Geological overview onshore

Croatia is located at the crossroads of Central Europe, the Balkans, and the Mediterranean. It is a highly varied country with high mountains of the Dinaric Alps at over 1000 m to large river planes cut by the Sava, Drava, Kupa and Danube rivers.

Geologically the country is split into two main onshore provinces, the Pannonian Basin Province and the Dinarides. These areas are very different both in geographical appearance and in geological terms.
The Pannonian Basin is located within the arcuate Carpathian mountain chain that encircles the basin; this chain joins the Alpine mountain system to west and the Dinaric mountain system to southwest.
The basin is internally segmented by the intra-Carpathian mountain ranges and basement uplifts, but its continuity at the Tertiary level is not disrupted (the Graz Sub-basin and Vienna Basin in Austria, as well as the East Slovakian Sub-basin in Slovakia and Ukraine and the Transylvanian Basin in Romania show geological continuity with the central part of the Pannonian Basin).

The Croatian sector of the basin covers some 26,000km² and is split into 4 main sub basins – Sava, Drava, North west Croatia and Slavonija. The Dinaride region is present just outside of the basin limits to the South west.

Structural evolution

The present tectonic setting of the Pannonian Basin Province is characterized by Palaeozoic-Mesozoic sedimentary successions deposited upon older metamorphic basement.

The basement blocks were assembled during the Paleogene by extrusion from the west along strike-slip faults and were mainly subjected to sub-aerial erosion during the Maastrichtian-Paleocene. During the Cretaceous compressive deformations were common in the basin, caused by the closure of the Tethys Ocean.

During the Middle Miocene, extensive strike slip faulting began leading to pull-apart basins which rapidly evolved into rifts. Extension rates of up to 200% are often associated with reactivation of earlier compressional faults. The time of rifting varied according to the location – basins in the north and west are inferred to have been rifting as the earliest.

Many of the Miocene extensional basins were initially starved, but were subsequently filled by two major deltaic systems prograding from the west, north and southeast (draining the Carpathians). Combined thickness of the Palaeozoic-Recent series exceeds well over 10,000 m. Thickness of the Neogene succession alone is reaching over 7,000 m.

Tecto - stratigraphic overview

Seven units have been identified in the Pannonian basin, the oldest is pre-Permian and represents the basement. It consists of magmatic metamorphic and rarely sedimentary rocks, with granites, gneisses, schists and various low grade metamorphic rocks. These units appear to have been affected by both the Caledonian and Variscan Orogenies. The unit is referred as unit 7 on the figure below.

Schematic lithostratigraphic section of terciar and quartar complex in deep wells

Inside the shallow depressions along the basin margins and in the central area, an angular unconformity separates the underlying basement rocks with the overlying sediments and carbonates related to platforms and troughs which were formed during the late Permian, Triassic and Jurassic periods. Sedimentary basement (unit 6 on the figure) consists of these shallow water carbonates, dolomitic limestones and breccias up until the mid-Triassic when associated volcanism indicates the fragmentation of west Tethys.

Younger basement sediments largely compromises carbonate sediments that were deposited in a gradually deepening platform basin and seamount conditions. The end of the phase is marked by the late Jurassic obduction of ophiolites. Subsequent local uplift and erosion occurred. The collision of Apitus and Rhodope began during the late Cretaceous- Palaeocene period. The beginning of unit 6 is therefore marked by the initial Dinaride thrusting and the development of longitudinal flysch and then molasses, with troughs oriented parallel to the movement of the over thrust system.
In deeper parts of the depression, especially in Sava and Drava, sediments of unit 5 of upper and lower Miocene age unconformably overlay the basement rocks. In other depressions they are in anomalous contact with Mesozoic or Paleogene strata. Formation of these units is related to the Miocene wrench pull-apart extension which occurred with response to rotation of the Apulian plate; it seems that wrench faults followed the weak zones formed during the previous thrusting process. In this way fault bounded basins (Sava and Drava) with Dinaric orientation were created. These strong movements induced rapid subsidence and fault controlled marine and non-marine sedimentation that lasted up until the end of the Sarmatian. At the beginning large amount of breccias, conglomerates and various sandstones were deposited in alluvial fans, braided rivers and near shore areas. Simultaneously considerable discharge of lavas and pyroclastics occurred and were related to fundamental wrench fault activity along the new faulted basin margins. Later some parts of the carbonates were reworked as thick carbonate breccias. Initial fault related subsidence began to slow by Carpathian times and in marginal marine areas limestones formed in complete back-reef, reef and fore-reef settings.

