GEOL. CROAT. 49/2 127 1 - 275 1 2 Figs. ZAGREB 1996 1 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 Klj ucnc rijcci: geo[oska struktura, potencijalne mati­ rocks, Reservoir rocks, Structure traps, Oil-gas cne stijene, kolektorske stijene, strukturne zamke, fields. naftno-plinska polja.

Abstract Saietak [n (ile Slovcnian part of the Pannonian basin the oil-gas field U slovcnskom dijclu Panonskog bazcna II protck 10m jc razdobljll Pclisovci , the gas fields Oolil1<1 and Gancani, and lhe oil fields f-ilov­ otkriveno naftno-plinsko polje Petisovci, plinska polja Dolina i ei, Bakovci and Bukovci were discovered. With the exception of Gancani, IC naftna polja Filovci, Bakovci i Bukovci. Od spomcnutih PclisoYci, the mentioned fields afC current ly nOll prodllclivc. On [his nalazista d,lllas je II proizvodnji samo potje Petisovci. Na temetjll pri­ basis, and as a consequence of recent results of oil geo logical investi ­ je spomcnlltog i s obzirom na rezuttate novijih istrazivanja, prikazan gat ions, lhe lithostratigraphic description of lhe Neogene beds of Ihis jc litostratigrafski sastav ncogcnskih naslaga ovoga dijcta Panonskog part of the Pannonian basin is presented, together wilh occurrences or bazena, Ie pojave maticnih stijena i strllktul'lla gracla podrucja. II'.nije- source rocks in the given strat igraphic horizons, as well as the struc­ 10 je i misljenje 0 nesto vecoj perspektivnOsli istrazivanja na nekim tural charactcristics or thc tcrrilOry. The resul ts highlight the some­ podrucjirna, slo bi bilo i ekonomski naj vaznijc. what hi gher potcntial 01" ccrtain arcas, which is also of importance economicall y.

I. INTRODUCTION ber of deep boreholes were drilled. The results of these investigations included the lithological and stratigraphi­ The Sloven ian part of the Pannonian basin is known cal study of Neogene deposits, determination of the as Ihe Mura depression (AKSIN el aI., 1981). This rep­ lithological composition of Pre-Tertiary basement, and resents one of the deep depressions in the marginal area tectonic subdivision of the Mura depression into indi­ or the southern and southwestern parts of the Pannonian vidual uni ts. In terms of petroleum geology the source basin. rocks were identified from which the hydrocarbons The Mura depression is bounded in the west by the mi grated into favourable reservoir rocks and pools. A massifs of the Eastern Alps, Pohorje and Kobansko number of structural problems were solved, and also the with Kozjak, and along the southwestern side by the general structures and tectonic units of the known oil extensions of the Eastern Karavanke Mountains (Fig. and gas fields were determined. I). The eastern extensions of these mountain chains This li thostratigraphical and tectonical framework is were submerged during the Neogene when the basin presented, together with the petroleum geological char­ was formed and in which the Neogene beds were acteristics, and the economically optimal areas for deposited. The total area of the Slovenian part of the future investigations are highlighted. 2 Mura depression is approximately 2,640 km • At the end of the 19th century oil was discovered at Selnica in the neighbouring Croatian part of the Mura 2. LITHOSTRATIGRAPHY depression. In the Slovenian pan of the depression dur­ ing the Second World War two field s were discovered: The geology of the Mura depression can be separat­ the Peli sovci oil-gas field and the Dolina gas field. ed into three distinct units: the Pre-Neogene basement, Arter the war, the gas field Gancani was discovered, as the Tertiary sediments (mostly Miocene-Pliocene), and well as th e three oil fields, Filovei, Bakovci and Bukovci the Quaternary cover. (Fig. I). The Pre-Neogene basement consists of metamorphic In the past, geological and geophysical investiga­ rocks and Mesozoic sed im entary rocks. Among the tions over 11 wider region were performed, and a num- metamorphics gneisses prevail, with mica schists, Slovenia. lain by phyllites (Ordovician) and Silurian-Devonian 272 Geologia Cromiea 49/2

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Fig. 1 Sketch-map of th e tectonic p units of the Mura depression. o H 0 R ) 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.

