GEOL. CROAT. 
1 

49/2 127 1 - 275 1 2 Figs. ZAGREB 1996 

Review po{)er 

Geological Characteristics of the Oil Fields in the Sloven ian 
Part of the Pannonian Basin 

Pero MIOC and Miro ZNIDARCIC 
PROCEEDINGS 

Key words: Geological structure, Potentia! source 
rocks, Reservoir rocks, Structure traps, Oil-gas 
fields. 

Abstract 
[n (ile Slovcnian part of the Pannonian basin the oil-gas field 

Pclisovci , the gas fields Oolil1<1 and Gancani, and lhe oil fields f-ilov­
ei, Bakovci and Bukovci were discovered. With the exception of 
PclisoYci, the mentioned fields afC current ly nOll prodllclivc. On [his 
basis, and as a consequence of recent results of oil geo logical investi ­
gat ions, lhe li thostratigraphic description of lhe Neogene beds of Ihis 
part of the Pannonian basin is presented , together wilh occurrences or 
source rocks in the given strat igraphic horizons, as well as the struc­
tural charactcristics or thc tcrrilOry. The resul ts highlight the some­
what hi ghe r potcntial 01" ccrtain arcas, which is also of importance 
economicall y. 

I. INTRODUCTION 

The Sloven ian part of the Pannonian basin is known 
as Ihe Mura depression (AKSIN el aI., 1981). This rep­
resents one of the deep depressions in the marginal area 
or the southern and southwestern parts of the Pannonian 
basin . 

The Mura depression is bounded in the west by the 
massifs of the Eastern Alps, Pohorje and Kobansko 
with Kozjak, and along the southwestern side by the 
extensions of the Eastern Karavanke Mountains (Fig. 
I). The eastern extensions of these mountain chains 
were submerged during the Neogene when the basin 
was formed and in which the Neogene beds were 
deposited . The total area of the Slovenian part of the 
Mura depression is approximately 2,640 km 2

• 

At the end of the 19th century oil was discovered at 
Selnica in the neighbouring Croatian part of the Mura 
depression. In the Slovenian pan of the depression dur­
ing the Second World War two field s were discovered: 
the Peli sovci oil-gas field and the Dolina gas field. 
Arter the war, the gas field Gancani was discovered , as 
well as the three oil fields, Filovei, Bakovci and Bukovci 
(Fig. I). 

In the past , geological and geophysical investiga­
tions over 11 wider region were performed, and a num-

Geological Survey, Dimiceva 14, 1000 Ljllbljana, Slovenia. 

Klj ucnc rijcci: geo[oska struktura, potencijalne mati­
cne stijene, kolektorske stijene, st rukturne zamke, 
naftno-plinska polja. 

Saietak 
U slovcnskom dijclu Panonskog bazcna II protck 10m jc razdobljll 

otkriveno naftno -plinsko polje Petisovci, plinska polja Dolina i 
Gancani, IC naftna polja Filovci , Bakovci i Bukovci. Od spomcnutih 
nalazista d,lllas je II proizvodnji samo potje Petisovci. Na temetjll pri­
je spom cnlltog i s obzirom na rezuttate novijih istrazivanja, prikazan 
jc litostratigrafski sastav ncogcnskih naslaga ovoga dijcta Panonskog 
bazena, Ie pojave maticnih stijena i strllktul'lla gracla podrucja. II'.nije-
10 je i misljenje 0 nesto vecoj perspektivnOsli istrazivanja na nekim 
podrucjirna, slo bi bilo i ekonomski naj vaznijc. 

ber of deep boreholes were drilled. The results of these 
investigations included the lithological and stratigraphi­
cal study of Neogene deposits, determination of the 
lithological composition of Pre-Tert iary basement, and 
tectonic subdivision of the Mura depression into indi­
vidual uni ts. In terms of petroleum geology the source 
rocks were identified from which the hydrocarbons 
mi grated into favourable reservoir rocks and pools. A 
number of structural problems were solved, and also the 
general structures and tectonic units of the known oil 
and gas fields were determined. 

