GEOL. C ROAT. 49/2 1135 - 1431 8 Figs. 1 3 Tabs. 1 1 ZAGREB 1996 

Geochemical Study of Oils and Oil Source Rock from the Eastern Drava and 
Siavonija-Srijem Depressions, Pannonian Basin, Croatia 

Anda ALAJllEG',.T. Michael MOLDOWAN2
, Gerard.T. DEMAISON', Vladimir JELASKA", 

Zeljka llRODlC-JAKUPAK', Dubravko SVILKOVIC' and Bradly J. HUIZINGA6 

Key words: Biomarkers, Geochemical parameters, Oil , 
Oil source rock, Organic geochemistry. 

Abstract 
A Middle Miocene oi1 source rock has been identified by geo­

chemical logging of exp loration wells in the Eastern Drav(l depres­
sion (EDD) and in the Siavonija-Srijclll dep ression (SS D) al the 
soulh-cas t margin o f Ihe P<UHlonian Basin, CrO:lI ia. The source rock 
contains Type II to II / III kerogen. ,mel reaches carly maturity stage at 
<Icplhs of about 2400 m. The Mesozoic seq uences wefe found either 
to be absent or \0 be poor in organic mallcr within Ihe analysed wells . 

Acco rd ing to C27-CZ~-C'9 stcranc and C27-C2~-C2' !11onoaromati c 
steroid distributions , gtl !11m:lcerane indices, C.15 homohopanc indiccs 
and prcsence of C X! stefancs, the EDD and SSD o il s wcre generated 
from the same marine SOtlrce rock . The distribut ion o f homo hopanes 
and high Ni/(Ni + V) porphyri n r~Hios indicates Ihat the related source 
rock was deposited under s uboxic conditions. The oi ls appear to have 
been generated from Ihe sou rce rock during the early to midd le stages 

of maturity. T he oi ls are waxy, wilh pom points up to 30°C and sul­
phur contents not exceedi ng 0.7%. 

J. INTRODUCTION 

The Eastern Drava (E DD) and Siavon ija-Srijem 
(SSD) depressions share the common geologica l history 
or lhe sOll lh Pannonian Bas in (RO YD EN & HOR­
VATTl , 1988). This area has been studied by l-lERNITZ 
( 1983) and PANDZIC (1 986). The Basement, which is 
most ly composed o r a l11elamorphic complex intruded 
by He rcynian grani tes (PAMIC, 1986), is overlain by 
Ihe Palaeozoic phyllites, quarlzites, lurfs, eXlrusivcs and 
clastics. 

Kljucne rijeci: biomurkeri, geokemijski pa rametr i, 
nana, naftnomaticna stijena, organska geokem ija. 

Sazctak 
Telllcljem geokcmijskih podat~\ka za istrazile busotinc identitic i­

rana je srcd njomioccnska naftnomalicna slijcna U ]slocnodravskoj i 
Siavons ko -srijcmskoj POlol il1i, nl! jugoislocnom ru bu Panonskog 
baz:ena , H.-vats ka . Maticna stijena sad rz i kerogen li pa II. do 11 ./1 11. 
S t<ldij ra lle zrclos ti mal icna s tijcna dosci.c otprilikc na c1t1 bini zalijc­
ganja od 2.400 m. U ispitivanim busotinama Zit mezozoj skc jc slojeve 
utvrdeno cia Sli siromasn i organskolll tvari iIi da slojcvi nc(!osl<Iju. 

PrcJ11a raspodjcli Cn- C2~-C29 sterana i C27 - C2S-C2~ mo noaro­
m<1l sk ih stero ida , Ie gamaccfallskih indeksa, C35 hOlll o!lopans kih 
indeksa i prislitnosli C30 stefana, EDD i SSD naftc 51! OlpllS\C nC od 
iSle marins ke maticnc stijcne. Raspodjela homohopan:l i visok i por­
!"i .- ins ki omjeri Ni/(N i +V) ukazuju da je odnosna maticna s lijCll<l 
ista lol.ena u suboksicnim llvjclima. Nafte Sll otpustcnc od nmticne sli ­
jcnc 1I siad iju njezine rane do srcdnje zrclosti. T o Sli visokoparafinskc 
naftc sa sti ni slcm do 30°C, a sadr7.aj Sll mpora nc prclaz i 0.7 %. 

