Sava Depression Research Articles

Mineralogical and geochemical characteristics of Miocene pelitic sedimentary rocks from the south-western part of the Pannonian Basin System (Croatia): Implications for provenance studies

Fifty-two samples of Miocene pelitic sedimentary rock from outcrops on Medvednica, Moslavačka Gora and Psunj Mts., and boreholes in the Sava Depression and the Požega Sub-depression were investigated. These sediments formed in different marine (with normal and reduced salinity), brackish, and freshwater environments, depending on the development stage of the Pannonian Basin System. Carbonate minerals, clay minerals and quartz are the main constituents of all pelitic sedimentary rocks, except in those from Moslavačka Gora Mt in which carbonate minerals are not present. Feldspars, pyrite, opal-CT, and hematite are present as minor constituents in some rocks. Besides calcite, dependent on the sedimentary environment and diagenetic changes, high-magnesium calcite, aragonite, dolomite and ankerite/Ca-dolomite are also present. Smectite or illite-smectite is the main clay minerals in the samples. Minor constituents, present in almost all samples, are detrital illite and kaolinite. In some samples chlorite is also present in a low amount. Major elements, trace elements and rare earth elements patterns used in provenance analysis show that all analysed samples have a composition similar to the values of the upper continental crust (UCC). The contents of major and trace elements as well as SiO2/Al2O3, K2O/Al2O3, Na2O/K2O, Eu/Eu*, La/Sc, Th/Sc, La/Co Th/Co, Th/Cr, Ce/Ce* and LREE/HREE ratios, show that the analysed pelitic sedimentary rocks were formed by weathering of different types of mostly acidic (silicic), i.e. felsic rocks.

SUBSURFACE MODELING AND UNCERTAINTY MODELS OF THE NEOGENE-QUATERNARY SEDIMENTS IN THE PART OF SAVA DEPRESSION BASED ON DIGITALIZATION OF LEGACY MAP DATA

Aim of the research was the accurate digitalization and subsurface modelling of the legacy data, namely paper-based geological maps. The research area covers approximately 1053 km 2 and is located in Croatian part of Pannonian Basin System, Sava Depression, i.e. in the surrounding of Stružec Oil Field. Data input for the structural analysis were structural maps which were previously made in the 1980. Set included five structural maps based on regional E-log markers and one map of pre-Neogene surface. All were digitalized in program ArcGIS, and later modelled in Petrel. Structural contours and total of 134 faults were exported into Petrel software where they were further processed and regrouped. From such digitalized and processed data the geological model (geomodel) was made using complex fault framework. The study concerned the advantages and errors that occur during creation of such geomodel. It clearly displays relations between major and minor faults and fault slips, geological structures and structural traps in 3D (e.g. anticline of the Stružec Field). Another advantage is multicolored presentation of depths and faults, which enables easier recognition of geological structures and potential traps for hydrocarbons. Errors and issues can occur during digitalization of contour lines, selection of the appropriate algorithm and those ones that arise when model is created. Low error between two approaches indicates accuracy of digitalization of data and subsurface modelling. Uncertainty models (maps of statistical percentage deviation) were made in the Petrel Software for the display of the deviation between handmade maps and Petrel models for each E-log marker and one pre-Neogene top.

Stratigraphic and paleogeographic significance of lacustrine mollusks from the Pliocene Viviparus beds in central Croatia

The mollusk fauna from the Pliocene Viviparus beds of Vukomericke Gorice hills in central Croatia was investigated at four sites in the region of Kravarsko, S of Zagreb. The region represents a Pleistocene dome-anticline at the southern margin of the Sava depression. Sediments are dominated by clay, bearing some sand, gravel and lignite intercalations. The mollusks, comprising 11 gastropod and 2 bivalve species, prove the studied deposits to derive from the long-lived, highly endemic Lake Slavonia. The taxonomic revisions include the introductions of Viviparus kochanskyae n. sp. for specimens from Lake Slavonia previously identified with V. fuchsi NEUMAYR, 1872 and Prososthenia ? praeslavonica n. nom. replacing the primary homonym Hydrobia vitrella BRUSINA, 1897 non Stefanescu, 1896. Recognized as an independent phase in the geodynamic evolution of the Pannonian Basin, the new regional stage Cernikian is introduced for the succession, defined by the complete depositional sequence of the Viviparus beds. Two stratigraphic horizons detected in the studied sites are constrained by the Lower Cernikian Viviparus kochanskyae and the Upper Cernikian Viviparus hoernesi zones and stay in perfect agreement with previous regional data. Timing of the Lake Slavonia history is enabled through several zonal markers calibrated to the Geological Time Scale in the Dacian Basin. Accordingly, the Lower Cernikian transgression dates to c. 4.3 Ma, the Upper Cernikian to c. 3.1 Ma, indicating strong alteration of the lacustrine depositional settings during the Pliocene, most likely related to changes in the regional climate. Interestingly, the second transgression of Lake Slavonia is marked by the evolution of strongly sculptured viviparid shells and coincides with the Pliocene Climate Optimum.

