Josipa Velić

Tertiary Subsurface Facies, Source Rocks and Hydrocarbon Reservoirs in the SW Part of the Pannonian Basin (Northern Croatia and South-Western Hungary)

The Neogene sedimentary successions of the Drava, Sava and Slavonija–Srijem depressions in the SW part of the Pannonian Basin Systemare built up of three 2nd order megacycles separated by four majorerosional unconformities. The first megacycle contains terrestrial tomarine syn-rift and early post-rift sediments of Early to Mid-Mioceneage. The second is built up of Late Miocene Lake Pannon deposits,while the third contains those sediments which were deposited inthe remnants of Lake Pannon and in the subsequent fluvial systems,in areas of continuous subsidence associated with basin inversionfrom the Pliocene onwards. Most of the petroleum source rocks andreservoir rocks are of Miocene age and were formed during the firstand second depositional megacycle. Conditions for the accumulationand preservation of large quantities of marine and terrigenousorganic matter were most favourable during the Badenian, Sarmatianand Early Pannonian, in deep basin settings, partly associated withrifting. The generation of hydrocarbons was promoted by relativelyhigh geothermal gradients during the initial and subsequent thermalsubsidence. Various sedimentary environments produced depositswith good reservoir characteristics: e.g. fault-related talus breccia(mainly Lower Miocene), reefs (mainly Badenian), coastal, shallowmarine (Karpatian, Badenian) and deltaic (Pannonian–Pontian) sandbodies or turbiditic sand lobes (mainly Pannonian). The hydrocarbon(HC) migration paths were often provided by the major unconformitiesbounding the three megacycles, as well as by faults, particularlyaround the basement highs.

Tectonic activity in the Croatian part of the Pannonian basin

Abstract Three main stages in structural development of the SW Pannonian basin are differentiated: the onset of extensional tectonics between the Oligocene and the Early Miocene, main extensional processes in the Early and Middle Miocene and prevailing transpression in the Pliocene and Quaternary. The neotectonic phase is given special attention in this paper. Structural fabrics allow to subdivide the area into three large structural zones: corresponding to the Western, Southern and Central marginal part of the Pannonian basin in Croatia. These zones are bounded by important faults: the Periadriatic–Drava fault, the Medvednica fault zone and the southern marginal fault of the Pannonian basin. The recent displacements within the Dinarides and the Eastern Alps form the boundary conditions for active transpressional deformation in the Western marginal part of the Pannonian basin. Dextral transcurrent displacement is evident in the zone of the Periadriatic–Drava wrench fault. The formation of the new types of structures is observed, especially in zones of compression along the faults. Narrowing of the area between the Sava and Drava rivers causes structural changes in the marginal parts of the corresponding basins, i.e. in the areas close to the Slavonian Mts. Recent tectonic activity is marked by the occurrence of earthquakes which are most common in the northwestern part of Croatia. The spatial distribution of hypocentres and of seismotectonically active zones is used to reconstruct probable displacements and the structural style of deformation in depth. The earthquake sources in the vicinity of the Medvednica fault and the Periadriatic–Drava fault are given special attention.

Neogene Tectonics in Croatian Part of the Pannonian Basin and Reflectance in Hydrocarbon Accumulations