This li thostrat igraphical and tectonical framework is 
presented, together with the petroleum geological char­
acteristics, and the economically optimal areas for 
future investigations are highlighted. 

2. LITHOSTRATIGRAPHY 

The geology of the Mura depression can be separat­
ed into three distinct units: the Pre-Neogene basement, 
the Tertiary sediments (mostly Miocene-Pliocene), and 
the Quaternary cover. 

The Pre-Neogene basement consists of metamorphic 
rocks and Mesozoic sed imentary rocks. Among the 
metamorphics gneisses prevail, with mica schists, 
<lmphibolites and lenses of eclogite, marble and 
quartzite (Middle Austroalpine). These rocks arc over­
lain by phyllites (Ordovic ian ) and Silurian -Devonian 



272 

\ 

i 

\l 
s 

p 
o H 0 R ) 

dark and greenish schists. These beds were overlain by 
Permo-Triassic red sandstones (Upper Austroalpine). 
The Tri assic is represented by limestones and dolo­
mites, and the youngest Mesozoic member is represent­
ee! by Senonian limestones, breccias and marl s 
(PLENICAR , 1954, 1979; FLUGEL, 1988; MIOC el 
aI., 1993). 

The Tertiary sedimentary rocks consist of Neogene 
beds from Ollnangian - Ponlian age (RIJAVEC, 1976). 
The Oltnangian-Carpathian deposit s include conglom­
erate, sandstone and silt itc. In the lower part of these 
beds individual layers occur (2-10 m lhick) of bilumi­
nous black marl that was identified as the source rock 
of the hydrocarbons. In the upper part clacitic-andesitic 
tuff occurs between marls and sandstones. The Baden­
ian beds afC represented in their lower part by poorly 
consolidated coarse grained breccia and conglomerate 
with sand, overlain by marl and lithothamnian lime­
stone. During the Sarmat ian clastic deposi tion contin­
ued, and the marine env ironment became gradually 
brackish. Marls, sands and conglomerates are the char­
acteriSlic lithological members (RIJAVEC, 1978). This 
clastic deposition persisted throughout the Pannonian 
and Pontian. 

Lilhostratigraphically the Neogene beds can be su b­
divided into three formations: the Murska Sobota for­
mation, from Lower Miocene to Lower Pannonian, the 
Lendava formation, from Upper Pannon ian to Lower 
Pontian , and the Mura form ation, from Dacian to 
Romanian. 

South of the Drava River, in the area of Haloze and 
westerly, in the Southern Karavanke, older, Upper 
Oligocene (Egerian) beds also occur in which the 
anclesitic tuffs of the Smrekovec series are especially 

Geologia Cromiea 49/2 

Fig. 1 Sketch-map of th e tectonic 
units of the Mura depression. 
Legend: 

R - Radgona depression; 
M - Murska Sobota massif; 
P - Ptuj-Ljutomer Depression; 
0- Ormo7.-Seln ica antiform; 
a-b - trace of the profile; 
- - - - supposed borde r between 

tectonic units; 
- oil-gas fields. 

characteri sl ic (ANICIC & JURISA, 1985; MIOC, 
1978). 

3. OVERVIEW OF THE GEOLOGICAL 
STRUCTURE 

The wider region of the Mura depression Jies, in the 
geotectonic sense, in the eas tern extension of lhe East­
ern and Southern Alps that represent the Pre-Tertiary 
basement of this territory. After intensive tangential 
movements and overthrusts the basement disintegrated 
into individual blocks (tectonic units) before the 
Miocene. In the areas of subsided units basins formed 
in which rapid deposition took place. Tectonic activ ity 
of various degrees also continued during deposition of 
the Neogene beds, as evidenced by the variable thi ck­
ness of the Neogene sediments. Vertical and horizontal 
movements and weak overthrustings al reverse faults 
(e.g. the Ljutomer fault) are characteristic. In this man­
ner units were form ed in the shape o[ depressions (e.g. 
the Radgona depression) and uplifted blocks - horsts 
(e.g. the Murska Sobota massif that passes into PohOIje 
in the west). 

