Reservoir geology , hydrocarbon reserves and production in the Croatian part of the Pannonian Basin System

Approximately 104 x 106 m3 of oil (39 fi elds), 6.93 x 106 m3 of condensate (11 fi elds), and 64.92 x 109 m3 of gas (52 fi elds), were recovered in the Croatian part of the Pannonian Basin System during 64 years of exploitation (1941– 2005). The production peak was attained between 1980–1989, when exploitation began in 12 new fi elds. Based on their cumulative production, the Croatian oil and gas fi elds can be divided into four groups, and the condensate fi elds into three groups. Such a division has been supported by analysis of recovery, number of reservoirs, porosity and permeability, age and lithology of reservoir rocks. The longest production period is assumed for the fi rst group of fi elds; for oil it is approximately 55 years, for condensate 46 and gas 36 years. In the favourable fi rst group the average number of reservoirs is 16 for oil and 11 for gas. Lithological composition is highly favourable, because reservoirs are represented mostly by sandstones of Pannonian and Pontian age with high porosities and permeabilities. A relatively homogeneous sandstone lithology, including good regional seals like marls, enables an increase in recovery through the use of secondary and tertiary recovery methods. Also, water-fl ooding will remain the dominant secondary-recovery method for increased production in the future.


INTRODUCTION
This work presents a detailed overview of Croatia's oil, gas and condensate reserves.The data is derived from oil and gas fi elds located in the Croatian part (CPBS) of the Pannonian Basin System (PBS), and to cover the whole country's reserves, the Adriatic gas fi elds were also added.However, as the main topic is the CPBS, the most well know data on the evolution of the entire Pannonian Basin System are presented by way of an introduction, followed by further review of the Croatian part.These data are crucial to understand the location, age and recovery factors from all types of reservoirs as well as assumptions about future production.
The Pannonian Basin System (PBS) is a back arc basin system that belongs to an area in the past covered by the Central Paratethys, and younger brackish and fresh-water environments formed from Paratethys.PBS generation started with convergence and subduction of the Apulian Plate under the Dinarides during the Styrian orogenic phase, developing the southern (Periadriatic-Vardar lineament) and northern (Outer Carpathian) boundaries.However, the southwestern boundary of the Central Paratethys area located further to the south from the Periadriatic-Vardar lineament was represented by an array of small, fresh-water Neogene basins within the Dinarides that were never invaded during

