GEOLOGICA CARPATHICA, JUNE 2008, 59, 3, 225—236
www.geologicacarpathica.sk
Organic geochemistry of Jurassic-Cretaceous source rocks
and oil seeps from the profile across the Adriatic-Dinaric
carbonate platform
ŽELJKA FIKET
*1
, ANĐA ALAJBEG
2
,
SABINA STRMIĆ PALINKAŠ
3
, VLASTA TARI-KOVAČIĆ
4
,
LADISLAV PALINKAŠ
3
and JORGE SPANGENBERG
5
1
Center for Marine and Environmental Research, “Ru er Bošković” Institute, Bijenička 54, PP 180, HR-10002 Zagreb, Croatia;
*
zeljka.fiket@irb.hr
2
INA- Strategic Development, Research and Investment Sector, HR-10002 Zagreb, Croatia
3
Institute of Mineralogy and Petrography, Faculty of Science, University of Zagreb, HR-10000 Zagreb, Croatia
4
INA-Exploration, HR-10000 Zagreb, Croatia
5
Institute of Mineralogy and Geochemistry, University of Lausanne, BFSH-2, CH-1015 Lausanne, Switzerland
(Manuscript received November 23, 2006; accepted in revised form November 21, 2007)
Abstract: Organic geochemical and stable isotope investigations were performed to provide an insight into the deposi-
tional environments, origin and maturity of the organic matter in Jurassic and Cretaceous formations of the External
Dinarides. A correlation is made among various parameters acquired from Rock-Eval, gas chromatography-mass spec-
trometry data and isotope analysis of carbonates and kerogen. Three groups of samples were analysed. The first group
includes source rocks derived from Lower Jurassic limestone and Upper Jurassic “Lemeš” beds, the second from Upper
Cretaceous carbonates, while the third group comprises oil seeps genetically connected with Upper Cretaceous source
rocks. The carbon and oxygen isotopic ratios of all the carbonates display marine isotopic composition. Rock-Eval data
and maturity parameter values derived from biomarkers define the organic matter of the Upper Cretaceous carbonates
as Type I-S and Type II-S kerogen at the low stage of maturity up to entering the oil-generating window. Lower and
Upper Jurassic source rocks contain early mature Type III mixed with Type IV organic matter. All Jurassic and Creta-
ceous potential source rock extracts show similarity in triterpane and sterane distribution. The hopane and sterane
distribution pattern of the studied oil seeps correspond to those from Cretaceous source rocks. The difference between
Cretaceous oil seeps and potential source rock extracts was found in the intensity and distribution of n-alkanes, as well
as in the abundance of asphaltenes which is connected to their biodegradation stage. In the Jurassic and Cretaceous
potential source rock samples a mixture of aromatic hydrocarbons with their alkyl derivatives were indicated, whereas
in the oil seep samples extracts only asphaltenes were observed.
Key words: Jurassic, Cretaceous, Adriatic-Dinaric carbonate platform, biomarkers, potential source rock, oil seep.
Introduction
The Adriatic-Dinaric carbonate platform (ADCP) (Fig. 1), de-
veloped during the Alpine evolution of the Dinaric parts of the
Tethys (Pamić 1993), is composed of up to 8 km thick carbon-
ate and shallow-water carbonate-evaporite sequences (Fig. 2).
Potential and effective source rocks in the ADCP include:
(1) Carboniferous mudstones and shales and Middle Permian
laminated limestones and shales; (2) Middle Triassic marls,
shales and dolomicrites; (3) Upper Jurassic “Lemeš” beds –
sediments deposited in a deeper bay of Tethys, represented by
thin-bedded to platy limestones interbedded with cherts, in
some places with rich ammonite assemblages (Furlani 1910;
Chorowicz & Geyssant 1972) and (4) Lower and Upper Creta-
ceous carbonates. Some Lower and Middle Eocene limestone
formations contain immature or early mature source rocks
(Pamić 1993). Oil seeps in the ADCP were correlated to
source rocks of various stratigraphic levels ranging from the
Middle Triassic to the Upper Cretaceous (Moldowan et al.
1992; Jerinić et al. 1994).