Periods of gentle subsidence at the beginning of the Badenian (unit 4) prompts migration of the reefs towards marginal parts of the basin and the formation of thick packages of organic rich marl in the central areas. At the end of this phase during the Sarmatian slow thermal uplift and extensive weathering of the marginal parts of the depression led to the formation of a regional unconformity.
Subsidence occurred immediately after the uplift due to the cooling of the lithosphere with relatively fast filling of the newly formed basin with lake characteristics. Regressive tendencies that predominate in the period from the end of the middle Miocene till present facilitate progression of deltaic and turbiditic depositional systems (unit 3) and the infilling of depressions. This cycle begins with the deposition of the lower Pannonian dark organic rich anoxic marls in the deeper parts of the basin. Light grey marls were simultaneously deposited in the marginal areas.
These sediments are followed by the upper Pannoinan and lower Pontian sandstones of turbiditic origin which are deposited only in the deepest parts of the depression. Alteration of lower Pontian argillaceous limestones and siltstones of delta slope and sandstones of delta front follow. Generation of oil started in the upper Plio-Quaternary sands and clays of the alluvial plain with common coal intercalations (unit 2).
The final tectonic phase began at the end of the Pliocene, with the formation of the strong transgressive regime. Marginal faults of the basin are transferred into reverse faults with common occurrence of positive flower structures and associated anticlinal forms. Since this tectonic phase coincides with the main phase of the oil and gas generation it is assumed that newly formed anticlinal traps were immediately filled with hydrocarbons. Unit 1 overlies the fore mentioned succession and represents modern sediments.

Petroleum systems

Source rocks

  • The deltaic sequences of the Pannonian system itself. The lower part of the sequence was generated in deep-sea and is considered a very good source for oil and gas. The upper part (deltaic complex) is rather gas-prone and it is suggested that it has never been buried deeply enough to reach oil window. However, it is very difficult to separate the respective sources of hydrocarbons in the basin (Clayton et al., 1988).
  • Late Neogene source rocks with type II and III kerogen content were deposited as marine or lacustrine clays and marls deposited in restricted lagoonal settings, TOC average 1–2 wt.% (up to 5 wt.%).
  • Early Miocene siltstones and mudstones with kerogen type III are mature to over-mature and produce dry wet and gas condensate.
  • Upper Triassic Kössen black shales and marls are considered the principal sources of oil and natural gas in deep systems.
  • Dinaride sources are oil prone formed within carbonate and carbonate ­ evaporitic sequences of the region with TOC values (up to 21.8%).

Reservoirs

The major reservoirs in the basin include:

  • Triassic limestones and dolomites (porosity up to 38%).
  • Cretaceous carbonates.
  • Neogene series of the Pannonian Basin (virtually all porous sands within these successions may form reservoir units given good structural/stratigraphic position).
  • Miocene-age, near-basement basal clastic bodies (conglomerate). The series represent an excellent reservoir (porosity values may reach up to 61%).
  • Oligocene sands within the “Paleogene basins” (e.g. turbidites of the Kiscell Formation).
  • Fractured metamorphic and igneous basement rocks of Precambrian-Palaeozoic age.
  • Dinaride resevoirs are generally composed of carbonate with Reefoidal limestone of the Upper Jurassic , Bio-accumulated limestone of the Cenomanian , Onkoidal and oolitic limestone (Aptian-Albian) and Succroidal dolomite and dolomite breccia of the Middle Jurassic.

Seals

The most important seals in the entire basin are provided by intra-formational mudstones of pro-delta and delta-top mudstones. In majority of the fields, the argillaceous interval above the reservoir usually forms the sealing horizon. It may also be the source rock for the next reservoir. The Neogene series are considered as top seal to all older reservoirs of the Pannonian.

Seal rocks in the dinarides are formed of sediments of hypersaline lagoonal Sabkha facies (anhydrite, early digenetic dolomite) of Albian-Cenomanian age.

Plays

  • There are five main groups of plays discerned in the Pannonian Basin, best classified according to their structural affiliation (majority of fields involve reservoirs at many stratigraphic levels).
  • Miocene-Pliocene clastic and/or carbonate series draped over basement highs form the most significant play types in the basin (such play types are represented by post-rift compactional drape varieties).
  • Fractured and/or weathered basement (highs) with or without Miocene basal conglomerate and sealed by Miocene mudstones form a very important group of plays (Play types 1 and 2 are sometimes found together).
  • Miocene extension-related, tilted fault blocks containing reservoirs of various ages form an important group of plays (Pre-rift/Basement compactional drape).
  • Inversion-related play types are found in the anticlines associated with reactivation of strike-slip faults and include Miocene-Pliocene clastic reservoirs.
  • Syn-rift-related, Miocene sandstone series truncated and sealed by post-rift mudstones (this play type formally may belong to type 1 plays).
  • In general, combined structural-stratigraphic plays are most typical in majority of the fields in the basin. The above-specified types of plays are valid in all countries sharing the area of the Pannonian Basin.