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Geologia Croatica 65/1 their evolution by marine transgressions in Paratethys PAVELIĆ (2002).These basins have never been considered as parts of the Mediterranean or Central Paratethys, although sediments and fauna can be correlated with some PBS characteristics.
Inside PBS, numerous basins, depressions and subdepressions were formed along dextral and sinistral strike-slip faults (Fig. 1A, modifi ed after ROYDEN, 1988).The terms "basin, depression or subdepression" described areal geometry of opened areas.The very fi rst extensions were initiated in the Ottnangian (ROYDEN, 1988;RÖGL, 1996RÖGL, , 1998)), but sediments of this age were only deposited and preserved locally.The extensional period continued through the Karpathian when restricted lacustrine and fl uvial sediments were deposited in the Apuseni Mts., Carpathians and Podolian Upland as well as on the margin of the Alps and the Bohemian massive (e.g., RÖGL 1996RÖGL , 1998)).
During the Badenian, mostly marine sedimentation and maximum extensional displacements are characteristic for the entire PBS (e.g., RÖGL, 1996RÖGL, , 1998)), including the CPBS (e.g., VRBANAC, 1996).The real marine environment with "normal" salinity was dependent on the existence of connections with large "oceanic" areas, i.e. with the Mediterranean in the south-west and the Indo-pacifi c to the southeast through Trans-Tethyan trench corridors.The Lower Badenian transgression covered the entire basin, from Austria to Romania (Transylvania) and from the Carpathians to the Dinarides.The Carpathians and the Apuseni Mts., remained perhaps the two most prominent areas with island arcs.However, in the Middle Badenian, the marine environ-ment of normal salinity was limited to the western part of Central Paratethys (i.e.parts of Poland, Hungary, Slovenia and Croatia).Elsewhere, sediments of a regional regression phase and evaporites have been proven.Deposition of thick evaporate sediments signal the Middle Badenian salinity crisis in the Transcarpathian Basin, Transylvanian Basin andCarpathian Foredeep (KOVAČ et al., 2004, 2007).The last regional fl ood in the PBS occurred during the Late Badenian, when the connections with the Mediterranean and Indo-pacifi c were interrupted (e.g., STEININGER et al., 1978).Some areas on the margins of the PBS are characterised in the Upper Badenian by evaporites, including the Carpathian Foredeep (e.g., PERYT, 1997PERYT, , 1999;;MǍRUNTEANU, 1999;ANDREYEVA-GRIGOROVICH et al., 1997) and the Transylvanian Basin (e.g., CHIRA, 2000).
In the CPBS, PAVELIĆ ( 2001) described the period of the Late Badenian as the transition between the extensional and post-extensional stages.Similarly, VRBANAC (1996) considered that Northern Croatia remained a marine environment at that time, where the present-day mountains represented islands or remarkable submarine highs.
Consequently, the next Sarmatian age is the period when a continuous marine environment began to be progressively reduced (e.g., RÖGL, 1996RÖGL, , 1998)), resulting in isolated areas that eventually became brackish and fresh water lakes during the Late Miocene.Psammitic, and pelitic, clastic sedimentation in littoral environments, was sporadically interrupted by deposition of large quantities of coarse-grained carbonate material, originating from eroded, near-shore reefs.ROYDEN (1988) noted a major phase of extension  that fi nished over most of the Pannonian Basin System in the Early Pannonian.In the post-extensional phase, the generator of dynamism was thermal subsidence, triggered by cooling of the lithosphere.Subsidence was stronger in the central part of the basin system (approx.2 km in 10 Ma), and alkali volcanic activity occurred locally.From the Early Pannonian, continuing into the Pontian, sedimentation took place in brackish and fresh water lacustrine environments (e.g., RÖGL, 1996RÖGL, , 1998)).Sediments were transported through delta and prodelta systems, or by turbiditic mechanisms capable of moving detritus dozen of kilometres.Turbidites, as the main transport mechanism in the CPBS, were periodically activated in response to gravitational and/or tectonic instability on the structural "ramps" (e.g.MALVIĆ & VE-LIĆ, 2011).The Eastern Alps were the main clastic sediment source for the Croatian depressions in the Late Miocene.

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Eventually, the Late Pontian, and especially the Pliocene and Quaternary were characterised by fl uvial, lacustrine and marshy sedimentation along with aeolian sediments in the youngest parts of the Quaternary.These periods completed the basin infi ll and resulted in formation of the present-day relief.The tectonic regime was compressional, indicated by reverse faults and overthrusts (ROYDEN, 1988), and often along reactivated older fault planes with inversion of displacement as proven by the numerous relative young hydrocarbon traps.Also in the CPBS, tectonic and depositional properties were similar (e.g., VELIĆ, 2007).Interestingly, extensional and compressional phases in the PBS during the Neogene and Quaternary, and recently in the CPS, can be divided into two transtensional (Badenian and Pannonian-Early Pontian) and two transpressional (Sarmatian and Late Pontian-Quaternary) phases (in MALVIĆ & VELIĆ, 2011).

GEOLOGICAL SETTINGS AND HYDROCARBON PROPERTIES IN THE CPBS
The area of the CPBS is divided into the Drava, Sava, Mura, and Slavonija-Srijem depressions (Fig. 1B).Although each of these has their local characteristics, the general evolution of their depositional environments, transport mechanisms, tectonics and dominant lithologies is very similar, and can be described as a unique area of the PBS.The boundaries between the depressions are mountains and massifs in Northern Croatia, or their subsurface extensions, which can be recognised as uplifted structures or buried hills covered by Neogene and Quaternary sediments (VELIĆ et al. 2002;VELIĆ 2007).They are recognizable in present-day relief or in the subsurface via well or seismic data.