dark and greenish schists. These beds were overlain by characterislic (ANICIC & JURISA, 1985; MIOC, Permo-Triassic red sandstones (Upper Austroalpine). 1978). The Tri assic is represented by limestones and dolo­ mites, and the youngest Mesozoic member is represent ­ ee! by Senonian limestones, breccias and marls 3. OVERVIEW OF THE GEOLOGICAL (PLENICAR, 1954, 1979; FLUGEL, 1988; MIOC el STRUCTURE aI., 1993). The Tertiary sedimentary rocks consist of Neogene The wider region of the Mura depression Jies, in the beds from Ollnangian - Ponlian age (RIJAVEC, 1976). geotectonic sense, in the eastern extension of lh e East­ The Oltnangian-Carpathian deposit s include conglom­ ern and Southern Alps that represent the Pre-Tertiary erate, sandstone and siltit c. In th e lower part of these basement of this territory. After intensive tangential beds individual layers occur (2-10 m lhick) of bilumi­ movements and overthrusts the basement disintegrated nous black marl that was identified as the source rock into individual blocks (tectonic units) before the of the hydrocarbons. In the upper part clacitic-andesitic Miocene. In the areas of subsided units basins formed tuff occurs between marls and sandstones. The Baden­ in which rapid deposition took place. Tectonic activity ian beds afC represented in their lower part by poorly of various degrees also continued during deposition of consolidated coarse grained breccia and conglomerate the Neogene beds, as evidenced by the variable thi ck­ with sand, overlain by marl and lithothamnian lime­ ness of the Neogene sediments. Vertical and horizontal stone. During the Sarmatian clastic deposition contin­ movements and weak overthrustings al reverse faults ued, and the marine environment became gradually (e.g. the Ljutomer fault) are characteristic. In this man­ brackish. Marls, sands and conglomerates are the char­ ner units were form ed in the shape o[ depressions (e.g. acteriSlic lithological members (RIJAVEC, 1978). This the Radgona depression) and uplifted blocks - horsts clastic deposition persisted throughout the Pannonian (e.g. the Murska Sobota massif that passes into PohOIje and Pontian. in the west). Lilhostratigraphically the Neogene beds can be su b­ During prospecti ng [or oil and gas, special attention divided into three formations: the Murska Sobota for­ was paid to solving the stratigraphic-tectonic and struc­ mation, from Lower Miocene to Lower Pannonian, the tural problems of the wider region. These problems Lendava formation, from Upper Pannonian to Lower were studied by numerous authors, among them Pontian, and the Mura formation, from Dacian to PLENICAR (1954), SIMON (1973), KISOV AR Romanian. (1977), MIOC & ZNIDARCIC (1989), DURASEK & South of the Drava River, in the area of Haloze and HARI (1992), and olhers. After geophysical investiga­ westerly, in the Southern Karavanke, older, Upper tion and deep drilling the following, more important Oligocene (Egerian) beds also occur in which the units were di stinguished from north to south: the Rad­ anclesitic tuffs of the Smrekovec series are especially gona depression, the Murska Sobota massif, the Ptuj- Mioc & Znidareic: GeolO~ i c31 Char.IC[eristics of [he Oil Fields in the Sloven ian Pa n of the P:mnonian Aa sin 273

a ® + @ + ® + @ b -- Fig. 2 Schematic c ross-section of the territory - between th e Radgona depression and (he Onnoz-Sclnica antiform. Legend : N) Neo- 3 gene (mostl y sand, sa nd stone. marl and 5 clay): T ) Triassic (limestoncs, dolomi tes and dolomitc brcccia): MA ) Mi (ldlc and 10 2Q :xl 40 50 km Upper Au stroalpine (gnciss. amphibolit e, phylite): -) Fault.