Prospects for CO2 carbonation and storage in Upper Miocene sandstone of Sava Depression, Croatia

Upper Miocene sandstones in the Sava Depression (Northern Croatia) are potential targets for carbon dioxide (CO 2 )-based enhanced oil recovery (EOR) processes or mineral carbonation with reservoir brine. In general, sandstone lithology, even rich with aluminosilicate minerals, is not the most favourable rock medium for efficient sequestration of CO 2 in minerals. However, CO 2 is naturally sequestered in minerals when CO 2 is injected into sandstone reservoirs and subdued to carbonation. The timescale of such sequestration is on the order of 104–105 years. Upper Miocene sandstones in the Sava Depression could incorporate up to 25% of aluminosilicate minerals (10% K-feldspars) and up to 20% dolomites and often laterally transition between pelitic and psammitic lithofacies, rich in clay minerals (15% micas). Total volumes are approximately 107 and 62 million m 3 (approximately 268 and 155 million t of rocks), respectively for the potential injection reservoirs in the Ivanić Field. Oil saturation in the injection intervals is estimated to be approximately 14.8 and 4.1 million m 3 respectively. Geochemical analogies and mineralogy can be used to support predictions for the low percentages of injected CO 2 that may be stored during carbonation and form secondary minerals.

Overview and Reinstatement of Some Geophysical Interpretations in the Eastern Sava Depression

The eastern part of Sava Depression is bounded on the south with the Sava River and on the north with Psunj Mountain, Požeška and Dilj Hills. In this part thicker Neogene-Quaternary sediments than in the western part were found, and there is almost never reached thermal maturity sufficient to catagenesis. Therefore, after the first seismic survey and exploratory drilling, a new additional research never continued. The overview and assessment of the reconstruction of the part of geophysical measurements was prepared in the eastern part of the Sava Depression. In some previous interpretations rocks between the Upper Pannonian and Upper Pontian are described as result of deltaic systems, with the clinoforms. Regional deposition at the time of the Early Pannonian indicates the beginning of significant changes in the environment in Croatian part of the Pannonian Basin System (CPBS), with a distinct domination of turbidites. Clinoforms are possible results of especially psammitic detritus deposited toward the margins of the depression, and partly in the direction material transport. In the Early Pontian regionally depositional environments started to be local. In the Quaternary can be expected the existence of the delta or alluvial fans.

Characterization of clastic sedimentary enviroments by clustering algorithm and several statistical approaches — case study, Sava Depression in Northern Croatia

This study demonstrates a method to identify and characterize some facies of turbiditic depositional environments. The study area is a hydrocarbon field in the Sava Depression (Northern Croatia). Its Upper Miocene reservoirs have been proved to represent a lacustrine turbidite system. In the workflow, first an unsupervised neural network was applied as clustering method for two sandstone reservoirs. The elements of the input vectors were the basic petrophysical parameters. In the second step autocorrelation surfaces were used to reveal the hidden anisotropy of the grid. This anisotropy is supposed to identify the main continuity directions in the geometrical analyses of sandstone bodies. Finally, in the description of clusters several parametric and nonparametric statistics were used to characterize the identified facies. Obtained results correspond to the previously published interpretation of those reservoir facies.

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.

Sediment characterization and its implications for arsenic mobilization in deep aquifers of eastern Croatia

Sediments from two boreholes drilled in the Drava and Sava depressions were analyzed to determine the origin of the elevated arsenic (As) concentrations in groundwaters in eastern Croatia. The composition of the sediments was investigated by total chemical analysis and sequential extraction. In addition, groundwater samples were taken from eight producing wells, and hydrochemical and trace element and As speciation analyses were performed. The concentrations of As in the sediment were high (up to 471 and 65.1mg/kg, in the <0.065mm fraction in the boreholes in the Drava and Sava depression respectively) supporting the hypothesis proposed in a previous hydrochemical study that the Middle and Upper Pleistocene sediments are the main source of As in eastern Croatian groundwaters. Mineralogical and chemical analyses showed that sediments from the Drava and the Sava boreholes differed, supporting different origins of the sediments in the boreholes concerned: the provenance of PVc-3 sediments being the Alps, and of G1 sediments, the Bosnian mountains. In both boreholes, the large amount of As was bound to amorphous Fe and Mn (hydr)oxides (on average 32% and 60% in boreholes PVc-3 and G1 respectively). Sediment and groundwater analyses indicate that reductive dissolution of iron (hydr)oxides and concomitant release of sorbed or co-precipitated As into the groundwater is ready to occur. Since the groundwaters in eastern Croatia are generally anoxic and reducing conditions, in which S2− and Fe2+ are stable, prevail the concentration of As released by reductive dissolution of iron (hydr)oxides may be controlled by the secondary formed sulfides which could act as a sink rather than as a source of As in the groundwater. Depending on local geomorphological and hydrogeochemical conditions, in the zones with a longer groundwater retention time, pH dependant As desorption from iron (hydr)oxides can occur and other trace elements such as Mo may become mobile.