Depositional megacycles of Neogene and Quaternary systems in the CPBS
Neogene and Quaternary sedimentary systems are subdivided into three megacycles (VELIĆ et al., 2002).Neogene and older outcrops in the CPBS are rare as most of the surface is covered by Holocene sediments (VELIĆ, 2007).At the margins of the depression, the thickness of Neogene sediments varies between 500-1500 m, rising towards the cen-tres, to 3500 m thick in Slavonia-Srijem, 5500 m in the Sava and Mura and almost 7000 m in Drava Depressions respectively (Fig. 1B; VELIĆ et al., 2002;SAFTIĆ et al., 2003).The isochore 0 m is proven along the highest mountains in Northern Croatia.These central basin zones of maximum deposit thickness are locations of thick intervals of cyclic, i.e. rhythmic deposition (FILJAK et al., 1969;ŠIMON, 1980).VELIĆ et al. (2002) described three sedimentation megacycles in the Neogene and Quaternary.These cycles consist of sequences of well defi ned lithostratigraphic formations.Each megacycle includes sediments deposited during cycles of relative sea level change.In the 1 st part of the cycle the water level is gradually rising, and in the 2 nd it is suddenly dropping (e.g., MITCHUM 1977).Generally, each megacycle is composed of a regular/ordered superposition of well defi ned lithologic units (formations and/or members).A particular cycle is measured in tens or hundreds of metres (e.g., MATHUR, 1981), but an entire megacycle in the CPBS can reach more than 1000 m (VELIĆ et al., 2002;VELIĆ, 2007).
The oldest, or 1 st megacycle (Fig. 2), comprises Middle Miocene sediments, i.e. the Prečec Formation (Fm.) in the Sava Depression, Moslavačka Gora Fm. in the west Drava Depression, the Vukovar Fm. in the east Drava and Slavonia-Srijem Depressions and the Murska Sobota Fm. (except the formation top) in the Mura Depression.Lithologically it is a very heterogeneous sequence.It comprises coarse grained clastics (breccia, conglomerates and sandstones), fi ne grained clastics (clays, marls and calcareous, sandy, argillaceous marls) and carbonates (limestones and calcareous sandstones).According to ĆORIĆ et al. (2009) an initial marine transgression occurred later in the Croatian part of the Pannonian Basin -1 Ma after the beginning of the Badenian.This was dated using Nannofossil Marker Species, especially

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Geologia Croatica 65/1 the nannoplankton NN5 zone, resulting in sediments previously regarded as Karpathian now being of Badenian age.All sediments that belonging to 1 st megacycle but have Lower Miocene age, also are of very restricted areal extension and it is why they are excluded from this study.
The 2 nd megacycle (Fig. 2), is of Late Miocene age (belonging to the Pannonian and Pontian stages).It comprises sediments of the Sava Group (Ivanić-Grad, Kloštar Ivanić and Široko Polje Formations) in the Sava Depression.In the western Drava Depression, the Bilogora Formation (Late Pontian) is equivalent to the Široko Polje Formation.In the east Drava and Slavonia-Srijem Depressions, equivalent formations are the Vinkovci and Vera Formations.Finally, in the Mura Depression, this megacycle comprises the Lendava Formation and the lower part of the Mura Formation.
The 3 rd megacycle (Fig. 2), is of Pliocene, Pleistocene and Holocene age.According to the lithostratigraphic nomenclature it comprises the Lonja Formation in the Sava and the west Drava Depressions, the Vuka Formation in the east Drava and Slavonia-Srijem Depressions and the Mura Formation in the Mura Depression.

Hydrocarbon reserves in the CPBS
Analysis of the number of discovered and active fi elds (39 oil, 11 condensate and 52 gas fi elds) between 1941-2005 (Fig. 3) showed that the number of new fi elds rapidly increased in the sixties for oil, and in the eighties for gas.This period was followed by a drastic decline in new discoveries.Generally, the 30 years between1959-1989 can be described as a highly successful exploration period, when potential structural and combined traps could be discovered with the use of several easily applied methods such as gravimetric and seismic surveys.These were often discovered by the very fi rst exploration well.Improved seismic survey methods, focused on depths greater than 2000 m, played a critical role in the discovery of the largest Croatian gas-condensate reservoirs in the Drava Depression in the eighties.Reservoirs are described as oil, condensate and gas pools.