This paper presents data on Rock-Eval pyrolysis, the stable
isotopic composition of carbonates (
δ
13
C,
δ
18
O) and kerogen
(
δ
13
C), as well as the molecular distribution of saturated and
aromatic compounds from the Jurassic and Cretaceous poten-
tial source rocks and oil seeps, collected along the 400 km
long profile in ADCP (Fig. 1).
The aim of this paper is geochemical characterization of the
depositional environments and maturation history of the or-
ganic matter (OM) in carbonate deposits of the ADCP during
the Jurassic and Cretaceous Periods.
Geological setting
The studied area extended along the Croatian coast-line, in-
cluding Hvar and Brač Islands, from Metković (43.05°N,
17.65°E) to Senj (44.99°N, 14.90°E) (Fig. 1).
The Dinarides form a complex fold, thrust and imbricate
belt which developed along the northeastern margin of the
Adriatic (Dewey et al. 1973) or Apulia microplate (Ricou et
al. 1986; Dercourt et al. 1993). The Dinarides, which can be
traced along-strike for about 700 km, merge in the north-west
with the Southern Alps and in the south-east with the Helle-
nides (Pamić et al. 1998). The largest part of the Central Di-
226
FIKET, STRMIĆ PALINKAŠ, PALINKAŠ, TARI-KOVAČIĆ, ALAJBEG and ŠPANIĆ
narides, despite their complex fold, thrust and imbricate struc-
ture, is characterized by a regular zonal pattern in the spatial
distribution of characteristic Mesozoic—Paleogene tectonos-
tratigraphic units developed during the Alpine evolution in the
Dinaric parts of the Tethys (Pamić et al. 1998). From the
south-west to the north-east, that is from the Adriatic micro-
plate towards the Pannonian Basin, the following tectonos-
tratigraphic
units,
originating
in
different
Tethyan
environments, can be distinguished: (1) Adriatic-Dinaric car-
bonate platform (ADCP) – the External Dinarides (Fig. 1);
(2) carbonate—clastic sedimentary rocks, in some places with
flysch signatures, of the passive continental margin of the Di-
naric Tethys; (3) ophiolites associated with genetically relat-
ed sedimentary formations (the Tethyan open-ocean realm);
(4) sedimentary, igneous and metamorphic units of the Eu-
roasian active continental margin; (5) Paleozoic—Triassic
nappes, which are thrust onto the Internal Dinarides units;
their frontal parts directly overlying the northeastern margin
of the ADCP. The tectonostratigraphic units 2 to 4 define the
Internal Dinarides.
From the Liassic period to the Middle Eocene, the ADCP
was an isolated platform surrounded by the Tethys Ocean. The
final disintegration of the ADCP started in the Senonian with
regional tectonic movements resulting in uplift, partial regres-
sion and flysch deposition (Pamić et al. 1998). Tangential tec-
tonics reduced transversally the area of the ADCP to the about
700 km long and 50—250 km wide thrust belt commonly
named the External Dinarides (Velić et al. 2001). From the
Late Triassic to the Middle Eocene, for almost 150 million
years, the ADCP was a relatively stable, shallow-marine plat-
form; global sea-level changes and synsedimentary tectonics
influenced both platforms periodically but not contemporane-
ously, generating about 5 to 8 km thick carbonate sequences.
The carbonate rocks and carbonate-evaporite shallow-water
deposits in the ADCP include effective and potential source
rocks, oil seeps, and ore deposits associated with organic mat-
ter of different ages starting from the Carboniferous and con-
tinuing up to the Paleogene.
Samples and methods
Sampling and geochemical analysis
A total of 13 Jurassic and 23 Cretaceous samples of poten-
tial source rocks and oil seeps were collected from fresh rock
surface exposures at 12 localities along a profile in the ADCP
(Fig. 1). The studied Jurassic and Cretaceous potential source
rock samples are represented by carbonates which contain
autochthonous organic matter, kerogen and associated bitu-
Fig. 1. Map of the Adriatic-Dinaric
carbonate platform with marked lo-
calities of sampled potential source
rocks and oil seeps. ( – Creta-
ceous potential source rock; –
Cretaceous oil seep;
– Jurassic
potential source rock).
227
ORGANIC GEOCHEMISTRY OF SOURCE ROCKS FROM THE ADRIATIC-DINARIC CARBONATE PLATFORM
men. The Cretaceous oil seep samples are represented by
carbonates with pores, fissures and cavities filled with mi-
grated bitumen.
