4 4.3 13.0
E-7 Ewa-7 341±331.5 Ciezkowice Sand. 861.4 8.00 0.33 44.2 32.4 22.1 1.3 1.4 3.3
E-13 Ewa-13 272±239 I Ciezkowice Sand. 860.3 2.27 0.32 42.6 37.5 18.4 1.5 1.1 4.0 Po-11 Poznan 445±354 I Ciezkowice Sand. 902.6 24.00 0.34 36.6 40.6 21.1 1.7 0.9 3.4 MA-3 Mac Allan-3 415.6±375.6 I Ciezkowice Sand. 880.3 12.99 0.30 49.3 30.8 18.2 1.7 1.6 4.0 K-41 Kronen-41 524±521.5 II Ciezkowice Sand. 898.2 20.32 0.19 59.2 26.6 13.0 1.2 2.2 6.1 T-10 TrzesÂnioÂw-10 268±263.7 Intermenilite Sand. 844.2 8.10 0.34 64.3 23.5 11.0 1.2 2.7 7.3
with an accumulation of gas saturated oil. It was pos-tulated that only the saturated vapour migrates along the fault and this will change the bulk properties of the oil as well as certain important ratios in the light ends themselves; particularly enhancing the aromaticity and naphtenic nature of the light ends of the oil. As an aid to recognising this eect of evaporative fractionation, Thompson (1988) proposed the use of a number of
ratios based on low molecular weight hydrocarbons. For example, the residual oils are typically enriched in aromatics such as benzene and toluene.
The chromatogram of the typical non-front-end loa-ded oil from Moderowka-6 well (residual oil) is shown in Fig. 8.
Curiale (1996) suggested that migration-fractionation leads to changes in parameters traditionally considered
to be predominantly source-in¯uenced (e.g. Pr/Ph,
sul-phur/nitrogen,d13C dierences greater than 1.0%) and
maturity-in¯uenced (the ratio of short-chain to long
chain triaromatic steroid and the ratio of the C23
-tri-cyclic terpane to hopane).
In this study, the characterisation of the oils by GC± MS has shown that the biomarker ®ngerprints are almost identical for all of the oils examined, regardless of their degree of biodegradation or condensate content, suggesting that in all probability these oils were all
derived from the same source (Fig. 9). The m/z 191
chromatograms are generally similar, 17a21b
(H)-hopanes are dominated by C30 hopane. A decreasing
intensity from C31 to C35 17a21b(H) 22S and 22R
homohopanes is observed indicating the prevailing
sub-oxic depositional environment. Them/z217
chromato-grams demonstrate very similar distribution of regular steranes. The dominance of hopanes over steranes in examined oils, excluding Trzesniow oil, (hop/ster ratio >2.3) suggests that prokaryotic organism were impor-tant in the source for these oils. The presence of higher plant biomarker, e.g. oleanane, indicates that the source rocks of these oils are Upper Cretaceous to Tertiary in age. Biomarker maturity parameters indicate that the oils of the Upper Cretaceous reservoirs in Roztoki, Sobniow, Polmin and Potok ®elds are the most mature
oils. Relative to other oils, mass fragmentograms m/z
191 and m/z 217 for these oils from west part of the
Potok Fold indicate they have low amount of hopanes and steranes (Table 4). The low amounts of polycyclic alkanes in these oils is probably a function of high thermal maturity. Trzesniow oil, reservoired in the Oli-gocene Menilite sandstone, appears to be normal maturity oil based on its whole oil parameters. Its low maturity sterane patterns (not presented in this paper) may be due to ''contamination'' from low maturity Menilite Shales.
With this in mind, and the fact the region is heavily faulted with reservoirs occurring at many depths as a consequence of a major tectonic activity in the region leads to the proposal that migration fractionation is probably an active mechanism in the region. Over-pres-suring of the deeper reservoirs can lead to fracturing of the cap rocks and escape of the lighter components from the reservoirs and subsequent release of the pressure. The lighter components subsequently migrate in to the shal-lower reservoirs. Variations in the cyclohexane and tolu-ene ratios following the same trends initially proposed by Thompson (1988) support this hypothesis (Table 3).