During prospecti ng [or oil and gas, special attention 
was paid to solving the stratigraphic-tectonic and struc­
tural problems of the wider region. These problems 
were studied by numerous authors, among them 
PLENICAR (1954), SIMON (1973), KISOV AR 
(1977), MIOC & ZNIDARCIC (1989), DURASEK & 
HARI (1992), and olhers. After geophysical investiga­
tion and deep dri ll ing the followin g, more important 
units were dist inguished from north to south: the Rad ­
gona depression, the Murska Sobota massif, the Ptuj -



Mioc & Znidareic: GeolO~ ic31 Char.IC[eristics of [he Oil Fields in the Sloven ian Pan of the P:mnonian Aasin 273 

a ® + @ + ® + 

3 

5 

km 
10 2Q :xl 

Lj utomer depression and Ihe Orm oz-Sclni ca antiform 
(Figs. 1 and 2). 

The gcolog ical struc t.ures of the Pre-Tert iary base­
ment that ex tend in the NE-SW direction are not late ral ­
ly equi va lent , and deposition of the Neogene beds did 
not commence at the same time in all localities. 

Tn the Pre-Tertiary basement of the Radgona depres­
sion Older Palaeozo ic shales, Perm o-Triass ic sand­
stone, and limestone with dolomite, as well as Creta­
ceous beds occur. Thi s basement was overl ain by Neo­
gene sediments. Their thi ckness inc reases towards the 
northeast where it exceeds 4,500 m. The ex tension of 
this unit toward s th e southwest is represent ed by thc 
Ribniea-Sclnica grabell (MIOC, 1977). 

The Murska Sobota massif lies to the south of the 
Radgona dcpress ion. Tn the geolog ical structure of Ihe 
Pre-Tertiary basement gneisses, mica schi sts, amphibo­
lites (CIGIT, 1958) and evell ec logites were de te r­
mined. T he mass if was tota lly submerged during the 
Badenian aner which the first Neogene sediments were 
deposited ( ill the apical part), and they exceed 2,000 m 
in thickness. The southwestern extension of this unit is 
th e Pohorj e mass if. Th c geologica l stru cture of the 
wieleI' surroundi ngs indicates the poss ibility of erosi onal 
rcmnant s of carbonate beds (Upper Triass ic) in the 
so uthern margin of this un it. 

T he Ptuj-Ljut ome r depression li es to the south of 
the Murska Sobota mass if and it represent s thc sub­
me rged part of the te rritory between thi s mass if in the 
north and the Ormoz-Sclnica antiform in the south. The 
Pre-Tertiary basement predominantl y consists o f meta­
morphic rocks, gneisses, amphibolites, and possibly 
also phyllit es. Indiv idual " patches" o f c rus hed to 
mylonitized dolomite (Upper Triassic) are preserved on 
the metamorphics. The thi ckness of the Neogenc beds 
<-ltlains 4,000 III in the borehole ncar Ljutomer. The 
c rushed dolomit e, approximatcly 40 m thick, beneath 
these beds, represents a tec tonic remnant of the over­
thrust on gneiss. The PlUj -Ljutomer tec ton ic unit con­
tinues towards the soulilwest, and its ex tension is prob­
ab ly the Mezica- Vi tanje graben wh ich li es between 
Po hOlje in the north and 1 orthern Karavanke in the 
south (M IOC ct at., 1993). 

The Onnoz-Se lniea antiform is the most southerly 
or the described units. The northern limit with the Ptuj­
Ljutomer depression is of tectonic charac tcr, represent­
ed by the Ljutomer reverse fault Ihat dips to the south. 
The Pre-Te rt ia ry basement of the Orllloz-Seiniea 

@ b 
-- - Fig. 2 Schematic c ross-section of the territory 

40 

between th e Radgona depres sion and (he 
Onnoz-Sclnica antiform. Legend : N) Neo-
gene (mostl y sand, sa nd stone. marl and 
clay): T ) Triass ic (li mestoncs , do lomi tes 