Main reservoir lithofacies in the CPBS
The oldest fi elds are the Gojlo, Šumečani, Bunjani, Kloštar and Dugo Selo fi elds, (production began between 1941 and 1957).The youngest fi elds are Ðeletovci and Privlaka (1984), Bizovac and Leticani (1989) and Galovac-Pavljani (1991).The group of "the oldest fi elds" is located in the Sava Depression (Fig. 1B) and includes three different reservoir lithofacies: two of them are Neogene sandstone and coarsegrained clastics while the third is in the Palaeozoic magmatic and metamorphic rocks of the crystalline basement.Almost all of these fi elds are of the "buried hill" type trap (Šumećani, Bunjani, Kloštar, Ðeletovci, Privlaka, Galovac-Pavljani), where signifi cant quantities of oil and gas are accumulated across an erosional unconformity between the crystalline basement and the coarse-grained Badenian sediments.
Quantitative data and conclusions from previous studies (BASSIOUNI & VELIĆ, 1996;BELOŠIĆ, 2003;DRAGAŠ et al., 1995;HERNITZ et al., 2002;MALVIĆ & VELIĆ 2010;MALVIĆ et al., 2005;JÜTTNER et al., 2001;SAFTIĆ et al., 2001;VELIĆ et al., 2002)  Observing the volume of Neogene and Quaternary sediments explored by drilling, there is good potential for successful exploration of the remaining hydrocarbons, especially in areas among the discovered fi elds (VRBANAC et al., 2010).Younger sediments (upper part of the Upper Miocene, Pliocene, Pleistocene and Holocene) have not been explored at all, although the presence of gas (especially biogenic methane) has been identifi ed in several fi elds and prospects.The oldest Neogene rocks, older than Lower Pannonian, probably contain signifi cant undiscovered hydrocarbon reserves of oil and thermogenic gas, because these deposits constitute the principal sequences of Neogene source rocks (especially in the Badenian, i.e., the Middle Miocene, TRO-   SKOT-ČORBIĆ et al., 2009;SAFTIĆ et al., 2003;VELIĆ et al., 2000;ZEČEVIĆ et al., 2010).The potential of these deposits as unconventional reservoir rocks remains to be explored.

Remaining recoverable and probable reserves in the CPBS
A regional study was recently published concerning exploration and production (E&P) ventures in Eastern and Central Europe, which also gave the potential of remaining recoverable hydrocarbon liquids in this area (DOBROVA et al., 2003).This study reports that "E&P activities were particularly successful, in terms of having discovered economically viable oil and gas reserves and having achieved incremental production, in Poland, Lithuania, Hungary, the Czech Republic and Romania".Of course, new discoveries of hydrocarbons are small to moderate, but it is expected that they will greatly contribute to the reserves' replacement in these countries.Cited paper reveals that Croatia's resources are 6% of the total recoverable resources in Central and Eastern Europe, which places Croatia in third place along with Austria and Belarus, after Romania and Ukraine.DOBROVA et al. (2003) stated two conclusions of particular interest: (a) improved drilling success ratios were achieved mainly by state companies, since they preferred to drill appraisal and/ or low risk prospects while largely reduced drilling frontier wells; and (b) current production growth will come mainly from traditional producers' activities, not from those of newcomers.This means that the decline of oil and gas production/proven reserves in Croatia may be slowed or could cease with the addition of the production from satellite reservoirs of existing fi elds rather than from new discoveries.MALVIĆ & RUSAN (2009) calculated the minimum size of a commercial discovery of oil in the Croatian part of the Pannonian Basin System as being of 2 x 10 5 m 3 of recoverable oil.Such discoveries can be expected even in marginal areas (like the Bjelovar Subdepression) of the Drava or Sava Depression that are the largest depressions in Croatia.They calculated, that to discover one new fi eld with an expected profi t of more than 2 x 10 6 USD (net present value, 13.52 x 10 6 USD, and risk adjusted value of discovery 35%), approximately 50 x 10 6 USD should be invested in exploration of the geological area where such discoveries are assumed.MALVIĆ & RUSAN (2009) also pointed out that more prospective plays for new fi elds would be in rocks of Lower and Middle Miocene age, or in the Mesozoic or Palaeozoic basement.It should be mentioned that between the Miocene and the Mesozoic-Palaeozoic rocks is a great tectonic and erosional unconformity, across which large secondary pore volumes in basement rocks are developed, as a result of a long period of weathering, dissolution and fracturing.
In any case, there is strong evidence that the Croatian part of the Pannonian Basin System contains signifi cant unproven (probable & possible) hydrocarbon reserves, in existing reservoirs.It is especially valid in clastic reservoirs (breccia or sandstone).Better use of secondary or tertiary recovery methods might make it possible to recover part of these trapped or bypassed reserves as well as supporting the extended production from presently proven reserves in certain reservoirs.