Samples were chosen based on the literature and knowledge
accumulated at the Croatian Oil Company INA in order to rep-
resent carbonate formations of different depositional environ-
ments developed during the Jurassic and Cretaceous Periods.
To remove superficial contamination from handling and
weathered material, the rock samples were cut into slabs with
a water cooled saw, washed with deionized water and distilled
ethanol and dried at 50 °C for 48 h. The cleaned samples were
milled in an agate ball-mill, and analysed for distribution of
hydrocarbons and isotopic composition of carbonates and ker-
ogen according to the procedure described by Spangenberg &
Macko (1998).
All the samples were subjected to TOC and Rock-Eval anal-
ysis in order to investigate basic potential source rock proper-
ties and select a reduced number of samples for further
detailed analysis. Rock powders were submitted to total or-
ganic carbon (TOC) and Rock-Eval analysis at the Humble
Geochemical Services Division (Humble, TX 77347).
An aliquot of selected samples (150—200 g) was extracted
with dichloromethane (DCM) (200 ml) for 6 days, with a
change of solvent after the first 48 h in the case of samples
Fig. 2. Stratigraphic column of Cretaceous and Jurassic carbonate deposits in the Adriatic-Dinaric carbonate platform. ( – Cretaceous
potential source rock; – Cretaceous oil seep; – Jurassic potential source rock).
228
FIKET, STRMIĆ PALINKAŠ, PALINKAŠ, TARI-KOVAČIĆ, ALAJBEG and ŠPANIĆ
with low TOC ( < 1 %), or Soxhlet extracted (ca. 50 g of sam-
ple) for 72 h with DCM in the case of samples with high TOC
( > 1 %). The extractable organic matter (EOM) was desulphur-
ized with activated Cu (24 h, room temperature). The extracts
were fractionated by silica-alumina liquid chromatography into
saturated, aromatic and polar compounds. Prior to liquid chro-
matography asphaltenes were precipitated from oil seeps and
potential source rock extracts with TOC > 2 wt. % with hexane
at room temperature. Chemical characterization of saturated and
aromatic hydrocarbons was performed with an Agilent Tech-
nologies 6890 GC coupled to an Agilent Technologies 5973
quadrupole mass selective detector (gas chromatography-
mass spectrometry – GC/MS) using a HP-ULTRA-2 fused-
silica capillary column (50 m
×0.20 mm i.d. coated with
0.11
µm cross-linked 5%-diphenyl—95%-dimethyl siloxane as
stationary phase) and He as carrier gas. The samples were in-
jected splitless at 280 °C. After an initial period of 1 min at
70 °C, the column was heated to 280 °C at 5 °C/min followed
by an isothermal period of 20 min. The MS was operated in the
electron impact mode at 70 eV, source temperature of 250 °C,
emission current of 1 mA and multiple-ion detection with a
mass range from 50 to 700 amu. Compound identifications are
based on comparison of standards, GC retention time, mass
spectrometric fragmentation patterns and literature mass spectra.
Isotope analysis of kerogen
The insoluble organic matter (kerogen) was obtained
by acidification of the extracted sample with 6 N HCl for
24 h and HF for 48 h. The oven-dried residues (consisting
mostly of kerogen, a little quartz and clay) were analysed
for carbon isotopic composition by using a Carlo Erba
1108 EA connected to a Finnigan MAT Delta S IRMS via
a Conflo II split interface (EA/IRMS). The isotopic com-
position is reported in delta (
δ) notation as the per mil
(‰) deviation relative to the Vienna Pee Dee Belemnite
(V-PDB). The reproducibility of the EA/IRMS analyses,
assessed by replicate analyses of a laboratory standard
(glycine (—25.8‰), urea (—43.1‰
δ
13
C) and USGS24
(—15.9‰
δ
13
C)), was better than 0.1‰.