North west croatia - geological summary

Two smaller depressions exist in the north-western part of Croatia, the Mura and Hrvatsko Zagorje.

The Mura Depression is morphologically divided into three smaller sub-depressions with northwest orientation. The young, neotectonic Ormoz-Selnica anticline and older Cakovec plateau separate depressions from each other. The basement rocks have not been drilled but Mesozoic carbonates and breccias are abundant similar to those observed in the Drava Depression. Effusive rocks and pyroclastics are present in the early Miocene time. Unusually thick sandstone packages occur in the early Pannonian within the post rift Upper Miocene sediments. Several smaller fields are located in the Mura Depression. The main reservoir rocks are early Miocene conglomeratic sandstones and, to a lesser extent, Upper Pannonian and Pontian sandstones. The existence of a relatively unexplored faulted Mesozoic carbonate uplift, and younger anticlinal forms indicate favourable conditions for the discovery of new oil and gas accumulations in this region.

Mura depresion

Mura depresion

Hrvatsko Zagorje is a smaller depression characterised by a north east orientation. It is situated between the local Medvednica, Ivansvcica and Kalnik mountains. Carbonates dominate the Mesozoic sedimentary complex along with thick deposits (1200 m) of Lower Miocene coarse grained clastics; these have been drilled in the deepest part of the depression. Within the post rift Upper Miocene deposits, Lower Pannonian marls are present and Upper Pontian sandstones are missing but deltaic Pontian sandstones are well developed. This is one of the least explored regions in which no field has ever been found. However, borehole HZ-1 from the deeper part of the depression has shown oil and gas occurrences.

Drava - geological summary

Young neotectonic uplift of Bilogora divides the Drava exploration area in to the Drava Depression and Bjelovar sub-depression.

The Drava Depression, which is also the largest in the region, covers the majority of the area; the depression has NW-SE orientation and asymmetrical outline with gently dipping north-eastern and steep south-western margin that follows the north-eastern slopes of the Bilogora. Basement of the Tertiary deposits consists either of metamorphic and intrusive rocks or carbonates, evaporite and flysch-molasse sediments of the Mesozoic and Paleogene.

Miocene and Pliocene deposits have a typical synrift end post rift development. Occurrences of volcanic end pyroclastic rocks are frequent in the Lower and Middle Miocene. The Lower Pannonian is in a discord with the usual development in the other parts of the Pannonian Basin, it consists of thick sandstone packages. Upper Pannonian and Lower Pontian are represented by the well-developed turbiditic sandstones while alteration of deltaic and alluvial limestones and marl predominates in the Upper Pontian. Thickness of the deposits in the deepest parts of the depression totals over 6000 m.

The Main productive zones are in the Mesozoic carbonates and carbonate breccias, as well as in Lower to Middle Miocene pyroclastic breccias, extrusives and lithothamnium Limestones. Portions of the Upper Pannonian and Pontian sandstones are also productive for gas condensate; the Molve, Kalinovac and Stari Gradac are the largest fields in this area. The biggest oil field, Beničanci, is located in the deep central parts of the depression. Several smaller and shallower gas and oil fields are located within the vast and complex positive flower structure of the Bilogora Mountain.

The Bjelovar sub depression is located on the south-western side of the Bilogora Mountain. It is divided into several paleomorphological uplifts without a distinct orientation. Stratigraphic settings are similar to the Drava Depression but the thickness of the Tertiary deposits is reduced and totals less than 3000m. Upper Pannonian turbidite sandstones are missing and marly and silty deposits have been deposited instead. Several oil fields at the depths of less than 1200 m have also been recently discovered. Main reservoirs are weathered parts of the basement rocks, Middle Miocene biocalcarenltes and Upper Pontian sandstones of the delta front.

Potential for the discovery of the new fields exists in the hidden buried hill structures as well as in the stratigraphic and structural­ stratigraphic traps associated with coarse grained clastic deposits of the alluvial plains, braided rivers and shore facies of the Lower and Middle Miocene. Pinching out of the Upper Miocene and Upper Pannonian turbidite sandstones towards the edges of the basin and basin uplifts, are also potential hydrocarbon traps.