Prom Penno-'fr iassic to Latc T riassic a subs iding 
ca rbonate platform was widespread, gene rat ing lime­
stones and dolomites. Pe lagic sediment ati on in the 
Jurassic mainly produced muddy limestones. 

Cretaceous lithofacies include both shelf and deep 
sea clast ics . In th e Tethyan region, rclal cd to ocean 
anoxic events, "Scag lia" fac ies were genera ted in the 
Upper Cretaceous. Compressional tectonics, which had 
started duri ng Ihe Jurassic, init iated upl ifl and altered 
pallerns of sedimentation. Therefore remarkable uncon­
formities occur along th e Basin margins in Ihe Creta-

I INA-Induslrija nane, Rese;;lrch & Developl11elH, P.O. Box 1014, II R-I 0000 Zagreb. Croatia. 

2 Chevron Oil Field Research Co., P.O. Box 1627. Rich mond. CA 94802-0627, USA. Present Add ress: Stanford Un iversit y. Department of Geo­
logical and Environmental Sciences. Stanford, CA 94305-2115, USA. 

J Chev ron Overseas Petroleum Inc., P.O.Box 5046, San Ramon. CA 94583 -0946, USA . Present Address: P.O.Box IR77. c;'lpitola. CA 95010, 
USA. 

I Uni versi ty of Zagreb. Faculty of Natural Sciences, Zvonil11irova Street 8/1 1, HR-I 0000 Zagreb, Croatia. 

5 INA-Industrija nafte, Naft'lplin. P.O.Dox 554, IIR - I 0000 Zagreb, Croatia. 

(. Chevron Overseas Petroleum Inc., P.O. Box 5046, San Ramon, CA 94583-0946, USA. Present Address: ARCO International Oi l and Gas Co., 
2300 Wesl Plano ParkwilY, Plano, Texas 75075-8499, USA. 



136 

• • • • • 024 
A 01S ••• 

0'9 ... 

enlarged area 

E. 

... 

... 

• 022 

N 

A 
"" . 

• • 023 

Geologia Cro:nica 49n 

Fig. I Sample loca ti on 
map. Legend: • - oil 
we ll s; .... - explo ra­
tion we ll s (only the 
well s subjected ( 0 

runher siudy arc 
add itiona ll y label­
led) . 

cco us. Above the ullcomformities Cretaceous flysch 
deposit s a rc found which cons ist of reworked pelagic 
material, terrigenous clays and turbidites. 

Shallow mari ne carbonate sedimentation cont inued 
in the Palaeogene only in a small area and was followed 
by a period of extensive erosion of the Mesozoic strata. 

During the Neogene, when compressional tec tonics 
turned [ 0 an extensional regime, rifting and subsidence 

took place, associated with taphrogcnic fo ld ing and vol­
cani sm, During the Mid-Latc Miocene, rapid sed imen­
ta ti on accumulated up to 2 km thi ck sequences in the 
depoccntcrs. Frequent vertica l and lat era l fa c ies 
changes are characteri stic of thesc deposits, mainly due 
to differences in the geological structures o f o lder sur­
rounding strata which supplied mate ria l to the Miocene 
depressions. The rapid sedime nt ati on las ted until the 

SAMPLE 02 016 019 07 024 08 02 01 03 022 023 

PHYSICAL PROPERTIES 

gravi ty, kg/I 0.874 0.871 0.860 0.843 0.877 0.859 0.899 0.951 0.882 0.897 0.870 
A PI , a1 15°C 29 3 1 35 35 31 33 27 35 28 26 30 
pour poi III , °C 27 26 30 24 27 30 30 24 2 1 -9 19 

ISOTOPE DEPLETION 

bl~C, ppt -25 .6 -25.5 -26.0 -26.4 -25 .8 -26.5 -25.9 -26.1 -24.6 -24.8 -25.3 

8 D, ppl - 158 - 160 - 148 - 138 - 14 1 - 152 - 143 - 14 1 - 154 - 154 - 152 

ELEMENTAL COMPOSITION 

II/C 1. 8 1.7 1.8 1.9 1.9 1.9 1.8 1.8 J.7 1.6 1.8 
S,% 0.7 0.7 OA 0.2 0.3 0.5 0.6 0.2 0.3 OA OA 
Ni/(Ni+V) 0.9 0.8 * • 1.0 0.9 1.0 0.9 1.0 1.0 1.0 