Neogene and Quaternary tectonics in the Croatian basin is very complex, due to two phases of transtension (Badenian and Late Pannonian-Early Pontian) and two of transpression (Sarmatian- Early Pannonian and Late Pontian-recent). Transtensions were periods of main sediment accumulation and transpression of uplifting and structural forming. Consequently, there are lithological heterogeneities laterally and vertically as well as occurrence of numerous fault zones. Such faults separate regional tectonic blocks and very often bordering hydrocarbon field structures, acting as partial or complete seals. Pressure and production anomalies can be a useful indicator of fault sealing, observing at wells located on opposite blocks. Middle Miocene was period when a numerous strike-slip negative flowers had been formed inside regional depressions of CPBS. Such places were depositional centres of alluvial fans, where coarse-grained sediments from local sources of clastics, with good reservoir properties, were deposited. Moreover, numerous intra-reservoir micro-zones of secondary porosity are present in basement of such reservoirs owing to the complex Middle Miocene transtensional tectonic reflected in Palaeozoic and Mesozoic basement rocks. Results are single hydrodynamic units of heterogeneous reservoirs of Palaeozoic, Mesozoic and Middle Miocene (rarely Lower Miocene), where unconformities play the most important role. Stratigraphycally, such unconformities are loci for lengthy exposure at the surface before Badenian transgression. Heterogeneity and the ages of reservoir rocks, which span several geological erathems, strongly emphasise diagenesis as a highly important process in Mesozoic basement rocks. Rocks of these sediments today represent the 2nd reservoir unit according with importance of remaining and potential hydrocarbon reserves. The modern resolution of petroleum exploration techniques today make possible to detect also smaller, but economically interesting reservoirs as the new discoveries. Upper Miocene encompasses turbiditic depositional mechanism strongly characterised by single source area located at western margins of PBS (Eastern Alps). Reservoir rocks are sandstones, which reached the highest thicknesses in central parts of CPBS depressions. The major influence on sandstone reservoir quality had increasing of silty and marly components in marginal depression parts and decreasing of thickness. Also, mechanical diagenesis, as a process of compaction, caused a decrease in porosity for depth difference more than 400 meters in the same lithostratigraphic member (Malvic et al., 2005). It happened especially in deeper parts, but has minor influence compared with increasing of silty and marly components in marginal parts. The third process, also of minor influence, had been chemical diagenesis including several processes caused by dissolved ions, pH value, pressure and temperature. in any case, sandstone reservoirs contain the largest proven hydrocarbon reserves. Also, there are projected the highest possible (undiscovered) quantities of oil and gas, but for difference of Badenian coarse- grained reservoirs, in sandstones such reserves would be located close to existing fields in the so called subtle traps, i.e. as satellite reservoirs. Such targets were not explored in the past, due to smaller areal extension and often not so favourable lithological composition (transitional lithofacies). But, as total recovery increase thanks to new technologies, such reservoirs start to be important hydrocarbon source. Total remaining hydrocarbon potential in CPBS makes this area interesting for future exploration. Remaining reserves are probably at least 8x106 m3 of oil, 3.80 x106 m3 of condensate and 36 x109 m3 of gas (Velic et al., 2010). Middle Miocene sediments probably hide some undiscovered smaller structures on depressions margins, and in Upper Miocene subtle traps are probably remained as satellite structures around existing larger sandstone reservoirs. The majority of remaining hydrocarbons is assumed in Upper Miocene sandstones. The strike-slip tectonics played crucial role in shaping CPBS with contemporary structures and hydrocarbon fields. These fault systems play two important roles. In transtensional phases they formed negative flower structures where sediments had been accumulated (both in Middle and Upper Miocene). On contrary, in transpressional phases they were changed in positive flower structures, especially in 2nd transpressional period, forming traps for hydrocarbon accumulations as well as migration pathways (e.g. in Malvic, 2003 or Velic, 2007). The main fault displacement happened along the bordering strike-slip faults, between which structure had been formed. Also, tectonic and sedimentation in CPBS, which occurred through 2 transtensional and 2 transpressional phase, can be analysis additionally through three depositional megacycles. Properties of such megacycles can be followed on seismic sections, well cores, logs, and outcrops on surface, even in large scale (Blaskovic et al., 1984). The 1st megacycle corresponds with 1st transtensional phase and mostly included coarse- grained reservoir sediments in older, and pelites (often source rocks) in younger part. The 2nd megacycle corresponds to 2nd transtensional phase of Late Miocene, and included the sandstones that are the main reservoirs regarding volumes and recoverable hydrocarbons in CPBS. These rocks resulted from periodical, strong turbiditic currents that generally moved from NW/N toward SE/S, and which direction had been strongly determined by marginal depression’s faults, local strike-slip structures and uplifted, subaqueous palaeoreliefs remained from 1st transtensional phase. The 3rd megacycles is connected with 2nd transpressional phase, which took place the most of time in continental environment. This phase has importance for final structural evolution of CPBS but also in the last decade some researching results opened possibilities for economical biogenic methane accumulation in Pliocene and Lower Quaternary sediments. Some projections indicate that such accumulations are often located above existing reservoirs as represent mixture of thermogenic and biogenic gases. Croatian part of Pannonian Basin System is large and well geologically described Neogene and Quaternary regional basin system. Many analyses offered enough data and results for describing geological evolution of this area and transfer methods and conclusion in other similar geological provinces. This basin system is also well known area of numerous hydrocarbon reservoirs, where some of them are classified as very large in world scale. Although this province is today considered as mature petroleum basin, there is still enough remaining reserves for production in next several decades, what is here described numerically for CPBS. But, technological improvements also make possible increasing of recovery from discovered reservoirs as well as discovering some smaller and subtle traps. Presented analysis for CPBS makes easier to understand in which stratigraphical units and tectonical environments such traps can be found in CPBS.