Isotope analysis of carbonates
Thirty-six carbonates were separated for stable isotope
analyses. Extraction of CO
2
from the carbonates was
done by reaction with 100% phosphoric acid (4 h, 50 °C)
in a closed reaction vessel (McCrea 1950). Carbon and
oxygen isotopic compositions were measured via dual in-
let on a Thermoquest/Finnigan Delta S mass spectrome-
ter. The results were corrected for carbonate-phosphoric
acid fractionation using the factors of 1.010600 for dolo-
mite (Rosenbaum & Sheppard 1986) and 1.009311 for
calcite (Friedman & O’Neil 1977). The stable C and O
isotope ratios are reported in delta (
δ) notation as the per
mil (‰) deviation relative to the V-PDB international
standard. Analytical uncertainty, assessed by replicate
analyses of the laboratory standard (Carrara marble,
δ
13
C= + 2.1‰ and
δ
18
O= + 29.4‰), is less than ± 0.05‰
for
δ
13
C and ± 0.1‰ for
δ
18
O.
Results
Total organic carbon and Rock-Eval pyrolysis
The total organic carbon (TOC) values of Lower and Upper
Jurassic carbonates range from 0.04 to 0.13 % (Table 2). For
Upper Cretaceous and Cretaceous-Paleocene samples higher
values were recorded, up to 3.1 % and 6.0 %, respectively
(Table 3).
The Rock-Eval pyrolysis data (S
1
, S
2
, and S
3
) are used to as-
sess the temperature of maximum hydrocarbon generation
(T
max
) and to determine the hydrogen (HI), oxygen (OI) and
production (PI) indices for chemical characterization of prima-
ry in situ sedimentary organic matter (Peters 1986). The re-
sults of Rock-Eval pyrolysis are presented in Table 2 and
Table 3, conventional HI vs. OI (Fig. 3) and HI vs. T
max
(Fig. 4) plot.
Acyclic hydrocarbons
In gas chromatograms of all fractions of saturated hydrocar-
bons which did not undergo severe biodegradation, normal al-
kanes and acyclic isoprenoids pristane (Pr) and phytane (Ph)
are the main resolvable compounds.
Table 1: Stable carbon and oxygen isotope data of the analysed carbonates.
229
ORGANIC GEOCHEMISTRY OF SOURCE ROCKS FROM THE ADRIATIC-DINARIC CARBONATE PLATFORM
In Jurassic potential source rock extracts the n-alkanes are
easily noticed in C
12
—C
31
range, maximized at n-C
16—18
(Fig. 5). Phytane is generally dominant over pristane with
Pr/Ph ratio values between 0.59 and 0.82. The only exception
is the sample from Velić (Lower Jurassic) with Pr/Ph = 2.34.
The ratios between acyclic isoprenoids (Pr and Ph) and their
neighbouring n-alkanes n-C
17
and n-C
18
range from 0.24 to
0.92 for Pr/n-C
17
ratio and from 0.46 to 1.09 for Ph/n-C
18
ratio.
In Upper Cretaceous potential source rock extracts the n-al-
kanes are found to be present in the C
11
—C
25
range. In samples
Table 2: The Rock-Eval pyrolysis parameters of the analysed Jurassic potential source rock samples.
Table 3: The Rock-Eval pyrolysis parameters of the analysed Cretaceous potential source rock and oil seep samples.
from Sućuraj, Hvar Island and Selca, Brač Island n-alkanes
exhibit a bimodal distribution maximizing
at n-C
13
and
n-C
18—19
,
whereas in samples from Hajdukovića Mlin and Prapatnica
(Fig. 5) n-alkanes maximize at n-C
21
and n-C
18
,
respectively.
The Pr/n-C
17
(0.59—2.38) and Ph/n-C
18
(0.78—3.10) ratio val-
ues are higher then for Jurassic samples with Pr/Ph ratios
ranging from 0.32 to 0.99.
In oil seep extracts from Vrgorac (Upper Cretaceous), Pak-
lenka (Upper Cretaceous) and Dračevo (Cretaceous-Pale-
ocene) no n-alkanes were identified. Only oil seep samples
230
FIKET, STRMIĆ PALINKAŠ, PALINKAŠ, TARI-KOVAČIĆ, ALAJBEG and ŠPANIĆ
from Škrip, Brač Island (Upper Cretaceous) contained n-al-
kanes displaying a bimodal distribution in the range of C
16
—
C
35
with a major mode centred at n-C
21
and a secondary mode
centered at n-C
27
(Fig. 5).