Ljutomer depression and Ihe Ormoz-Sclni ca antiform antiform, as suggested by th e regional analys is of the (Figs. 1 and 2). geological structure, consists of Mesozoic , predomi ­ The gcological struct.ures of the Pre-Tert iary base­ nantly Triassic beds of th e Northern Karavankc. Thcse ment that extend in the NE-SW direction are not lateral­ beds arc overthrusted on the metamorphic basement in ly equi va lent , and deposition of th e Neogene beds did the Northern Karavanke Mountains. Tn the course o f not commence at th e same time in all localities. forming of the Pannonian basin these struc tures sub­ Tn the Pre-Tertiary basement of th e Radgona depres­ sided. The Neogene sediments that were deposi ted o n sion Older Palaeozoic shales, Perm o-Triassic sand­ th em were deformed already partly during th eir deposi­ stone, and limestone with dolomite, as well as Creta­ ti on, and to a larger degrce later, during reactivation of ceous beds occur. This basement was overl ain by Neo­ the Pre-Terti ary basement. This is indicated by reverse gene sediment s. Their thi ckness in creases towards the faults in the northern rim of the Ormoz-Selnica northeast where it exceeds 4,500 m. The extension of antiform, and in pan by ovenhrusting on th e neighbour­ this unit towards th e southwest is represent ed by thc ing Ptuj-LjulOmcr depression. Ribniea-Sclnica grabell (MIOC, 1977). The Ormoz-Selnica antiform is widcst in its north­ The Murska Sobota massif lies to th e south of the west part, and subslanlially narrower towards the south­ Radgona dcpression. Tn the geological structure of Ihe east, in th e area of the Dravinja vineyards and Halozc. Pre-Tertiary basement gneisses, mi ca schi sts, amphibo­ The major part of this unit , approximately between Ptuj lit es (CIGIT, 1958) and evell eclogites were de ter­ and Dornava in the southwes t, and west of Peti sovei mined. T he massif was totally submerged during the and Sclniea in thc nOl1heast, is somewhat uplifted. Fur­ Badeni an aner which the first Neogene sediments were thermore the surface structure of the southeast area dips deposited (ill the api cal part), and they exceed 2,000 m relatively gently, and only in the southwestern part, in in thickness. The southwestern extension of this unit is the area of the Dravinja vineyards, are the dip anglcs th e Pohorje massif. Thc geological stru cture of the stecp, vertical or even overturned. It is important to wieleI' surroundi ngs indicates the poss ibility of erosi onal note th at th e Ormoz-Selni ca antiform exists at present rcmnants of carbonate beds (Upper Triassic) in the disint egrated into a /lumber of smallcr anticlinal units so uthern margin of this un it. that arc in places sevcral square kilometres in size. T he Ptuj-Ljutome r depression li es to the south of Based on th c geological relations in the wider area, the th e Murska Sobota massif and it represent s thc sub­ Triassic carbonate rocks may be ex pected in th e cores me rged part of the territory between thi s massif in the of these units. north and the Ormoz-Sclnica antiform in th e south. The Pre-Tertiary basement predominantly consists of meta­ morphic rocks, gneisses, amphibolites, and possibly 4. OIL GEOLOGY also phyllites. In dividual " patches" of cru s hed to mylonitized dolomite (Upper Triassic) are preserved on In the wider region of th e Sloven ian pan of the the metamorphics. The thi ckness of th e Neogenc beds Mura depression the potential source roeks were estab­ <-ltlains 4,000 III in th e borehole ncar Ljutomer. The lished among the Neogcne sediments. Favourable reser­ cru shed dolomite, approximatcly 40 m thick, beneath voirs and traps also occur together with favourabl e th ese beds, represent s a tectonic remnant of the over­ structural conditions. thrust on gneiss. The PlUj -Ljutomer tecton ic unit con­ The potential source rocks outcrop in th e northeast­ tinues towards th e soulilwest, and it s ex tension is prob­ ern pDankovci). In the area of th e ment rocks occur that arc bad reservoirs in certain cir­ Murska Sobota massif smaller anticlincs occur in which cumstances, and others with the best reservoir proper­ hydrocarbons accumulated in Neogene reservoi r rocks. ties. With respect 10 the older tectonic history and [or­ It is estimated that the O rmoz-Selnica unit repre­ mati on or th e Pre-Tertiary basement parts or enti re sent s the most favourab le area for occurrence of units o f Ihe tectonically deformed Pre-Tertia ry base­ deposits according to its geological struc ture and oil ment may be of interest for accumulation of oil and gas, geology