BASIC RESERVOIR CHARACTERISTICS
In Croatia, about 3610 wells were drilled prior to 2002.All fi elds are grouped by their cumulative production, into large, medium, small and very small fi elds, and those currently not productive (Tables 1-4).For oil and gas, this grouping is as follows: (a) Large fi elds, which produced more than 10 6 m 3 of oil/ condensate or more than 10 9 m 3 of gas; (b) Medium fi elds, which produced 10 5 -10 6 m 3 of oil/condensate or 10 8 -10 9 m 3 of gas; (c) Small fi elds, which produced 10 4 -10 5 m 3 of oil, <10 5 condensate or 10 7 -10 8 m 3 of gas; (d) Very small fi elds, which produced less than 10 4 m 3 of oil or less than 10 7 m 3 of gas.
As expected, the large fi elds (group 1) accounted for 93.07% of total oil production up to 2005, from group 2 another 6.67%, and from groups 3 and 4 the amount of production was negligible.For condensate, the reservoirs of group 1 accounted for 82.13%, and from gas reservoirs group 1 recovery was 90.75% of total production.The life-cycle of commercial production is generally long; e.g., the average production period for such fi elds is estimated at 55 years.Such fi elds characterised by an extremely long production period include the Mramor Brdo (80 years), Okoli (74 years) and Stružec (72 years).Tables 3 and 4 may indicate why fi elds from group 1 are more productive than others, suggesting the relevance of a higher number of reservoirs, deeper traps, higher porosity and permeability etc.Data from Tables 1 (oil), and 2 (gas and condensate) supported the classifi cation of fi elds in groups by analysis of petrophysical and statistical data as well as other geological information including depression, age, lithology and trap.

Reservoir properties of oil reservoirs
Data are taken from 39 oil fi elds, 52 gas fi elds and from 8 fi elds that were never productive.Data for condensate reservoirs are from 11 active fi elds and 1 recently discovered fi eld.Some earlier statistical analysis for data mostly from sandstone reservoirs of Lower Pontian age, were also included in this summary (MALVIĆ et al., 2005;SAFTIĆ et al., 2001).
For oil fi elds, group 1 reservoirs are sandstones of Pannonian and Pontian age, except for the Beničanci Field, where the reservoir is a Badenian breccia.Groups 2, 3 and 4 include reservoirs of heterogeneous lithology and ages: deposits in the Drava Depression consist of effusives, sandstones, breccias and conglomerates of (possible Karpathian) and Badenian ages.Other reservoirs include carbonates of Triassic, Cretaceous and Oligocene(?)-Miocene ages.In the Slavonia-Srijem Depression, reservoir rocks are represented

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Geologia Croatica 65/1 by Palaeozoic granite, gneiss, schist and diabase as well as conglomerates, breccias and sandstones of Badenian age.

Reservoir properties of gas and condensate reservoirs
The gas and condensate fi elds are also classifi ed in four and three groups respectively (Table 2), regarding the statistics of geological, and petrophysical data.
The four groups of gas fi elds (Table 2), are based on the recovered volumes of gas.Reservoirs are in Palaeozoic and Mesozoic rocks in 10 fi elds, Middle Miocene rocks in 22 fi elds and in the Upper Miocene rocks in 20 fi elds.However, there is no regularity of reservoir ages in the four groups.Predicted periods of production vary between 10 and 31 years.Not counting the gas fi elds in the Adriatic Sea, the production periods would be between 5 and 23 years.The largest fi elds could have the longest production, similar to oil fi elds.Other parameters listed for gas fi elds are characterised by similar trends observed for oil fi elds.The condensate is added to gas fi eld's data which mostly have the same characteristics.The main differences are in the deeper subsea depth of condensate reservoirs and number of analysed reservoirs (Table 2).