Cyclic hydrocarbons
In all potential source rock extracts, triterpanes (m/z = 191)
(Fig. 6) and steranes (m/z = 217) (Fig. 7) show a similar distri-
bution pattern. The m/z 191 mass chromatograms are dominat-
ed by three groups of pentacyclic terpanes: Tm, norhopanes
and hopanes (Fig. 6). The tricyclic terpanes are found only in
source rock samples from Velić (Lower Jurassic) and Svilaja
Mt (Upper Jurassic) and two oil seep samples from Vrgorac
(Upper Cretaceous) and Dračevo, Metković (Cretaceous—Pa-
leocene). When present, tricyclic terpanes are found in the
C
19
—C
24
range, maximized at C
23.
Among pentacyclic terpanes
C
31
and C
32
17
α(H)-homohopanes are found and character-
ized with predominance of 22S over 22R epimers. The con-
centrations of homohopanes decrease with an increase of their
molecular mass which is characteristic for suboxic conditions
during OM deposition (Peters & Moldowan 1991). The ap-
Fig. 3. Hydrogen index (HI) vs. oxygen index (OI) plot of the analy-
sed samples.
Fig. 4. Hydrogen index (HI) vs. T
max
plot of the analysed samples.
Fig. 5. Ion chromatograms showing the distribution of alkanes (m/z
71) in potential source rock (a,b) and oil seep (c) samples. Peak
identifications: C
x
= n-alkanes with x carbon number; Pr = pristane;
Ph = phytane.
231
ORGANIC GEOCHEMISTRY OF SOURCE ROCKS FROM THE ADRIATIC-DINARIC CARBONATE PLATFORM
Fig. 6. Ion chromatograms showing the distribution of hopanes (m/z
191) in potential source rock (a,b) and oil seep (c,d) samples. Iden-
tification of the labeled compounds is given in Table 6.
plied lower final GC oven temperature (280 °C) resulted in
peak broadening and height lowering, causing the difference
to the results published by Moldowan et al. (1992). All Juras-
sic samples have C
29
hopane
as the most abundant hopane,
while in Upper Cretaceous potential source rock extracts C
30
hopane is dominant (Fig. 6).
Regarding regular steranes predominantly composed of C
27
and C
29
homologs (Fig. 7), Jurassic extracts are characterized
by dominant content of C
27,
while in Cretaceous extracts C
29
regular sterane is the most abundant (Fig. 7). In potential
source rock extracts from the Velić (Lower Jurassic) and
Svilaja Mt (Upper Jurassic) (Fig. 7) a remarkable amount of
pregnanes is found.
The Cretaceous oil seeps can be divided into two subgroups
based on their hopane and sterane distribution pattern (Fig. 6
and Fig. 7). Hopane (m/z 191) mass fragmentograms of oil
seep extracts from Vrgorac (Upper Cretaceous), Škrip, Brač
Island (Upper Cretaceous) and Dračevo, Metković (Creta-
ceous-Paleogene) are characterized by C
27
—C
30
17
α(H)-ho-
panes maximized at C
29
and presence of C
31
and C
32
17
α(H)-homohopanes with predominance of 22S diastere-
omers. Regular steranes in samples from Vrgorac (Upper Cre-
taceous) are below the detection limit. In oil seep extracts
from Paklenka (Upper Cretaceous) hopane distribution is
characterized by C
27
—C
30
17
α(H)-hopanes maximized at C
30
and again with C
31
and C
32
17
α(H)-homohopanes 22S diaste-
reomers predominating. Sterane distribution in all oil seep
samples is similar to those of Cretaceous potential source
rocks comprising predominantly C
27
and C
29
homologs maxi-
mized at C
29
. All oil seep extracts have one feature in com-
mon, they all contain pregnanes (Fig. 7).
Aromatic hydrocarbons
The aromatic hydrocarbon fractions of all potential source
rocks are dominated by alkyl derivatives of naphthalene,
phenantrene and dibenzothiophene. The methylphenantrene
index (MPI-1) values obtained from GC/MS data of aromatic
hydrocarbons are listed in Table 4. In oil seep extracts only as-
phaltenes were observed.
Isotopic composition of carbonates and kerogen
The
δ
13
C values of Lower Jurassic carbonates range be-
tween —1.0 ‰ and 1.7 ‰ V-PDB (Table 1). The
δ
13
C values
of Upper Jurassic carbonates display higher scatter ranging
between —3.9 ‰ and 4.3 ‰ V-PDB. The whole set of Jurassic
δ
13
C isotopic values falls within those documented for marine
limestones (Marshall 1992; Mahboubil et al. 2002).