50 
a nd dolo mitc brccc ia): MA ) Mi (ldl c and 
Upper Austroalpine (gnciss. amphibolit e, 
phylite): -) Fault. 

antiform , as suggested by the reg ional anal ys is of the 
geo log ica l structure, cons ists of Mesozoic , predomi ­
nantl y Triassic beds of the Northern Karavankc. Thcse 
beds arc overthrusted on the metamorphic basement in 
the Nort hern Karavanke Moun tain s. Tn the course o f 
formin g of the Pannoni an bas in these struc tures sub­
sided. The Neogene sediments that were deposi ted o n 
them were deformed already partly during the ir deposi­
tion, and to a larger degrce later, during reactivation of 
the Pre-Terti ary basement. This is indicated by reverse 
faults in the northern rim of the Ormoz-Seln ica 
ant iform, and in pan by ovenhrusting on the neighbour­
ing Ptuj -LjulOmcr depression. 

The Ormoz-Selnica antiform is widcst in its north­
west part , and subslanlially narrower towards the south­
east, in the area of the Dravinja vineyards and Halozc. 
The major part of this unit , approximately between Ptuj 
and Dornava in the sout hwes t, and west o f Pe ti sovei 
and Scl niea in thc nOl1heast, is somewhat upli fted. Fur­
thermore the surface struc ture of the southeast area dips 
relative ly gently, and only in the south western part, in 
the area of the Dravinja vineyards, are the dip anglcs 
stec p, ve rtical or even overturn ed. It is important to 
note that the Ormoz-Selni ca antiform exi sts at present 
di sint eg rated into a /lumber o f smallcr anticlinal un its 
that arc in places sevcral square kilometres in s ize. 
Based on thc geological relat ions in the wider area, the 
Triassic carbonate rocks may be ex pected in the cores 
of these units. 

4. OIL GEOLOGY 

In the wider region of the Sloven ian pan of the 
Mura depression the potentia l source roeks were estab­
lished among the Neogcne sediments. Favourable reser­
voirs and traps also occ ur together with favourabl e 
structural conditions. 

The potential source rocks outcrop in the northeas t­
e rn p<w of the area, in the region of Kozjak, and on the 
southern margin they outc rop in the Haloze area. They 
a re represented by dark bituminous marls to siltites that 
oeeur in the lower part o f the Ollnangian-Ca rpathian 
beds, and appear at several hori zons, varying in thick­
ness at individual levels from a few to more then ten 
mctres . S imilar marls have also been found in the 
Badenian deposits. These beds extend from the margin s 
to the cent ral part of the Mum depressi on where they 



274 

were probably the main source or hydrocarbons which 
migrated into the favourable structures of the reservoir, 
an cl fo rmed oil-gas fi elds. 

5. POTENTIAL RESERVOIR ROCKS 

Th e potential reservoirs were found among the 
rocks of the Pre-Tertiary basement , as well as in the 
Neogene beds. 

In the sequence of beds of the Pre-Te rtiary base­
ment rocks occur that arc bad reservoirs in certain cir­
cumstances, and others with the bes t reservoir proper­
ties . With respect 10 the older tecton ic history and [or­
mati on or th e Pre-Tertiary basement parts or en ti re 
unit s o f Ihe tectonically deform ed Pre-Tertia ry base­
ment may be of interest for accumulation of oil and gas , 
particularly phyllonites (c rushed gneisses), mylonites 
(crushcd dolomites, limestones, Pe rmian-Triass ic sand­
stone), breccias (dolomite and limestone) . 

The phyllonite zone can appea r a long the tectonic 
contact between phyllonites ove rthrust ing gneisses . In 
thi s process thc gneisses were crushed into fine fra g­
ments, and they can have bad reservoir propcrt ies. They 
may be expected in the area of the Murska Sobota mas­
sif and the footwall of the Radgona depression . 