In addition to these observations, characterisations of oils from west to east in the region shows that the sequence starts from Roztoki-Sobniow and Jaszczew ®elds with nondegraded oils located in deeper part of the producing trend. The second block in the sequence, Potok-Turaszowka has lost some n-parans, but
retained most of C18±C20isoalkanes. Furthermore these
Table 2
Geochemical ratios for oil samples based on gas chromatographic analysis
Code Well CPITotal CPI17ÿ23 CPI25ÿ31 nCmax Pr/Ph Pr/nC17 Ph/nC18
P-7 Polmin-7 1.04 1.05 1.01 17 2.83 1.00 0.43
S-27A SobnioÂw-27A 1.03 1.05 1.00 16 2.13 0.96 0.51
S-29 SobnioÂw-29 1.13 1.15 1.25 17 3.84 0.74 0.26
P-9 Polmin-9 1.14 1.22 0.76 16 1.92 1.85 1.28
R-37 Roztoki-37 1.08 1.11 0.76 17 2.08 0.85 0.54
Mo-6 ModeroÂwka-6 1.06 1.12 1.01 16 1.93 1.42 0.76
G-11 Gaz-11 1.04 1.00 1.05 19 1.79 1.15 0.54
M-2 Maksymilian-3 1.05 1.12 1.05 19 2.01 4.67 1.51
M-3 Maksymilian-2 1.04 1.04 1.03 17 2.23 0.71 0.40
W-1 Witold-1a 2.12
W-3 Witold-3a 2.72
W-6 Witold-6a 2.59
P-19 Potok-19 1.04 0.99 0.76 16 2.53 1.00 0.39
E-7 Ewa-7a 1.93
E-13 Ewa-13a 2.19
Po-11 PoznanÂ-11a 27
MA-3 Mac Allan-3a
K-41 Kronen-41a 17
T-10 TrzesÂnioÂw-10 1.07 1.05 0.76 19 1.89 1.20 0.57
Fig. 4. (a) Capillary gas chromatogram (whole-oil analysis) of crude oil from Ewa-7 well; (b) capillary gas chromatogram (whole-oil analysis) of crude oil from Witold-6 well (continued on next page).
Fig. 5. Total ion chromatograms of C15+saturated fractions of biodegraded oils from Potok-Turaszowka ®eld (Witold-6, Ewa-7).
Table 3
Geochemical ratios calculated from whole oil chromatogramsa
Code Well A B X C I F H
P-7 Polmin-7 1.51 1.15 1.00 0.59 0.99 0.45 17.63
S-27A SobnioÂw-27A 3.42 1.16 1.15 1.38 1.65 0.41 17.36
A-29 SobnioÂw-29 2.17 1.26 0.59 0.46 1.22 0.38 17.20
P-9 Polmin-9 3.08 1.30 1.00 1.15 0.89 0.45 17.50
R-37 Roztoki-37 0.70 0.95 0.49 0.52 1.66 0.50 20.64
Mo-6 ModeroÂwka-6 3.91 2.54 2.09 2.21 ± 0.46 21.95
G-11 Gaz-11 1.27 1.03 0.00 0.49 1.45 0.45 19.09
M-2 Maksymilian-2 1.88 1.13 0.58 0.46 1.37 0.44 19.35
M-3 Maksymilian-3 1.00 0.98 0.52 0.50 1.42 0.45 19.03
W-1 Witold-1 1.12 0.96 0.48 0.52 1.38 0.49 19.70
W-3 Witold-3 0.91 0.91 0.48 0.56 1.31 0.49 18.75
W-6 Witold-6 0.98 0.91 0.47 0.53 1.35 0.49 19.63
P-19 Potok-19 1.60 0.81 0.39 0.54 1.26 0.46 18.61
E-7 Ewa-7 0.24 0.90 0.56 0.75 0.66 0.58 18.50
E-13 Ewa-13 0.17 0.90 0.54 0.78 0.67 0.58 18.52
Po-11 PoznanÂ-11 1.43 0.16 0.89 0.30 0.51 0.26 9.93
K-41 Kronen-41 2.97 4.16 1.72 0.42 0.88 0.34 15.25
T-10 TrzesÂnioÂw-10 1.06 0.65 0.40 0.69 1.11 0.59 26.14
a De®nitions of compositional ratios: A, benzene/n-hexane; B, toulene/n-heptane; X, xylene (m- andp-)/n-octane; C, (n-hexane+n
-heptane)/cyclohexane+methylcyclohexane); I, [methylhexanes (2-+3-)]/[dimethylcyclopentanes (1c3-, 1t3-+1t2-)]; F, n-heptane/ methylcyclohexane; H, 100n-heptane/(cyclohexane through methylcyclohexane excluding 1c2-dimethylcyclopentane).