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.

Local sediment sources and palaeoflow directions in Upper Miocene turbidites of the Pannonian Basin System (Croatian part), based on mapping of reservoir properties

The source of the clastic sediments in the Croatian part of the Pannonian Basin System (CPBS) during the Late Pannonian and Early Pontian was the Eastern Alps. Clastic sediments were redeposited several times before they reached the Sava Depression. The depositional environment and sediment transport mechanisms have been subject to detailed analysis described in many publications, and this study builds on previous research. We have carried out geostatistical mapping of selected Upper Pannonian and Lower Pontian reservoir variables of the Klostar Field, located to the west of the Moslavacka gora Mt. (Croatia). This has shown that the Moslavacka gora Mt. was a secondary, local source of sediment, in contrast to the previous interpretation of a single, distant clastic source (Eastern Alps) for the CPBS during the Late Miocene. As the mineralogical composition of the Moslavacka gora Mt. and the Eastern Alps is very similar, the dominant direction of turbidity currents obtained by sequential indicator simulations are used to suggest that a modest amount of detritus was eroded from the Moslavacka gora Mt. and mixed with detritus sourced from the Eastern Alps.

Palinspastic reconstruction of synsedimentary tectonics of Neogene and Quaternary sediments in the Kloštar Field (Sava Depression, Pannonian Basin, Croatia)

Palinspastic reconstruction of synsedimentary tectonics of Neogene and Quaternary sediments in the Kloštar Field (Sava Depression, Pannonian Basin, Croatia)

Diagenesis of Miocene pelitic sedimentary rocks in the Sava Depression (Croatia)

AbstractMiocene pelitic sedimentary rocks from six wells in the Sava Depression (sub-basin in the south-western part of the Pannonian Basin System) were investigated in order to determine the degrees of diagenetic alteration.Qualitative and semiquantitative mineral compositions of samples and the content of smectite in illite-smectite (I-S) were determined by X-ray powder diffraction (XRD).Vitrinite reflectance and thermal alteration index (TAI) were measured in order to estimate the palaeotemperatures.Carbonate minerals, clay minerals and quartz are the main constituents of the pelitic sediments.Feldspars (albite), pyrite, opal-CT and hematite are present as minor constituents in some rocks. The mineral composition of the rocks, apart from previously known differences caused by various depositional environments and clastic material provenance, is dependent on the degree of diagenetic processes. At elevated temperatures and large burial depths the minerals formed by alteration processes or precipitated at the surface; smectite, kaolinite and calcite were gradually replaced by minerals formed by diagenetic processes, i.e. by illite-smectite, illite, chlorite, Ca-excess dolomite/ankerite and albite. Based on XRD data for smectite, I-S and illite, three stages of diagenetic development have been established. The early stage was characteristic of samples at depths &lt;1.8 km containing smectite, I-S of the random R0 type and detrital illite.The middle stage begins with the appearance of the orderedR1 type of I-S at the depths &gt;1.8 km and temperature above 80°C. At depths &gt;4.6 km with corresponding temperatures &gt;190ºC, the late stage began, characterized by the presence ofR&gt;1 I-S with &lt;10% smectite.