GROU P COMPOSITION 

SAT, % 68.0 67.9 72.4 70.7 67.7 65.3 0.53 0.74 60.7 41.8 59.5 
ARO,% 16.0 17.1 15.0 19.6 24.6 22.2 28.1 17.0 23.7 35.9 24.5 
NSO,% 12.5 11.0 8.6 7.5 6.3 10.0 14.5 5. 1 13.8 19.3 14.0 
ASPI-I ,% 3.5 4 .0 4.0 2.2 1.4 2.5 4.4 3.9 1.8 3.0 2.0 

GC PARAMETERS 

CPI 0.94 0.96 0.98 0.97 0.98 0.99 0.99 0.99 1.00 ** 0.97 
Prj Phyl 0.89 0.78 0.87 0.99 0.98 0.82 1.00 1.05 1.1 4 0.92 0.96 

Prjne ,? 0.87 0.68 0.67 0.54 0.66 0.64 0.95 0.63 lAO 5.68 0.77 

Table I Bulk properties of the EDD and SSD oi ls. Legend: :to = no porphyrins: ** = sample biodegraded. 



Abjbeg Cl al.: Geochernicnl Study of Oils and Oil Source Rock rrom the Eastcm Dravn and Sinvonija-S rijem Depressions .. , 137 

>­
f-
en 
Z 
w 
f­
Z 

Pr 

Pl iocene when depos its from coarse- to fine-grained 
clastics and pelites were deposited. 

Along the EDD and SSD the recent tempera tu re 
gradients are comparatively high (4-6°C/IOOm) proba­
bly as a consequence of a relat ively thin crust and the 
rifling characteristics of the Pannonian Basin wh ich is 
well known for high heal flow (e.g., DOYENYI & 
I-lORY krH, 1988). 

Commercial oils arc found in both the EDD and 
SSD. Thcy arc trapped in porous and/or fractured reser­
voirs, mainly in the Miocene Moslavacka Gora forma­
tion and in the Mesozoic strata, as we \! as in fractured 
basement. The related source rock has been studied 
recen[ly (ALAJBEG e[ al.. 1990; BARIC e[ al.. 1992; 
YULAMA, 1994; HERNITZ c[ aI. , 1995; BARIC e[ aI., 
1995). Thi s paper contributes detailed geochemical data 
as further evidence of this source rock. 

2. EXPERIMENTAL DATA 

Oils OS. 016, 019, 07, 024, 08, 02 and 01 from 
the EDD as well as oils 03 , 022 and 023 from [he 
SDD (Fig. 1) were studied for their physical properties: 
grav ity by ASTM D 1289 and pour point by ASTM D 
97; e lemental analysis: C , H, S, V and Ni; GC data: 

a 

b 

c 

RETE NT tON TtME 

Fig. 2 Examples of Ihe whole oil gas 
chromalograms a) 019: unimodal 
n-alkane diSlribulion - lite range 
of the inlensive peaks belongs to 
n-alkanes; b) 023: bimodal/tri ­
modal n-alkall e distribution - [he 
range of thc intensive peaks 
belongs to il-alkanes ; c) 0 22 : 
biodegraded oil sample - inten­
sive peaks belong 10 i-alkancs. 

whole oil gas chromatogram, carbon preference index 
(CPI) (BRAY & EYANS, 1961), pris[ane-[o-phy[ane 
ratio (Pr/Phyt) and pristane-to-normal heptaclecanc rat io 
(Pr/nC 17 ); group composition (T able I): saturates 
(SAT), aromatics (ARO), polars (POL) anel asphaltenes 
(ASPI-!) (PET ERS & MOLDOWAN, [993, p. 54) , as 
well as carbon and hydrogen isotope depletion 
(SCHOELL, 1984). 