Quaternary Deposits as the Hydrogeological System of Eastern Slavonia

The area of eastern Slavonia, situated between the Drava and Sava rivers, comprises three geotectonic units: the eastern part of the Drava depression in the north, part of the Slavonia-Srijem depression in the south and the central —akovo-Vinkovci plateau together with the Vukovar plateau. These units are separated by deep faults that reach the base of the Tertiary sediments. The first 200 m of Quaternary deposits are saturated with fresh water. The aim of this study was to find out whether the faults form impermeable boundaries separating the waterbearing deposits into independent hydraulic systems, or if a singular hydraulic entity exists. Results of the analysis indicate that lithological continuity of the aquifers exists along the fault zones on the margins of the —akovo-Vinkovci and the Vukovar plateaux, which means that there is no impermeable hydraulic boundary on the watershed between the Sava and Drava river valley. The part of eastern Slavonia between the Sava and Drava rivers is one hydraulic system consisting of zones with different transmissivity values. In the zones of reduced transmissivity, the hydraulic connections are weakened, but not broken. Such zones exist not only along the fault zones of the —akovo-Vinkovci plateau and the Vukovar plateau, but also within the Sava and Drava depressions. The terrain morphology influenced formation of both the surface and the underground watershed, parallel to the extension of the —akovo-Vinkovci and Vukovar plateau. Therefore, within this single hydraulic entity, when the drawdown reaches the watershed due to excessive pumping, the watershed will be displaced from its natural position. Saaeetak U prostoru istone Slavonije, izme﷿u Save i Drave nalaze se tri geotektonske jedinice i to na sjeveru istoni dio Dravske potoline, na jugu dio Slavonsko-srijemske potoline, a izme﷿u njih —akovakovinkovaki i Vukovarski ravnjak. Ravnjaci su u morfolo„kom i struk turno-tektonskom pogledu sloaeene timorske strukture, koje su od susjednih potolina odvojene sistemima dubokih rasjeda koji zadiru u podlogu tercijarnih sedimenata. Zbog morfologije terena se duae pruaeanja —akovako-vinkovakog i Vukovarskog ravnjaka proteaee povr„inska i podzemna razvodnica koja istraaeivani prostor dijeli u dva sliva: Dravski na sjeveru i Savski na jugu. Cilj ovog rada bio je istraaeiti da li rasjedne zone izme﷿u ravnjaka i potolina predstavljaju nepropusnu granicu koja naslage dijeli u dva odvojena hidraulika sustava ili one ine jednu hidrauliku cjelinu. Analizom su obuhvaEe ne kvartarne naslage koje sadraee vodu pogodnu za vodoopskrbu, a koje zalijeaeu do dubine od 200 m. Rezultati analize pokazali su da litolo„ki kontinuitet vodonosnih slojeva duae rasjednih zona —akovako-vinkovakog i Vukovarskog ravnjaka nije prekinut, te da na razvodnici savskog i dravskog sliva ne postoji nepropusna hidraulika granica. Prostor istone Slavonije izme﷿u Drave i Save smatra se jed nim hidraulikim sustavom unutar kojeg postoje zone s razliitim vri jednostima transmisivnosti. Duae zona smanjene transmisivnosti hidraulike veze su oslabljene, ali nisu prekinute. Takve zone nisu prisutne samo duae rasjednih zona —akovako-vinkovakog i Vukovarskog ravnjaka nego i unutar pojedinih depresija. U sluaju da sniaeenje uzrokovano crpljenjem dosegne razvodnicu zapoinje dotok vode iz podruja s druge strane razvodnice, tj. razvodnica se premije„ta u odnosu na prirodni poloaeaj udaljavajuEi se od mjesta crpljenja.

