GeolCarp_Vol59_No3_225

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

, ANĐA ALAJBEG

SABINA STRMIĆ PALINKAŠ

, VLASTA TARI-KOVAČIĆ

LADISLAV PALINKAŠ

and JORGE SPANGENBERG

Center for Marine and Environmental Research, “Ru er Bošković” Institute, Bijenička 54, PP 180, HR-10002 Zagreb, Croatia;

zeljka.fiket@irb.hr

INA- Strategic Development, Research and Investment Sector, HR-10002 Zagreb, Croatia

Institute of Mineralogy and Petrography, Faculty of Science, University of Zagreb, HR-10000 Zagreb, Croatia

INA-Exploration, HR-10000 Zagreb, Croatia

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 (

O) and kerogen

(

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-

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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

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).

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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‰

C) and USGS24

(—15.9‰

C)), was better than 0.1‰.

Isotope analysis of carbonates

Thirty-six carbonates were separated for stable isotope

analyses. Extraction of CO

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

C= + 2.1‰ and

O= + 29.4‰), is less than ± 0.05‰

for

C and ± 0.1‰ for

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

, S

, and S

) 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.

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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

—C

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

and n-C

range from 0.24 to

0.92 for Pr/n-C

ratio and from 0.46 to 1.09 for Ph/n-C

ratio.

In Upper Cretaceous potential source rock extracts the n-al-

kanes are found to be present in the C

—C

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

and

n-C

18—19

whereas in samples from Hajdukovića Mlin and Prapatnica
(Fig. 5) n-alkanes maximize at n-C

and n-C

respectively.

The Pr/n-C

(0.59—2.38) and Ph/n-C

(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

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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

—

with a major mode centred at n-C

and a secondary mode

centered at n-C

(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

—C

range, maximized at C

23.

Among pentacyclic terpanes

and C

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

= n-alkanes with x carbon number; Pr = pristane;

Ph = phytane.

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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

hopane

as the most abundant hopane,

while in Upper Cretaceous potential source rock extracts C

hopane is dominant (Fig. 6).

Regarding regular steranes predominantly composed of C

and C

homologs (Fig. 7), Jurassic extracts are characterized

by dominant content of C

27,

while in Cretaceous extracts C

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

—C

α(H)-ho-

panes maximized at C

and presence of C

and C

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

—C

α(H)-hopanes maximized at C

and again with C

and C

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

and C

homologs maxi-

mized at C

. 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

C values of Lower Jurassic carbonates range be-

tween —1.0 ‰ and 1.7 ‰ V-PDB (Table 1). The

C values

of Upper Jurassic carbonates display higher scatter ranging
between —3.9 ‰ and 4.3 ‰ V-PDB. The whole set of Jurassic
δ

C isotopic values falls within those documented for marine

limestones (Marshall 1992; Mahboubil et al. 2002).

For Upper Cretaceous carbonates

C values range from —

2 ‰ to 3.6 ‰ V-PDB. The

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

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

C of organic matter (i.e. variable contribution from marine

plankton, bacteria and algae) and variations in the productivity

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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

C-depleted dolomite in-

terbedded with shale from Hajdukovića Mlin, Plitvice lake
most likely indicate a contribution from biogenic carbon
sources.

The

O values of the Lower Jurassic samples show a high-

er scattering than the

C values, from —4.0 ‰ to —1.2 ‰

V-PDB (Table 1). The

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

O values ranging from

—4.5 ‰ to 4.2 ‰ V-PDB. Positive

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

, probably as a result of evaporation (Gill

et al. 1995).

The

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

C and

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.

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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

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.

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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

and C

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

ho-

molog is indicative of a predominantly marine input (Peters et
al. 2005).

In the m/z 217 mass fragmentogram of Jurassic extracts, C

steranes predominate over C

and C

steranes due to a high

contribution of algae to the organic source (Peters & Moldow-
an 1993). The relative abundance of C

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

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

and

0.55—0.60% R

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

) and Lower and Upper Jurassic potential

source rocks as marginally mature with R

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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