T hc oils 0 16, OJ 9 anel 024 from [he EDD anel [he 
022 and 023 oil s from the SSD, which were founel [0 

exp ress some differences in bulk properties , were 
selected for biomarker study; tritcrpancs, stcrancs, and 
monoaromatic and triaromatic steroids (SEIFERT & 
MOLDOWAN, 1986; PETERS & MOLDOWAN, 
1993, p. 151-202 and p. 227-250). Cuttings from eight 
exp loration wells (Fig. I) were examined for source 
rock identi ficatioll. This included total organic carbon 
(Pig. 8), free hydrocarbons yield (S I) ' kerogen pyro­
Iyza[c yield (S2)' hyelrogen index (HI~lOO S2(fOC) and 
the temperature of the S2-peak maximum (Tmax). The 
organic matter in Miocene strata from the exploration 
well Ew (Fig. l) was studied microscopically for vitri­
nite retlcctance determ ination (Ro). Pyrolysis in an off­
li ne lube rcacror (ALAJl3EG & STIPAK, 1985) anel gas 
chromatography of oil asphaltenc pyrolysates was also 
performed. 



138 

>­
t: 

'" z 
UJ 
t­
Z 

t­
a: 

~ 

C,O 

3. RESULTS AND DISCUSSION 

The study or the genetic relationships among the 
oils relied on their bulk properties and biomarkers. 

The EDD and SSD oils are found 10 be rather waxy 
as refleeled in pour points oC up to 30°C Crable 1). The 
exception is the pour point _9°C of 022 which was dis­
covered in a relatively shallow trap (depth <1000 In, 

trap temperature 65°C) where it underwent biodegrada­
tion. For this oil the biodegradation is confirmed by the 
high predominance of pristane over n-heptadekane 
(Pr/nCI7~5.68). a comparatively low H/C (1.6) and sat­
urates yield (41.8%) as well as in the low intensity of 
normal alkanes as shown in whole oil gas chro­
rnatogram (Fig. 2c). The other oils show an intensive n­
alkanes range, sometimes exceeding n-C3sHW arranged 
in a (rough ly) unimodal distri bution (e.g. 019, in Fig. 
2a, by biomarkers found to be comparativcly the most 
mature one) and in a non-unimodal (bimoda!/trimodal) 
distribution (e.g. 023, in Fig. 2b, found [0 be of the 
comparatively lowest malUrity lcvel). Almost all the 
oils show a slight predominance of even n-alkanes (Cpr 
slightly less than one - Table 1). 

The aliphatic structure of the studied oils is also 
reflected in the products of asphaJtene pyrolysis (e.g. 
019), which are dominated by the series of doublets of 
n-alkanes and the related l-alkenes (Fig. 3) as well as a 
H/C atomic ratio up to 1.9 (Table I). 

The waxy character and the type or n-alkane distrib­
ution, may indicate that organic matter of algal (or 
reworked algal) origin served as the predominating oil 
precursor, which was possibly deposited in carbonate-

RETENTION TIME 

Geologia Croatica 49/2 

Fig. 3 GC chromatogram of asphal­
lene pyrolysis products for 019; 
thc intcnsivc peak doublcts 
belong to J-alkenes and n-ulka­
ncs, c.g. C ;ij is related LO I-dccenc 
and C 10 Lo n-decane. 

rich sediments (PETERS & MOLDOW AN, 1993). In 
those oils where porphyrins are detccted, Ni porphyrins 
are found to predominate strongly over Y::::O por­
phyrins, suggesting a suboxic to oxic depositional envi­
ronment of the source rock which released the studied 
oils (e.g., MOLDOWAN et aI., 1986). Thi s is also 
reflected by the low to moderate sulphur content (0.2% 
to 0.7%) in oils. 

The SSD oils, in comparison to the EDD oils, are 
found to be isotopically heavier (Table 1) and lower III 

pour points and H/C ratios. 

3.1. BIOMARKER RESULTS­
- OIL/OIL CORRELATION 

For biomarker analysis, 3 EDD samples were cho­
sen: 019 as the example which reaches the high pour 
point (30°C), and high API gravity (33°) and has no 
measurable porphyrins; 024 as the example which con­
tains no V~O porphyrins [Ni/(Ni+V)~I)l and reaches 
high I-l/C ratio (1.9); 016 which is characterized by the 
lowest H/C ratio (1.7) among the studied EDD oils and 
comparatively high sulphur (0.7%). Furthermore 2 SSD 
oils were taken: the biodegraded 022, and 023 which 
was found to be similar in physical properties to 03 . 