Sedimentation of deep-water turbidites in the SW part of the Pannonian Basin

Sedimentation of deep-water turbidites in the SW part of the Pannonian Basin The Sava Depression and the Bjelovar Subdepression belong to the SW margin of the Pannonian Basin System, which was part of the Central Paratethys during the Pannonian period. Upper Pannonian deposits of the Ivanic-Grad Formation in the Sava Depression include several lithostratigraphic members such as Iva and Okoli Sandstone Member or their lateral equivalents, the Zagreb Member and Lipovac Marlstone Member. Their total thickness in the deepest part of the Sava Depression reaches up to 800 meters, while it is 100-200 meters in the margins of the depression. Deposits in the depression are composed of 4 facies. In the period of turbiditic activities these facies are primarily sedimented as different sandstone bodies. In the Bjelovar Subdepression, two lithostratigraphic members (lateral equivalent) were analysed, the Zagreb Member and Okoli Sandstone Member. The thickness of the Bjelovar Subdepression ranges from 50 meters along the S and SE margins to more than 350 meters along the E margin. Generally, detritus in the north-west part of the analysed area originated from a single source, the Eastern Alps, as demonstrated by sedimentological and physical properties, the geometry of the sandstone body and the fossil content. This clastic material was found to be dispersed throughout the elongated and relatively narrow Sava Depression and in the smaller Bjelovar Subdepression. Sedimentation primarily occurred in up to 200 meters water depth and was strongly influenced by the sub-aqueous paleorelief, which determined the direction of the flow of turbidity currents and sandstone body geometries. The main stream with medium- and fine-grained material was separated by two independent turbiditic flows from N-NW to the SE-E. Variability in the thickness of sandstone bodies is the result of differences in subsidence and cycles of progradation and retrogradation of turbidite fans.

Application of Neural Networks in Petroleum Reservoir Lithology and Saturation Prediction

The Klostar oil fi eld is situated in the northern part of the Sava Depression within the Croatian part of the Pannonian Basin. The major petroleum reserves are confi ned to Miocene sandstones that comprise two production units: the Lower Pontian I sandstone series and the Upper Pannonian II sandstone series. We used well logs from two wells through these sandstones as input data in the neural network analysis, and used spontaneous potential and resistivity logs (R16 and R64) as the input in network training. The fi rst analysis included prediction of lithology, which was defi ned as either sandstone or marl. These two rock types were assigned categorical values of 1 or 0 which were then used in numerical analysis. The neural network was also used to predict hydrocarbon saturation in selected wells. The input dataset was extended to depth and categorical lithology. The prediction results were excellent, because the training and prediction dataset showed little disagreement between the true and predicted values. At present, this study represents the best and most useful application of neural networks in the Croatian part of the Pannonian Basin.

Reservoir Geology, Hydrocarbon Reserves and Production in the Croatian part of the Pannonian Basin System

3 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 aver- age number of reservoirs is 16 for oil and 11 for gas. Lithological composition is highly favourable, because reser- voirs 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 recov- ery through the use of secondary and tertiary recovery methods. Also, water-fl ooding will remain the dominant sec- ondary-recovery method for increased production in the future.

Sedimentary bodies, forms and occurrences in the Tudorevo and Mirovo glacial deposits of northern Velebit (Croatia)