Nw drava depresion

Nw drava depresion

Sava - geological overview

The Sava Depression is oriented from the NW-SE and incorporates the majority of this exploration area. It has an asymmetrical shape with gently sloping south-western and extremely steep north-eastern flank.

A distinct wrench compression zone with frequent occurrences of positive flower structures delineates the north eastern margin of the depression. The remains of a Cretaceous carbonate platform and Cretaceous Paleogene flysch sediments are virtually absent inside the depression. They are present only in the southernmost part of the depression, inside the Karlovac and Glina sub depressions.

Maximum thickness of the Tertiary in the section is over 5000m. Lower and Middle Miocene deposits discontinuously and directly overlie basement igneous rocks. They have characteristics of the syn-rift sedimentation with rare occurrence of volcanic activity. Again, breccias conglomerates and conglomeratic sandstones as well as lithothamnium limestones with associated biocalcareites are present. Thick packages of the Badenian marls with excellent source rock characteristics are well developed. Post rift development is represented by the Lower Pannonian marly limestones, Upper Pannonian and Lower Pontian turbidite sandstones as well as Upper Pontian and Pliocene sediments of the deltaic and alluvial depositional systems. The effects of the youngest tectonic phase have been noted along the north-eastern margin of the depression and in its south-western and south-eastern parts (Vukomeričke Gorice, Dilj Gora), in these areas elements of the wrench compression are well pronounced.

Major hydrocarbon carriers are Upper Miocene turbidite sandstones as well as Upper Pontian sandstones of the delta front and channel fills of the delta plains. Weathered/fractured parts of the basement rock as well as conglomeratic limestones of the Lower Miocene that are present along the north-eastern edge of the basin and have potential to act as reservoir rocks. The majority of the accumulations belong to the structural and structural¬ stratigraphic traps located in the central and north-eastern areas along the extent of the wrench zone.

Remaining hydrocarbon accumulations can be expected in the areas where turbidite sandstones pinch out towards the edges of the basin and basin uplifts, as well as in coarsely grained Lower Miocene deposits of the alluvial fan, braided river, shore and reef environments. In the south-eastern parts of the depression there are strong indications for the tectonic inversion of the thickest and insufficiently explored central part of the Sava Depression, this would suggest potential existence of the structural traps with well-developed systems. Pozeka Kotlina is another small sub depression located north from the south-eastern margin of the depression with some potential for hydrocarbon accumulation.

Sava depresion

Sava depresion

East Slavonija - geological overview

The Eastern Slavonia region is represented by the Slavonia-Srijem Depression. It is geographically divided into the several smaller depressions and uplifts with diverse orientations (NW­ SE and SW-SE).

Uplifted blocks represent the remains of the Paleozoic basement as well as the remains of Mesozoic effusive and siliciclastic sediments with rare carbonate occurrences.

Maximum thickness of the Tertiary is around 3500 meters. Structurally positive flowers dominate the area, and are noted along the western margin of the depression, these are associated with the younger Pleistocene, wrench compressive tectonic activity.

The main productive zones are in metamorphic, intrusive and effusive rocks of the Paleozoic age as well as in the coarse clastic deposits (breccia-conglomerates and conglomeratic sandstones) of the Upper Miocene. Pontian sandstones are also well developed and productive. Undiscovered accumulations of hydrocarbons are expected along the margins of the basins, in the stratigraphic traps related to the pinching out of clastic bodies, in the buried hill structures of the poorly explored Mesozoic and (to a smaller extent) in younger anticlinal traps.

East drava depresion

East drava depresion

Dinarides - geological overview

The Dinarides are a wide NW-SE fold-and-thrust belt stretching from southwestern Slovenia to Montenegro along the Adriatic coast of Croatia and inland. The Dinarides formed as a consequence of subduction and collision processes in the border zone between Europe and Adria tectonic plates. Adria acted, and is still acting, as an indenter moving roughly towards stable Europe and causing the Meso-Cenozoic carbonate rocks of the Dinaric platform to over-ride onto the Adriatic lithosphere. The most prominent structures of the Dinarides are NW-SE oriented thrust faults and they directly bound the SW margin of the Pannonian.

Petroleum exploration in the Dinarides has been carried out for more than 30years encouraged by widespread hydrocarbon seeps and bituminous rocks. Geophysical surveys began with gravimetry in the 1940’s and became extensive in sixties and seventies continuing with magnetometry, geo-electric and seismic measurements. Despite encouraging oil and gas shows in some wells no discovery has yet been made, nor has this exploration work provided a definite model for the petroleum geology of the area, recent exploration in the Bosnian Dinarides may add potential to this area.

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Željka Rukavina