The biomarker source parameters for oil-to-oil cor­
relation of the five select oils from EDD and SSD indi ­
cate close correlation among the oils. The distributions 
of C27-C2S-C29 steranes were measured using metastable 
reac tion monitoring (MRM) GC MS. The steranes mea­
sured include the sum of isomers aaa20S, aaa20R, 
o:~~20S, o:~~20R, and the measured distributions clus-



Alajbeg ct al : Geochemical Study of Oi ls and Oi l Sourcc Rock rrOIll the Eastcrn Drava and SlavonijkSrijem Depressions ... 139 

c" 

LEGEND 

(:> O ll # 16 
t;I O il # 19 
a O IL#22 
fo O IL#23 
X O IL#24 

C" C" 

Fig. 4 $tcrane distribu ti on (m/z 21 7) ror se lect EDD and SSD oils ; 
Cn = cholestanes; C28 = ergostanes; C29 = stigmastanes. 

ter in a group indicating the similarity of oil for sterol 
precursors (F ig. 4, Table 2) . Steranc distributions 
refl ect the algal (eukaryoti c) input to the so urce 
(MACKENZIE, 1984). 

The distributions of C 27-C 2S-C29 C-ring monoaroma­
lie stero id s (MAS ), including the SLlm of isomers 
5a20S 5a20R 5~20S 5~20R dia-20R, dia-20S , cluster 

in a very tight group independently confirming the ste r­
ane results (Fig. 5). C-ring monoaromatic steroids are 
thought to be derived from sterols with a higher degree 
of unsaturation than is necessary [or the formation o[ 

steranes with, perhaps, a double bond in the side-chain 
(c.g. MOLDOWAN & FAGO , 1986; RIOLO ct aI. , 

1986), rellecting precursors other than the stcranes. The 

LEG END 

(:> O ll# 16 
>:'lOlL # 19 
o O IL#22 
~ O ll #23 
X O ll #24 

C'" C" 

fig. 5 MAS distribution (m/z 253) for select EDD and SSD oi ls; Cn 
= cho lestane type MA steroids; Cn = ergoslane type MA stero ids ; 
C2~ = stigmastane type MA steroids. 

steranes and MAS together indicate that the three EDD 
oils and two SSD oils examined were very probably 
genera ted from the same source. 

Homohopane concentrations in the different oils fol ­
Iowa similar pattern (Fig. 6), decreasing with increas­
ing C-number. Th is obscrvation togcther with a signi fi ­
cant preference for the C32 homolog, suggcst a mildly 
suboxic environment of deposition [or the source rock 
(MOLDOWAN et aI. , 1992). 24-n-Propyl choles tane 

was de tected by (MRM) GC MS (Fig. 7) indicating 

marine al gal input typical ly associated with a marine 
deposi tional environmenl (MOLDOWAN ct aI. , 1990). 

Low gammacerane indi ces [100 x gammace rane/17o., 

2 1~ (H) - hopanel suggest that the source rocks were 

SOURCE PARAMETERS 

SAMPLE STERANES MA STEROIDS A B 
a b c a b c 

016 0.39 0.37 0.24 0.2 1 0.45 0.34 0.72 8.7 
0 24 0.35 0.41 0.29 0.21 0.48 0.31 0.74 6.7 
019 0.34 0.43 0.23 0.23 0.46 0.32 0.93 B. ! 
022 0.37 0.39 0.24 0.23 0.45 0.32 1.08 7.4 

0023 0.39 0.33 0.28 0.21 0.46 0.33 0.95 10.2 

MATURITY PARAMETERS 

SAMPLE Ts/(Ts+Tm) C"HOMOHOPANE STIGMASTANE T AS,,/( T AS'8+ MAS,,) 
22S/(22S+22R) 20S/(20S+20R) 

016 0. 35 0.58 0.46 0.88 
024 0.38 0.56 0.45 0.82 
019 0.41 0.57 0.5 I 0.94 
022 0.42 0.56 0.43 0.89 
023 0.33 0.54 0.29 0.85 

Tab le 2 Biomarker parameters for the EOD and SSO oils. Legend: j\ == gammaeerane index; 13 = C35 homohopane index; a = Cn / LCn - CN : b = 
C2~ / L Cn - CN ; C == C29 / LCn - C29· 



140 

50 

40 
LEGEND 

() OIL# 16 

D OIL# 19 

0 OI L # 22 

<$ OIL # 23 

X OIL # 24 

10 

o+-----~------~------~------
C31 C32 C33 C34 C35 

Homohopane (22S+22R) 

Fi g. 6 Homohopallc diSlribulion fo r sc lec l EDD and SSD oi ls. 

deposi ted at normal or reduced, rathe r than elevated 

marine salinities. 