For Upper Cretaceous carbonates
δ
13
C values range from —
2 ‰ to 3.6 ‰ V-PDB. The
δ
13
C values from —2.5 ‰ to 2.5‰
have been interpreted as characteristic of Cretaceous marine
limestones and dolomitic limestones (Hudson 1977; Moss &
Tucker 1995), although relatively higher
δ
13
C values were
also documented for marine carbonates (Anderson & Arthur
1983). The significant scatter of these values, up to 5.6 ‰,
likely reflects the primary compositional variability in the
δ
13
C of organic matter (i.e. variable contribution from marine
plankton, bacteria and algae) and variations in the productivity
232
FIKET, STRMIĆ PALINKAŠ, PALINKAŠ, TARI-KOVAČIĆ, ALAJBEG and ŠPANIĆ
rate during deposition (Hollander & McKenzie 1991; Fogel &
Cifuentes 1993).
However, samples from Hajdukovića Mlin, Plitvice Lake
(Upper Cretaceous) which display values between —10 ‰
and —5 ‰ were also recorded. The
13
C-depleted dolomite in-
terbedded with shale from Hajdukovića Mlin, Plitvice lake
most likely indicate a contribution from biogenic carbon
sources.
The
δ
18
O values of the Lower Jurassic samples show a high-
er scattering than the
δ
13
C values, from —4.0 ‰ to —1.2 ‰
V-PDB (Table 1). The
δ
18
O values of Upper Jurassic carbon-
ates cover a narrower range, from —5 ‰ to —4.2 ‰ V-PDB.
The oxygen isotope behaviour of the Cretaceous samples
exhibits a considerable scatter with
δ
18
O values ranging from
—4.5 ‰ to 4.2 ‰ V-PDB. Positive
δ
18
O values are characteris-
tic of samples from Dračevo, Metković (Cretaceous-Paleo-
cene) and Sućuraj, Hvar Island (Upper Cretaceous). Since
these values are too high to have formed from normal sea wa-
ter these heavy carbonates must have been precipitated from
fluids enriched in O
18
, probably as a result of evaporation (Gill
et al. 1995).
The
δ
13
C values of kerogens and asphaltenes from Upper
Cretaceous carbonates (Table 5) vary between —26.2 ‰ and
—20.4 ‰ V-PDB and —26.9 ‰ and 22.3 ‰ V-PDB, respec-
tively, whereas the analysed Jurassic carbonates display
slightly lower values for kerogens in the range —27.9 ‰ to
—24.0 ‰ V-PDB. The significant scatter of these values is
most likely due to changes in the source of organic matter
during deposition.
Discussion
By combining geochemical and isotope analyses of selected
potential source rock and oil seep samples, especially the cor-
relation of certain biomarker compounds, we have attempted
to determine the depositional environments, origin and matu-
rity of the OM from the Jurassic and Cretaceous carbonates of
the ADCP.
The source and deposition of environment indicators
The
δ
13
C and
δ
18
O values of Lower and Upper Jurassic, as
well as Upper Cretaceous carbonates fall within the range
characteristic of marine limestone and dolomitic limestone.
The distinctive scatter of obtained values reflects variability in
the primary composition of organic matter (algae and/or bac-
teria) in the Jurassic and Cretaceous sedimentary environ-
ments.
The low organic matter content (TOC < 0.13 %) is a general
characteristic of the Jurassic sediments. Higher TOC values
were detected in Upper Cretaceous (up to 3.1 %) and Creta-
ceous-Paleocene (up to 6.0 %) sediments. The wide range of
obtained values (Table 2 and Table 3) is indicative of facies
variability of investigated Jurassic and Cretaceous potential
source rocks.
The pristane/phytane (Pr/Ph) ratio is an indicator of the re-
dox potential of the depositional environment (Didyk et al.
1978; Chappe et al. 1982), affected by source input, maturity
Fig. 7. Ion chromatograms showing the distribution of steranes (m/z
217) in potential source rock (a,b) and oil seep (c,d) samples. Iden-
tification of the labeled compounds is given in Table 6.