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Geologia Croatica 65/1 Table 3 gives data on production and recoveries obtained for oil, divided into groups of large, medium, small and very small fi elds.Table 4 gives data for production and recoveries of gas, divided into groups of large, medium and small fi elds as well as fi elds that are still not in production.
In the 1970's, when oil production peaked, recoveries by primary methods were as follows: 16-20% (dissolved gas drive) 20-25% (gas cap drive) 30-50% (water drive).The average recovery was 25%.Considering the largest fi elds with sandstone reservoirs, the following recovery was achieved: 16% in the Ivanić and Žutica Fields, 31% in the Kloštar Field and 39% in the Stružec Field.A maximum of 51% has been recovered from breccia in the Beničanci Field.The average total recoveries obtained until 2005 differ for fi eld sizes and fl uids.Generally recoveries are higher from the larger reservoirs.Fields were selected in four groups for oil and gas/condensate as in Tables 3 and 4, based on production to 2005.Three mechanisms are in use for oil production: Artifi cial lift (in a narrow sense, meaning a mechanical device inside the well, i.e., hydraulic pumping for transmitting energy to the bottom of the well); Gas lift (a widely used artifi cial sub-method), including injection of gas in the tubing to reduce the weight of the hydrostatic column; Natural fl ow production, when oil-reservoir pressure is suffi cient to transmit the hydrocarbons to the surface.
Production mechanisms used in Croatian fi elds were analysed with respect to fi eld sizes.The most often used mechanisms were: • For the largest fi elds, artifi cial lift and gas lift; for four fi elds, natural fl ow; • For medium fi elds, artifi cial lift; • For small fi elds, artifi cial and gas lifts; • For very small fi elds, artifi cial lift and natural fl ow.
The most frequently applied secondary recovery method for many Croatian fi elds is water injection.Approximately half of the present Croatian production comes from waterfl ooded reservoirs (e.g.Žutica, Ivanić, Beničanci etc.).Recovery from these reservoirs can reach 40%, thanks to a favourable oil/water viscosity ratio.In porous reservoirs, however, tertiary recovery methods, also called unconventional, enhanced or improved oil recovery (abbr.EOR or IOR) are successfully applied, as calculated for parts of the Ivanić and Žutica Fields (e.g.SEČEN et al., 2002).
During the last 20 years, the application of CO 2 injection was tested for 14 Croatian fi elds in the INA laboratories (GORIČNIK & DOMITROVIĆ, 2003).This method was found to be applicable in 33% of reservoirs attaining a complete fl uid miscibility, in 59% of reservoirs reaching a partial miscibility and in 8% of reservoirs with no fl uid miscibility.The amount of total bypassed oil in the water-fl ooded reservoirs of the 14 fi elds mentioned is about 140 x 10 6 m 3 , which means that appropriate tertiary methods could yield another 9-24 x 10 6 m 3 of liquid hydrocarbons.Gas injection (as gas lift) resulted in an approximate additional 14.5 x 10 6 m 3 oil recovered until the end of 1987.

DISCUSSION
The data presented are a valid source for estimating the hydrocarbon potential of the Croatian part of the Pannonian Basin System.Listed are in-place and recoverable quantities as well as cumulative hydrocarbon production data until 2005.They are related to geological, petrophysical and statistical data about all types of reservoir lithologies in the analysed area.Such a list should be useful when determining how to increase recovery from different types of lithologies, and how to select the most appropriate secondary and tertiary methods of recovery in the analysed lithologies.
The results obtained can be viewed in context with earlier published results (DOBROVA et al., 2003) in which the authors provided a valuable review of hydrocarbon reserves and potential over a large region of Central and Eastern Europe.According to available data, they reported comprehensive exploration and production results in Romania, Hungary, Czech Republic, Poland and Lithuania, covering recent successful ventures.These authors ranked Croatia in third place according to the remaining recoverable resources in Central and Eastern Europe at the end of 2002 (Romania fi rst, 39%; Ukraine second, 25%; and Croatia, Austria and Belarus third, 6% etc.).This is why we re-assessed our recoverable resources, cumulative production and reserves given in Table 2 of the previous report (DOBROVA et al., 2003), using 42-gallon-barrel units.The conversion of units has been accomplished using a 6.2898 barrel (USA) for 1 m 3 of oil.
More recent production data from Croatian hydrocarbon regions are lower, compared with data published in 2003.Even if condensate reserves are added to oil (i.e.liquid) reserves, new values of oil are still lower (122.79 vs. 141.18x 10 6 m 3 ), and consequently reserves in proven traps are 62.5% lower than previously published.It is also clear that remaining gas reserves are lower, at 56.6%.It is certain that new results, taken from data to 2005, are not overestimated, assuming that larger volumes can be expected only in new prospects, fi elds or satellite reservoirs.