c haracteristics. Along it s strike occur a number particularly phyllonites (crushed gneisses), mylonites of smaller anticlinal structures of several kilometers (crushcd dolomites, limestones, Pe rmian-Triassic sand­ extension. In the northeastern part of this unit oil-gas stone), breccias (dolomite and limestone). ficlds wcre di scovered in the vicinity of Peti sovci, and The phyllonite zone can appear along the tectonic in the Croatian part at Scln ica, Zebanec and the latest contact between phyllonites ove rthrust ing gneisses. In gas-fields Vukanovec and Vuckovec (RUNJIC et aI. , this process thc gneisses were crushed into fine frag­ 1995; SELJAN & PARLOY , 1995). ments, and th ey can have bad reservoir propcrt ies. They In the southweste rn extension of the Onnoz-Sclnica may be expected in the area of the Murska Sobota mas­ unit oil and gas were drilled at Bukovci, however with­ sif and the footwall of the Radgona depression. out any important economic results. From the stand­ Mylonites, tectonic breccias and fractured carbon­ point of more recent investigatio ns it was estimated that ates are more favourable as reservoir rocks, particularly at Bukovei the boreholes were drilled almost parallel to the deformed Triassic dolornites and limestones. The the ve rtical Neogene beds, whcre they did not find thickness va ri es from several tens to several hundred favourable stru ctures (MIOC & ZN lD ARCIC. 1989). metres. They may bc ex pected in the footwall of the Radgona depression between the Neogene beds and the unde rlying metamorphic complex. Especially favo­ 6. CONCLUSIONS AND RECOMMENDATIONS urabl e carbonate reservoirs are expected below the Neogene depos its in the area of the Onnoz-Selnica In conclusion of this short analysis of geological ant i rorm. These reservoir rocks are part of th e fi ssured and oil geological informat ion of the Mum depression, and deformed Northern Karavanke carbonate overthrust it may be stated: which consists of Triassic, massive and in part layered a) the Ormoz-Selniea struct ural unit appears as the limestones, and of Upper Triassic dolomite. most favourable for new petroleum discoveries, or potential reservoir valuc in the Neogene beds are b) the next potentially important structural unit is the the crushcd breccias, conglomerates and sandstones of R adgona depression, followed by Ottnangian-Carpathian age. In Badenian th e li tho­ c) individual areas of the Murska Sobota massif, in thamnian limestone and sand stone that is oftcn fi ssured which the possibilities of small e r occurrences of oil is also ravourablc. It a tt ai ns a thickness of several and gas exist, and of even more favourable geother­ metres to several tens of metres. Reservoir horizons can mal water sources. also be found in the Sannatian beds (sandstones, con­ glomcrates), as wcll as in thc Pannonian and Pontia n. Prior to planning further prospccting for petroleum, The structural traps could bc found in the Ottnan­ analysis and interpre tati on of ex isting data arc neces­ gian-Carpathian, Badenian and Pannonian, as well as in sary. From this, geophysical measurements and further POlllian deposits. Beds of marls, clayey marls and clays interpretation of the surface structural data are required occur in individual parts of these stratigraphic units, in order to facilitate a deep drilling programme. In this and they can form favourable traps. Their thickncsses manner the location of boreholes will be economicall y vary from several tens metres to a hundred metres. better established. 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 MIOC, P. & ZNIDARCIC, M. ( 19 89): Osnovna geo­ loska kart a 1:100000, Toll11ao za l ist Maribor in AKSIN, Y., DRAGASEVIC, T., KU RT, J., MARI­ Leibnitz. - Zvezni geol. zay., Beograci , 60 p. NOYIC, D. & MARTINEC, R. (1981): Tolmao PLENICAR, M. (1954): Obmurska naftna nahajalisca.­ kartc naftnih in plinskih nahajalisc v SFRJ Gcologija, 2, 36-93, Ljubljana. 1:500.000.- Zvczni gcoloski zavod, Beograd, 72 p. PLENICAR, M. (1979): PodJoga tcrcijarnih s loj cva ANICJC, B. & JUR ISA, M. (1985): Osnovna geoloska Murske i sjeverozapadnog dijela Dravske potoline.­ karla 1:100.000, '1'01111<1(': za list Rogalcc.- Zvczni Zbornik radova lV. znanslv. sk., Znanstv. savj. za gcoloski zavod, Beograd, 75 p. naftu HAZU, Stubicke Toplicc, 45-52, Zagreb. CIGIT, K. (1958): 0 geoloskih razmerah filovske RJJAYEC, L. (1976): Biostratigrafija miocena v nartnc strllklUrc. - Gcologija, 4, 171 - 187, Ljubljana. Slove nskih goricah.- Gcologija, 19, 53-82, Ljubl- DURASEK, S. & I-JARI, S. (1992): Growth faulting J