A small glacier of cirque-valley type existed during the Late Pleistocene Wurm Glacial in the Tudorevo and Mirovo karst valleys of Northern Velebit. Its source was in Tudorevo, and it moved through Dundovic Mirovo and Bilensko Mirovo to Baricevic Dolac, shaping U-valleys around 4 km in length. After melting, glacial deposits remained, composed of chaotic and unsorted till, composed of carbonate sand, debris and sub-rounded clasts, cobbles and blocks of predominantly Middle Jurassic and subordinately Lower Jurassic carbonate rocks. In Dundovic Mirovo and Bilensko Mirovo, where the largest masses of glacial deposits occur, terminal and recessional moraines can be found over the ground moraine, as well as some other features, mostly drumlins (drumlin field), eskers, erratic blocks, kettle holes and striations. Some erratics have been transported for more than 4 km from their primary outcrops, e.g. clasts of Lower Jurassic Toarcian Spotty limestone. A terminal moraine was deposited between Bilensko Mirovo and Baricevic Dolac, perpendicular to the glaciated U-valley and it forms the Bilo hill, the northern and southern foothills of which are composed partly of glaciofluvial deposits. Between Tudorevo and Mirovo, a recessional moraine occurs above the ground moraine. The glacier was subject to polyphase melting and freezing, and the youngest freezing events may be related to cirques in Tudorevo. During melting events, glacier lakes are supposed to have existed, initially in the Baricevic Dolac, later in Mirovo area and finally in Tudorevo. These discharged into the karst underground by percolation through till and by erosion to the karstified underlying Middle Jurassic carbonates.

Relations between effective thickness, gas production and porosity in heterogeneous reservoirs: an example from the Molve Field, Croatian Pannonian Basin

ABSTRACT The Molve Field is the most important gas-condensate reservoir in Croatia. This petroleum system is not typical for the Pannonian System, because it comprises several reservoir lithologies, relatively high structural closure and significant tectonic influence on the fields compartmentalization. Strike-slip extension in the Middle Miocene and younger Late Miocene and Pliocene tectonics formed the present-day tectonic setting. Reservoir stratigraphy includes four lithofacies (from Devonian to Neogene) with a unique gas-water contact. The lithologies encompass cataclased granite, gneiss, schists, quartzites, dolomites, limestones and grainstones. Source rocks were generated in lacustrine organic facies and migration occurred in the Late Miocene to Pliocene. Reservoir gas includes 4.5–15.7% C2+, but also non-hydrocarbon components. Analysed porosity data were approximated with a normal-distribution curve in lithofacies I, II and III, making it possible to calculate mean and variance easily by descriptive statistics. Moreover, gas production and effective thicknesses generally can be linked through a linear trend. However, significant deviations in the expected increased production rate with regard to greater reservoir thickness are observed for particular wells. This is a result of locally abrupt changes in effective porosities and permeabilities, and the size of the drainage area along the main fault zones. These faults resulted in significant compartmentalization of the field. Furthermore, owing to significant facies variations, permeability and porosity gradually change, especially in the vertical direction. Significant reserves of condensate (3 × 106 m3) and gas (43 500 × 106 m3) with a high recovery rate of 71% make this field significant for geological reservoir models. The well-established geological model for this field and its stable high pressure have maintained production rates at a present level of approximately 2900 m3 gas and 165 m3 condensate per day, thus providing a valuable example for other large heterogeneous reservoirs in the Pannonian Basin.

Interdependence of Petrophysical Properties and Depth: Some Implications of Multivariate Solution on Distinction Between the Lower Pontian Hydrocarbon-bearing Sandstone Units in the Western Part of the Sava Depression

Statistical analysis of reservoir data from the Lower Pontian clastics (the most important hydrocarbon reservoir rocks in the Sava depression), supports established knowledge of the interdependence of petrophysical properties and depth. Irrespective of the focus that the reservoir data may be studied and presented, depth always emerges as a fundamental reservoir descriptor. This is particularly evident when studying the differences between widely spaced oil and gas fields, when the numerical model completely separates the two sets of descriptor variables, indicating two different sources of their internal variability. Porosity and permeability belong to “intrinsic rock properties” while depth must be ascribed to other sources, e.g. tectonic subsidence. Discriminant function weighted with depth (DF1) has such group centroid values, that zones can be drawn within a particular field that coincide with structural relationships. On the function marked with reservoir properties (DF2), group centroid values are higher close to the axes of palaeotransport channels, where sandstone layers are the thickest and particles are best sorted. Group centroid values on the third function (DF3) depict the areas of relatively higher permeability in the apical parts of structures, possibly caused by fracturing due to folding, or by cementation of other parts of reservoirs, where the circulation of pore waters was more pronounced. In the case of the most thoroughly investigated Zutica field, the inverse relationship between depth and porosity becomes evident when compared with the direction of palaeotransport and thickness of reservoir rocks on the respective structure and thickness maps.