CONCLUSIONS FOR FUTURE EXPLORATION AND THE STRATIGRAPHIC TARGET
Some of the remaining hydrocarbons could still be recovered from the reservoirs/fi elds with a long production record in the Croatian part of the Pannonian Basin System (Fig. 5).

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Geologia Croatica 65/1 Also there are some less spectacular ("subtle") but potentially large reservoirs; this is also supported by recovery rates given in Tables 3 and 4. Such potential petroleum resources could add to proven reserves, especially in the Drava and Sava Depressions.
"Subtle" reservoirs can be recognized using improved geological models of entire petroleum systems.Signifi cant reservoir volumes can be expected in: • Neogene basement rocks, along depression margins and/or uplifted palaeorelief (buried hills).• Middle Miocene sediments, consisting of different lithofacies.These formations could be favourable for discovery of stratigraphic traps along the fl anks of buried hills or at abrupt facies changes in palaeo-depositional plains.• Typical heritage structures (or anticlines) above pre-Neogene buried hills.
Pannonian and Pontian sediments in which the main focus should be stratigraphic traps, typically along subtle or gentle anticlinal structures.Such potential traps can be observed on structural maps and especially on 3D seismic arrays by attribute analysis, searching for gradual transitions between channel sandstones and marls deposited in basinal plains.Those sediments have the most favourable petrophysical properties.
Differential compaction, which can be a signifi cant mechanism of folding, especially in areas where depths are quite different over small lateral distances, causing "false" or "compaction" anticlines.
Sediments of all ages, for which further research needs to be undertaken for identifi cation of possible traps along main basinal faults that infl uenced sedimentation.These faults had the main role in basin subsidence, but many faults also are proven migration pathways for hydrocarbons from source rocks to reservoir rocks.
Special attention should be paid to transform zones and related structural traps.
It is obvious that there is a need for renewing and/or intensifying geological and geophysical exploration and subsequent drilling in the Croatian part of the Pannonian Basin System.We believe this can be done in carefully selected areas, leading to successful prospects and plays.

Figure 2 :
Figure 2: Chronostratigraphic units with l ithostratigraphic units in the rank of formations valid for the Croatian part of the Pannonian Basin System (VELIĆ et al., 2002).
contribute to estimation of the exploration level of hydrocarbon bearing rocks in the Croatian part of the Pannonian Basin System.These rocks were classifi ed into two complexes with regard to their composition: (1) the rocks of Palaeozoic and Mesozoic ages represented by magmatic, metamorphic and carbonate rocks and (2) the clastics of Neogene and Quaternary ages.

Figure 3 :
Figure 3: Number of discovered oil (left) and gas (right) fi elds during the particular time periods.

Figure 4 :
Figure 4: Relationships between recoverable and recovered reserves of oil (left) and gas (right) during the particular time periods.

Figure 5 :
Figure 5: Status of the Croatian recoverable hydrocarbon reserves.

Velić et al.:
Reservoir geology, hydrocarbon reserves and production in the Croatian part of the Pannonian Basin System

Velić et al.:
Reservoir geology, hydrocarbon reserves and production in the Croatian part of the Pannonian Basin System

Table 1 :
Oil fi elds and reservoir groups, geological, statistical and petrophysical data.Reservoir geology, hydrocarbon reserves and production in the Croatian part of the Pannonian Basin System Velić et al.:

Table 2 :
Gas fi elds and reservoir groups, geological, statistical and petrophysical data, condensates are listed separately in italics or with additional marks* in geological data.

Table 3 :
Oil reserves and production (up to 2005).

Table 4 :
Gas reserves and production (up to 2005); condensates are listed separated in italic with marks* Reservoir geology, hydrocarbon reserves and production in the Croatian part of the Pannonian Basin System