Tomislav Malvić

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.

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.

Geological maps of Neogene sediments in the Bjelovar Subdepression (northern Croatia)

Abstract Please click here to download the map associated with this article. The methods applied in this paper constitute the conventional (without software packages) interpolations in subsurface geological mapping that pertain to basin analysis of the Bjelovar Subdepression in northern Croatia. The Bjelovar Subdepression covers about 2900 km, which is the scale of small basins within the Pannonian Basin System. The maps were constructed by interpolation of six stratigraphical and lithostratigraphical markers. These maps form the basis for explanations of (1) deposition and deformation evolution of sediments and structures, (2) spatial distribution of possible hydrocarbon traps and (3) evaluation of location of mature source rock. There are mapped large unconformities, especially in the eastern portion of the subdepression, two main synclines (western and eastern) and several anticlines (Pavljani, Dežanovec). Fault activity was easily observed, and the slight domination of normal faulting was proven in Badenian-Early Pontian (16.4–6.3 Ma) ages, and reverse faulting in the Late Pontian-Quaternary (6.3–0.0 Ma). Maximum throws were up to 300malong some stratigraphical borders.

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.

Comparison between the Middle Miocene and the Upper Miocene source rock formations in the Sava Depression (Pannonian Basin, Croatia)

The Sava Depression lies at the very south-western margin of the Pannonian Basin. There are 20 hydrocarbon fields altogether and 17 are still in production. The organic geochemistry data and their statistical analysis from the 25 exploration wells, indicate source rock formations in two stratigraphic levels, an older one of Middle Miocene age (Badenian and Sarmatian) and a younger one of Upper Miocene age (Lower Pannonian). Both source rock formations are composed of marls, calcitic marls, clayey limestones and shales. Source rock intervals lay at depths from 1200 to 3362 m. The Total Organic Carbon (TOC) of analyzed samples varies from 0.39 to 4.94%, while their total generative hydrocarbon potential is from 2.40 to 37.40 mg HC/g rock. The mean thickness of the intervals is 100–150 m. There is a regular linear increase of the maturity level with depth. Source rocks are mature, in the catagenetic phase of transformation that enables hydrocarbon generation. A favourable organic facies, mostly kerogen type II, (organic facies AB and B), with good hydrocarbon potential, dominates the north-western and central part of the depression. It can be connected with the deeper parts of the depression and/or protected, anoxic to dysoxic stagnant environments with a gradual transition from marine (Badenian/Sarmatian) to brackish depositional environments (Lower Pannonian). In the south-eastern part of the depression, the dominant kerogen type is II–III, (organic facies BC), which indicates a stronger influx of terrestrial material from the uplifted parts that are generally closer to the margins of the depositional basin. The Fisher test (F-test) of the variance similarity (homogeneity), clearly indicates that the Badenian/Sarmatian samples belong to a statistically different population from the Lower Pannonian ones, due to their different depositional environments.

Sequential Indicator Simulations maps of porosity, depth and thickness of Miocene clastic sediments in the Kloštar field, Northern Croatia

Data from selected Lower Pontian sandstone reservoir in the Kloštar Field, situated in the Sava Depression (Northern Croatia), were used for mapping with Sequential Indicator Simulations rather than using a classical approach. Such approaches offer better insight in distribution of geological variables or zonal uncertainties in the cases with larger datasets (15 points or more). Obtained maps of porosity and reservoir thickness are presented here along with probability maps of certain selected cut of values of petrophysical parameters. Maps showed distinct sedimentological features that can clearly be observed on the both sets of maps.

Kriging, cokriging or stochastical simulations, and the choice between deterministic or sequential approaches

This new research presented here for the first time in the Croatian geomathematical community, aims to establish several criteria which will aid in the selection of deterministic or stochastic geostatistical methods of estimation. This review associates the theoretical background of kriging, cokriging or stochastic simulations with some results obtained by mapping Badenian clastic reservoirs in the Drava Depression. The selected reservoirs are located in the Stari Gradac-Barcs Nyugat field, in the Western part of the Drava Depression, and at the Beni?anci field in the Eastern part of this depression. Both datasets (each with 14 points) include mean porosity values taken from well log analysis in the reservoir interval at the well sites. This resulted in significant uncertainties in the variogram models, especially with the determination of range (at the secondary variogram axis). The critical advantage was the availability of a seismic attribute that could be used as a secondary and co-regionalized variable (porosity is the primary variable). In the first example (the Beni?anci field) the seismic attribute was correlated with the average logged porosities. This made it possible to apply the cokriging method as the best interpolation option. The cross-validation result was 2.19 for 14 wells. In contrast, the existence of only a primary variable at the Stari Gradac-Barcs Nyugat field, forced the application of stochastic simulations as the better estimation tool, which can better describe the porosity changes in inter-well areas.

Thickness maps of Neogene and Quaternary sediments in the Kloštar Field (Sava Depression, Croatia)

The Kloštar Oil Field is situated at the north-western part of Sava Depression in the Croatian part of the Pannonian Basin. It is a typical geological structure that evolved through the Neogene and Quaternary and that is why the structural evolution is reconstructed using palinspastic mapping (i.e., using selected chronostratigraphic horizons as datum planes). The total map set includes six structural and 15 palaeostructural maps interpolated over five E-log markers and one border. The mapping has been performed using the Ordinary Kriging. The maps were used for the interpretation of geological evolution during the Neogene and Quaternary, and particularly a description of hydrocarbon reservoir formation and migration pathways. The structural development can be explained through two phases of transtension and two of transpression that existed regionally in the Sava Depression. However, the maps and cross-sections that are described locally show changes of dominant tectonic styles, particularly during the Quaternary when most of the field was a depositional centre.