233
ORGANIC GEOCHEMISTRY OF SOURCE ROCKS FROM THE ADRIATIC-DINARIC CARBONATE PLATFORM
Table 4: Distribution of molecular parameters determined from GC/MS analyses.
Table 5: Stable carbon isotope data of kerogen and asphaltenes
from the analysed samples.
and salinity (Goosens et al. 1984; ten Haven et al. 1987).
The low Pr/Ph ratio (0.32—0.99) found in all samples is con-
sidered to reflect the carbonate lithology and low TOC con-
tent (Hughes et al. 1995). The only exceptions are the
samples from the Velić (Lower Jurassic) with a Pr/Ph ratio
of 2.34 suggesting a different depositional environment.
The distribution of n-alkanes with predominance in the
C
14
—C
19
range indicates a dominant marine algal source of
OM for all Cretaceous and Jurassic potential source rocks.
The absence of n-alkanes in oil seep extracts from Vrgorac
(Upper Cretaceous), Paklenka (Upper Cretaceous) and
Dračevo (Cretaceous—Paleocene) can be attributed to bio-
degradation of migrating hydrocarbons in these samples (Pe-
ters & Moldowan 1993).
The composition of biomarker compounds, especially ste-
roid and triterpenoid derivatives, are of special interest be-
cause these compounds reflect the depositional environments,
origin and diagenetic/maturation history of sedimentary or-
ganic matter (Peters & Moldowan 1993; Peters et al. 2005).
The relative concentration of steranes and terpanes reflects the
eukaryotic and prokaryotic contributions to the organic matter
Table 6: Assignation of compounds in the m/z 191 and m/z 217
mass fragmentograms shown in Fig. 6 and Fig. 7.
234
FIKET, STRMIĆ PALINKAŠ, PALINKAŠ, TARI-KOVAČIĆ, ALAJBEG and ŠPANIĆ
of sediments (Peters et al. 2005). The higher contents of ho-
pane over sterane in all the studied samples, Cretaceous and
Jurassic, are indicative of organic matter deposited within the
carbonate platform (Marynowski et al. 2000).
The relative distribution of C
31
and C
32
homohopanes can
imply redox potential during and after deposition of carbon-
ates (Peters et al. 2005). Presence of short-chain tricyclic ter-
panes is indicative of a bacterial contribution to the organic
matter (Marynowski et al. 2000). High abundance of C
23
ho-
molog is indicative of a predominantly marine input (Peters et
al. 2005).
In the m/z 217 mass fragmentogram of Jurassic extracts, C
27
steranes predominate over C
28
and C
29
steranes due to a high
contribution of algae to the organic source (Peters & Moldow-
an 1993). The relative abundance of C
29
steranes in Creta-
ceous samples with 20S/20S+ 2 0R ratios of isomerization
around 0.47 (equilibrium values= 0 .55) indicates a relatively
low thermal maturity of samples (Yangming et al. 2005).
The overall higher proportion of pregnane relative to the
sterane content can be related to backbone rearrangement ca-
talysed by clay minerals (van Kaam-Peters et al. 1998). Dia-
sterane to sterane ratios do not correlate with clay content but
depend on the amount of clay relative to the amount of organ-
ic matter present (van Kaam-Peters et al. 1998). During early
diagenesis high clay/TOC ratios may favour backbone rear-
rangement over reduction of steranes.
The high abundance of dibenzothiophenes in the investigat-
ed samples is considered to be the result of a sulphurization
process of planktonic lipids and carbohydrates. The sulphur-
ization of algal-derived biolipids is suggested to be an impor-
tant mechanism for the selective preservation of these
molecules during early diagenesis (Kohnen et al. 1990; Russel
et al. 2000).
Thermal maturity
Plots of HI vs. OI as a van Krevelen-type diagram (Fig. 3)
and HI vs. T
max
(Fig. 4) were used for identification of the
type of organic matter, thermal maturity and its level of sec-
ondary alteration (Kenig et al. 1994).