Predominating Ni porphyrins also show that oxy­

gen was ava ilable during early diagenesis (LEWAN, 

1984; MOLDOWAN et aI. , 1986; BARWISE, 1990). 
Oleanane as the possible indicator of terrestrial plant 

in put to the source (after angiosperms appearance in 

Cretaceous) was not found in significant concentrations 

> 
I­
Ui 
Z 
UJ 
I­
~ 

Geologia Croatica 49/2 

in any of these oils. Low rearranged to normal slerane 
ratios r J3 ~, 17a(H), 20S + 20R]/ r 14a, 17a (H), 20S + 
20R + 14~, 17 ~(I-I) , 20S + 20R], point to a carbo nate 
mineral matrix for the sou rce rock (MOLDOW AN & 

FAGO, 1986; MELLO et aI., 1988; PETERS & MOL­
DOW AN, 1993). 

Biomarker maturity paramete rs (Table 2), which 
refl ec t thermal transformations of biomarkers, inc lud­
ing Ts / (Ts + Tm) , the isomer ratios of 22R / (22R + 
22S) and 17~, 2Ia(H) /17a , 21~(I-I) configurations in 
hopanes (i .e., moretancs/hopancs) and 14a, 17a(H) / [ 
14a, 17a(H) + 14~, 17~(H)J and the 20R / (20R + 20S) 
configurat ion in C29-steranes as well as s teroid aromati­
zat ion were used . The 22S / (22S + 22R) hOlllohopane 
ratios between 0.5 5 and 0.60 suggest full epimcriza tion 
and that the oils are morc mature than the initial genera­
tion stage (SEIFERT & MOLDOWAN, 1986). I-Iowev­
cr, the sterane isomerizat ion parameters are less than 
fully equilibrated suggesting that generation occurred 
before the peak of the oil window (PETERS & MOL­
DOW AN, 1993). Regarding the biomarker parameters 
the maturity level of the oils studied [or biomarkers can 
be ranked in the fo llowing order: 0 23 < 022, 016 and 
0 24 < 019, where 0 23 was generated in the late early 
oil window and 019 was generated near peak. Hi gh 
temperature gradients improved the maturity rank Qf the 
EDD and SSD oils. 

3.2. SOURCE ROCK STUDY 

In ord er to identify the oil so urce rock, cuttings 
from eight wells (Fig. I ) were studied . The organic 
matt er from the ex p lorat ion well Ew underwent addi­
tional s tudy as the reprcsentat ive sam pl e. In Ew the 
Miocene st rata between 2,400 m and 2,680 m arc found 
to be a so urce rock, with average TOC valucs ranging 
from 0.5 % to 1 %. Hydrogen indices range up to 475 in 

RETENTION TIME 

Fig . 7 Mass fragmell !Ogram m/z 414 
~ 2 17 from melaslable GC MS 
analys is for an EDD oil showing 
a dislribulion of 24-n-propylc­
holcslanes . 



Ahjbeg el '1 1 . Gcochcmic:ll Study of Oi ls .mel Oi l Source Rock from the E:lstem Dr:JY ;J and Slavon ij:l"Srijem Depressions .. '"' 

WELL TOC 5, HI mg/g TOC T max 
VITRINITE 

REFLECTANCE DATA % mg/g 5,.100 / TOC °C %Ro 

DEPTH w w 
z z z 0 m 0 0 oc 

w >= oc " a. 
OJ .. >-I I a. w (f) .. ::E OCOJ OJ -' >< .. oc w-

I (5 :iE 
0 

>I OJ ... 
" " " I.' I.' " '" ... 3" '" '" .. , ... ". '" 0.3 0.6 1.0 2.0 3.0 

• 

, 

• 

w 
z 
w 

" 0 ... 
500 on ......... ! w " -' 

~ z a. 0 
w -' 
z 
w 

" 
1000 

0 
:J 

, .. 
a. 