Table 4
Source and maturity related biomarker ratios of oil samples from the Potok Fold
Code Well Sterane (%) S/(S+R) bb/(aa+bb) S/(S+R) Ts/Tm M/C30hop O/C30hop hop/stera
C27 C28 C29 C29aaaster C29ster C32hop
P-7 Polmin-7 33 34 0.33 0.39 Low amounts of hopanes
S-27A SobnioÂw-27A Concentration below measurement limit S-29 SobnioÂw-29 Concentration below measurement limit P-9 Polmin-9 Concentration below measurement limit R-37 Roztoki-37 Concentration below measurement limit Mo-6 ModeroÂwka-6 Concentration below measurement limit G-11 Gaz-11 Concentration below measurement limit M-2 Maksymilian-2 Concentration below measurement limit M-3 Maksymilian-3 Concentration below measurement limit
W-1 Witold-1 32 34 34 0.42 0.42 0.57 0.82 0.15 0.17 2.26
W-3 Witold-3 31 34 34 0.38 0.40 0.56 0.87 0.15 0.20 2.54
W-6 Witold-6 33 31 36 0.39 0.40 0.57 0.83 0.16 0.16 2.38
P-19 Potok-19 Concentration below measurement limit
E-7 Ewa-7 30 33 37 0.35 0.36 0.56 0.77 0.16 0.13 2.82
E-13 Ewa-13 34 34 32 0.37 0.46 0.56 0.82 0.15 0.13 2.52
Po-11 PoznanÂ-11 35 33 32 0.38 0.37 0.59 1.14 0.13 0.10 3.05
MA-3 Mac Allan-3 32 32 36 0.37 0.42 0.56 1.11 0.12 0.18 2.70
K-41 Kronen-41 32 29 39 0.46 0.46 0.52 1.38 0.14 0.22 2.25
T-10 TrzesÂnioÂw-10 29 37 34 0.21 0.27 0.47 0.83 0.29 0.11 1.00
a hop/ster=17a(H)21b(H)C
30hopane (m/z 191)/5a(H)14a(H)17a(H) (20R+20S)C29and 5a(H)14b(H)17b(H) (20R+20S)C29
Fig. 6. Plot of aromaticity (B=toluene/nC7) vs. paranicity (F=nC7/methylcyclohexane) for Potok Fold oils (after Thompson, 1987;
Holba et al., 1996). Zones: 1, petroleum as generated; 2, higher maturity or source eect; 3, early phase fractionation and possible biodegradation; 4, heavy phase fractionation residual oil; 5, biodegradation.
Fig. 9. Mass fragmentograms (m/z191,m/z217) showing hopane and sterane distributions in two oils (saturated fractions GC±MS analysis) from Potok-Turaszowka and Jaszczew ®elds.
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degraded oils have an enhanced concentration of the light hydrocarbons in the condensate range. The third block in the sequence contains mainly degraded oils from Kroscienko ®eld, accumulated in the Ciezkowice Sandstones. The oil from the most eastern part, Trzes-niow, appears to be relatively immature. The suspected ``contamination'' of the oil Menilite biomarkers makes its assignment to any oil family dicult. Additionally, mixing of oils derived from shales with oils derived from other source-rocks might be possible, but this requires further work to be veri®ed.
5. Conclusions
Nineteen oil samples from Potok fold were analysed to identify oil families and to correlate oils from one tectonic unit but from separate tectonic blocks. According to the applied biomarker parameters, all oils seem to have originated from similar source.
The presence of higher plant biomarker, e.g. olea-nane, indicates that the source rocks of these oils are most likely the Lower Istebna beds.
The most mature from other oils are located in the west of the Potok fold and are buried at greatest depth. The Trzesniow oil reservoired in Oligocene Menilite sands indicates lower maturity. The biomarker traces suggest that this oil is contaminated with immature biomarkers from the Menilite Shale.
Biodegraded oils are located in the eastern part of the Potok fold (Potok-Turaszowka ®eld) and the intensity of biodegradation is almost constant in the oil ®eld. Some oils appear to have been biodegraded and then a second input of light oil has migrated into the reservoirs diluting the biodegraded oils.
The most probable explanation is migrational frac-tionation of light oils escaping from deeper over-pres-sured reservoirs.
The results presented in this paper, may be useful in clarifying probable mechanisms for oil mixing in this region, especially mixing of oils derived from Menilite shales with oils derived from other source-rocks.
Acknowledgements
We are indebted to Dr. M. Chiaramonte, Dr. Sedat Inan and an anonymous reviewer for constructive criticisms and suggestions that greatly improved the manuscript.
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