Mylonites, tectonic breccias and fractured carbon­
ates are more favourable as reservoir rocks, particularly 
the defo rmed Triassic dolornites and limestones. The 

thickness varies from several tens to several hundred 
metres. They may bc ex pected in the footwall of the 
Radgona depress ion between the Neogene beds and the 
unde rlying metamorphic compl ex . Espec ially favo ­
urabl e carbonate reservoirs are expected below the 
Neogene depos its in the area of the Onnoz-Selnica 
ant i rorm. These reservoir rocks are part of the fi ssured 
and deformed Northern Karavanke carbonate overthrust 
which consists of Triassic, massive and in part layered 
limes tones, and of Upper Triassic do lomite. 

or potential reservoir valuc in the Neogene beds are 
the crushcd breccias, conglomerates and sands tones of 
Ottnangia n-Carpathian age. In Badenian th e li tho­
thamnian limestone and sandstone that is oftcn fi ssured 
is al so ravourablc. It a tt ai ns a thickness of several 
metres to several tens of metres. Reservoir horizons can 
also be found in the Sannatian beds (sands tones, con­
glomcrates), as wcll as in thc Pannonian and Pont ian. 

The s truc tural traps could bc found in the Ottn an ­
gian-Carpathian, Badenian and Pannonian, as well as in 
POlllian depos its. Beds of marls, clayey marls and clays 
occur in individual parts of these stra tigraphic units , 
and they can form favourable traps. Their thickncsses 
vary from several tens metres to a hundred metres. 

Geologia Cm;llica 49(2 

Structures potentially favourable for oil appear in 
the region of the Ormoz-Selnica antiform andlhe Radg­
ana depression. In both areas carbonate roc ks tha t arc 
th e most favourable potential reservoir rocks occur in 
the Pre-Tertiary basement. Some also appear in Neo­
gene beds. 

The region of the Radgona depression disintegrated 
and individual smaller antiform features were formed. 
There were poss ibilities for the migration of hydrocar­
bons from the lower parts and their acc umulation in the 
mentioned structures (e.g. Dankovci). In the area of the 
Murska Sobota massif sma lle r anticlincs occur in which 
hydrocarbons accumulated in Neogene reservoi r rocks. 

It is estimated that the O rmoz-Selnica unit repre­
sent s the most favourab le area for occurrence of 
deposits according to its geo logical s truc ture and oil 
geology c harac teristics. Along its strike occur a number 
of smaller anticlinal s truc tures of several kilometers 
extension. In the northeas tern part of this unit oil -gas 
ficlds wcre di scovered in the vicinity of Peti sovci, and 
in the Croatian part at Sc ln ica, Zebanec and the latest 
gas -fields Vukanov ec and Vuckovec (RUNJIC et aI. , 
1995; SELJAN & PARLOY , 1995). 

In the southweste rn extension of the Onnoz-Sclnica 
unit oil and gas were drilled at Bukovci, however with­
out any important econom ic results. From the stand ­
point of more recent invest igat ions it was estimated that 
at Bukovei the boreholes were drilled almost parallel to 
the ve rtica l Neogene beds, whcre they did no t find 

favourable structures (MIOC & ZN lDARCIC. 1989) . 

6. CONCLUSIONS AND RECOMMENDATIONS 

In conclusion of this short analysis of geo logical 

and o il geological informat ion of the Mum depress ion, 
it may be stated : 

a) the Ormoz-Selniea st ruct ural unit appears as the 
most favourable for new petroleum discoveries, 

b) the next potentially important s tructural unit is the 
R adgona depression , fo llowed by 

c) individual areas of the Murska Sobota massif, in 
which the possibilities of smalle r occurrences of oil 
and gas exist, and of even more favourable geot her­
mal water sources. 

Prio r to plann ing further prospccting for petroleum, 
analysis and interpre tati on of ex is ting data arc neces­
sary. From this , geophysical measurements and further 
interpretation of the surface structural data are required 
in order to facilitate a deep dr illing programme. In this 
manner the location of boreholes will be economically 
better establi shed. 



Mi oc & Zilidarc ;c: Geologica I Ch~lr;IC1cris ti cs or the Oil Fields in the Slovcnian l'al1 of the Pannonian Basin 27) 

7 . REFERENCES 

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276 Gcologi;1 Croallca 49/2