1500 " ...... 

'" on 
0: 
0 

" 0 
-' 
CD 

2000 w ..... . .. , .... . . ..... 
z 
w 

.~. 
" 0 ovANle 

~. 
:;; 

IVANIC 

"'" 
· GRAD 

2500 

~ ,~ l! · --::; } 
>" -
:50: "§ · onO 

3000 0" 

" 
... z 
w 

" 3500 
w .... .. .. on 

" '" 

4000 .. . 

Fig. H Geochem ical log o f Ihe cxptor<u ion we ll Ew' Sia l' • thc Kril.cvc i r Ollll:lt ion. 

the deeper pan of the source rock (Fig. 8; Krilcvci Clan 
ill Mos lavacka Gora formation) and refl ec t the predom­
inating oil -prone characte r of the kerogen (Type II ). In 
overl ying strata , organic matte r turned to mixed oil/gas 
prone (Type II/ Ill) and then to gas prone (Type Ill) in 
the Ivanic Grad formation. 

Regarding the fac t that the sediments o f th is interval 
are predominantly represented by clasti cs, the change 
mi ght reflec t the upward increasin g input of non-oxi­
di zed te rrestrial maceral s. However, some c lasts of 
these c lastic sed iments were reworked from older for­
mati ons and they may contain variolls types of organic 

matte r of different diageneti c and thermal hi story. The 
non -uniformity of lhe organi c facies was re fl ec ted in 
three recognizable vitrini te popu lations: aut ochthonous 
v itrinit es, the values of whi ch increase with depth 
Cr able 3) in accordance with the geothermal gradient ; 
vitrini tcs with refl ectance values indicating they might 
have been earlier diagenclicall y (primarl y) partl y oxi ­
di zed o r secondarily rcworked; and vitrinit es with 
re fl ec tance va lues whi ch 1110st probably represent 
grains from older, reworked formations. 

The Miocene source rock is al so found in other 
ex ploration wells . Mesozoic stra ta arc irregul a rl y d is-



142 

DEPTH POPULATIONS 
(01 ) l sI 2nd 3rd 

2380 0.522 0.965 2.021 

2480 0.509 0.955 1.868 

2580 0.538 1.039 2.1 19 

2680 0.557 1.018 1.909 
2780 0.572 1.048 2.016 
2880 0.566 1.057 2.025 

2970 0.609 1.075 2.01 2 

Table 3 Vitrinite rcOcclanccs for organic matter in Miocene Sirala in 
the we ll Ewo 

tributcd in the EDD ancl in SSD, and they a fC poor in 
organic maHer. 

4. CONCLUSION 

Geochem ical evidence suggests that EDD and SSD 
oi Is have been generated from the same source rock, 
deposited in a carbonate mineral matrix under marine 
suboxic condi tions. The oils were generated at the early 
to 111idcl lc maturity stage of the source. DilTcrcnccs in 

oil properties should be ascribed to different migration 
and trapp ing hi sto ries or different maturity levels al 

variolls locations. 
A Miocene source rock is identified conta ining 

autochthonous and all ochthonous organ ic matter, pre­
dominant ly Type IT/ ITT. Mesozoic strata are found to be 
abse nt and with no recent oil gcnerating potential. 
Thus, the Miocene appears 10 be thc onl y cons istent 
source throughout the EDD and SSD that can account 
for the commcrcial oil in this basin. Becau se of the 
modest prolificity of th is source, a rather efficient trap­
ping system is post ul ated, including short mi gration 
distances and good cap rocks , in order to accou nt for 
the known oil acc umulations. These cond itions are con­
sistenl with Ihe geology of Ihe EDD and SSD. 

Acknowledgement 

The authors are gratefu l to the management of rNA 
and Chevron for approving the publi cation of these 
data, and they arc grateful to the colleagues who coop­
erated during this project, includ ing P. SUNDARARA­
MAN, K.E. PETERS, S. SVEL-CEROVECKI, I. 
LEB INEC, c.Y. LEE, M.M. PENA, L.A.WRAXALL 
and G. KOCH. 

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Manuscript received January 10, 1995. 
Revised manuscript accepted September II, 1996. 



'4' Geologia Cromica 49(2