According to the hydrogen and oxygen indices (HI = 34—767,
OI = 5—206), the organic matter of the Upper Cretaceous sam-
ples plot close to the maturation pathways typical of Type I
and Type II oil prone kerogen (Fig. 3 and Fig. 4). This is in
agreement with the biomarker composition data of these rocks
showing that their OM consists predominantly of algal materi-
al. The low HI values obtained for oil seep samples from
Škrip, Brač Island (Upper Cretaceous) and low HI and OI val-
ues for oil seep samples from Vrgorac (Upper Cretaceous), al-
though characteristic for Type III and Type IV OM, reflect
alteration of samples due to oxidation. Since relatively high
abundance of benzothiophene compounds is indicated in the
analysed Upper Cretaceous source rock and oil seep samples,
implying high organic sulphur concentration, kerogen types
might be additionally marked by S; namely Type I-S and
Type II-S. The Upper and Lower Jurassic samples, according
to HI (0—385) and OI (236—1178) values, plot along the ther-
mal evolution line characteristic for Type III and Type IV or-
ganic matter (Fig. 3 and Fig. 4).
The C
31
homohopanes 22S/(22S + 22R) ratio of the studied
samples ranges between 0.52 and 0.60 (Table 4), which is
below or equal to the equilibrium end point value of about
0.57—0.60 (Peters et al. 2005), indicating that these samples
are near or at the beginning of the oil-generating window
(Peters et al. 2005).
The MPI-1 values differ between the Jurassic and Creta-
ceous samples (Table 4) corresponding to a maturity range of
approximately 0.44—0.55% R
c
and
0.55—0.60% R
c
,
respective-
ly, implying thermal immaturity or low maturity of Upper
Cretaceous organic matter and the beginning of the oil-gener-
ating window for Jurassic organic matter. These low maturity
levels are in agreement with the low geothermal gradients ob-
served in that area (Cota & Baric 1998).
Since all studied kerogens contain reasonable amounts of
the organically bound sulphur, as reflected in the presence of
benzothiophenes, they might be expected to release hydrocar-
bons at relatively low maturity stage. In sulphur rich kerogen,
weak S—C bonds require significantly lower activation energy
during cracking, and therefore hydrocarbon generation occurs
at lower maturity levels (Orr 1974; Baric et al. 1988; Cota &
Baric 1998). The low T
max
(379 to 433 °C) values observed
are, therefore, probably related to the type of kerogen rather
than to immaturity of the organic matter.
The data presented here are based on a rather limited num-
ber of samples and cannot be used as representative of all the
processes influencing geological organic matter in the ADCP
during Jurassic and Cretaceous Periods. Therefore, further
more detailed research is required in order to obtain better in-
sight into depositional environments, origin and maturation
history of geological organic matter in the Jurassic and Creta-
ceous formations of the External Dinarides.
Conclusion
The distinctive scatter of carbon and oxygen isotope val-
ues in the studied samples is indicative of variation in the
sources of the organic matter. The Rock-Eval pyrolysis and
biomarker composition data characterize Upper Cretaceous
organic matter in the studied potential source rock samples
as Type I-S oil prone and Type II-S oil and gas prone kero-
gen at the immature to early mature oil-generating level. The
organic matter from the Lower and Upper Jurassic potential
source rocks is characterized as early mature Type III and
Type IV kerogen. Low Pr/Ph ratio values in all the analysed
samples mainly reflect suboxic to anoxic OM depositional
environments. The biomarkers distribution indicates the pre-
dominantly algal and bacterial marine organic matter input
with rare terrestrial organic matter. The sterane and triter-
pane maturity parameters confirm Upper Cretaceous poten-
tial source rocks as thermally immature, that is approaching
or just entering the oil-generating window (corresponding to
0.45—0.52 % R
c
) and Lower and Upper Jurassic potential
source rocks as marginally mature with R
c
values from 0.53
to 0.60 %. The high diasterane to sterane ratios found in
most of the studied samples reflect the backbone rearrange-
ment process catalysed by clay minerals. The presence of
dibenzothiophenes, as dominant compounds in most of the
235
ORGANIC GEOCHEMISTRY OF SOURCE ROCKS FROM THE ADRIATIC-DINARIC CARBONATE PLATFORM
aromatic fractions, reflects the sulphurization process of al-
gal-derived biolipids.
Acknowledgments: This research was supported by Suisse
National Science Foundation and the University of Lausanne
(SCOPES Project No. 7KRPJ065483.01). We thank to Vale-
rie Schwab and Jošt Lavrič for their help in preparation and
measurement of the samples, their patience and friendship.
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