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 depres­sion (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 sul­phur 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 & HOR­VATTl , 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 zalijc­ganja 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 noaro­m<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 lime­stones 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 uncon­formities 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 Geo­logical 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 ra­tion we ll s (only the well s subjected ( 0

runher siudy arc add itiona ll y label­led) .

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 vol­cani sm, During the Mid-Latc Miocene, rapid sed imen­ta 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 sur­rounding 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 f­Z

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) proba­bly 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 reser­voirs, mainly in the Miocene Moslavacka Gora forma­tion 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 - inten­sive 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 pyro­Iyza[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 vitri­nite retlcctance determ ination (Ro). Pyrolysis in an off­li ne lube rcacror (ALAJl3EG & STIPAK, 1985) anel gas chromatography of oil asphaltenc pyrolysates was also performed.

138

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t­a:

~

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 dis­covered in a relatively shallow trap (depth <1000 In,

trap temperature 65°C) where it underwent biodegrada­tion. 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 sat­urates yield (41.8%) as well as in the low intensity of normal alkanes as shown in whole oil gas chro­rnatogram (Fig. 2c). The other oils show an intensive n­alkanes 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 distrib­ution, 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 asphal­lene pyrolysis products for 019; thc intcnsivc peak doublcts belong to J-alkenes and n-ulka­ncs, 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 por­phyrins, suggesting a suboxic to oxic depositional envi­ronment 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 cho­sen: 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 con­tains 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 cor­relation 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 mea­sured 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 monoaroma­lie 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 r­ane 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 increas­ing 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

> I­Ui 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 & MOL­DOW AN, 1993).

Biomarker maturity paramete rs (Table 2), which refl ec t thermal transformations of biomarkers, inc lud­ing 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 aromati­zat 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 genera­tion stage (SEIFERT & MOLDOWAN, 1986). I-Iowev­cr, the sterane isomerizat ion parameters are less than fully equilibrated suggesting that generation occurred before the peak of the oil window (PETERS & MOL­DOW 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 addi­tional 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-propylc­holcslanes .

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 predom­inating 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-oxi­di zed te rrestrial maceral s. However, some c lasts of these c lastic sed iments were reworked from older for­mati 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, pre­dominant 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 trap­ping 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 con­sistenl 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 coop­erated during this project, includ ing P. SUNDARARA­MAN, K.E. PETERS, S. SVEL-CEROVECKI, I. LEB INEC, c.Y. LEE, M.M. PENA, L.A.WRAXALL and G. KOCH.

5. REFERENCES

ALAJBEG, A., BRITVlc, V., SVEL-CEROVECKl, S. , CORN FORD, CH., TODORlc, A" RA1KOVlc, 1., BARIC, G. & PUTNIKOVIc, A. (1990): Geo-

GCOlog i:l Croatica 49(1

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

'4' Geologia Cromica 49(2