Stochastic simulations of dependent geological variables in sandstone reservoirs of Neogene age: A case study of Kloštar Field, Sava Depression

The research presented herein is the first attempt to perform geostatistical simulations on three geologic variables, porosity, thickness, and depth to reservoir, in the Croatian Pannonian basin. The data were collected from a reservoir of Lower Pontian age in Klostar Field, located in the western part of the Sava Depression. All three variables were analyzed using sequential Gaussian simulations (SGS). Information regarding present-day depth, thickness, and locations of areas with higher porosity values were used to reconstruct paleo-depositional environments and the distribution of different lithotypes, ranging from medium-grained, to mostly clean sandstones and to pure, basin marls. Estimates of present-day thickness and depth can help to define areas of gross tectonic displacement and the role of major faults that have been mapped in the field. However, since mapping of the raw data (including porosities) does not allow the reconstruction of paleo-depositional environments, sequential indicator simulations (SIS) were applied as a secondary analytical tool. For this purpose, several cutoff values for thickness were defined in an effort to distinguish the orientation of depositional channels (main and transitional). This was accomplished by transforming porosities to indicator values (0 and 1) and by applying a non-linear “indicator kriging” technique as the “zero” map for obtaining numerous indicator realizations by SIS. In the SGS and the SIS approaches, the simulations encompassed 100 realizations. A representative realization was then selected using purely statistical criteria, i.e., two realizations were almost always chosen in accordance with the order of calculation. The 1 st and 100 th realizations were selected for each variable in the SGS and SIS and five “indicator kriging” maps were chosen for the thicknesses cutoffs.

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 BasinThe 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.

Crustal structure at the contact of the Dinarides and Pannonian basin based on 2-D seismic and gravity interpretation of the Alp07 profile in the ALP 2002 experiment

SUMMARY The area of this study includes the contact between the Dinarides and the Pannonian basin, as a relation between the Adriatic microplate and Pannonian segment. Our analysis was carried out on profile Alp07, which is a part of the ALP 2002 experiment. ALP 2002 was large international seismic experiment that focused on the lithospheric structure of the Eastern Alps and surrounding areas. Profile Alp07 is one of several refraction and wide-angle reflection profiles located in the transition from the Adriatic microplate, through the Dinarides, and into the Pannonian basin. This 300-km-long profile extends from Istra, Croatia to the Drava River at Hungarian–Croatian border in a WSW–ENE direction. It is oriented approximately perpendicular to the Dinarides, the main faults in the Adriatic region, and the contact between the Dinarides and Pannonian basin. 2-D seismic modelling was done using tomographic inversion and ray tracing techniques. The Moho depth is the greatest in the area of the Dinarides, reaching about 40 km and is shallowest (30–20 km) in the Pannonian basin area. At the boundary between these provinces, the depth changes rather suddenly. In order to obtain additional constraints on the crustal structure, 2-D gravity modelling was also performed. The layer boundaries were retained from the seismic model as there was no need to change them during modelling, since varying densities in the model produced a good fit to the data. A geological model was constructed based on both geophysical models. Three types of the crust were found along profile: the Dinaridic and the Pannonian crusts that are separated by a relatively narrow transition zone. The Dinaridic upper crust is characterized by low seismic velocities and densities, but its lower crust has high velocities and densities. The Pannonian crust can be seen as unique layer characterized by both low seismic velocities and densities. Large lateral and vertical changes in densities and seismic velocities can be found in the transition zone. Troughs in the seismic model at the level of the Mohorovicic discontinuity are interpreted as major faults in the lithosphere. Three main lithospheric faults were identified: in northeastern part of the Dinarides under the Sava depression and under the Drava depression. The first one may be considered as a result of subduction of the Adriatic microplate under the Pannonian segment. Similar movements are also defined within the transition zone, where the Pannonian segment is gradually rising over on the Adriatic microplate.

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.

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.

Selection of the most appropriate interpolation method for sandstone reservoirs in the Kloštar oil and gas field

The distribution of porosity data collected at the Kloštar field is analysed. This field is located in the north-western part of the Sava depression, and represents one of the most long-standing fields with respect to oil production in Croatia, where production has been ongoing for more than 50 years. Several reservoirs of very diverse lithology and stratigraphic age have been discovered. The oldest reservoirs (with minor oil reserves) occur in basement rocks of the Tertiary system (informally named the „temeljno gorje";;), then in Badenian coarse-grained clastics, as well as sandstones of Pannonian and Pontian ages. Major reservoirs, with respect to size, number and volume, are located in sediments of the Upper Pannonian and Lower Pontian. These are medium-grained sandstones, mostly quartz-mica grains, mutually separated by marls. Reservoirs of Lower Pontian age occur within whole structures (anticlines) at Kloštar, while the Upper Pannonian has been developed only in the south-west part. Earlier studies on the reservoirs have included only several measured values of reservoir porosity. These values have not been mapped, but only a unique average value, characteristic for the whole reservoir was calculated. Reinterpretation of e-logs increased the number of input values to 20 points of data. Rules are determined for the distribution of porosities in the largest reservoir of the Lower Pontian named ";;T";;. These regularities can be translated to the other sandstone reservoirs. Several interpolation methods were selected (inverse distance, nearest neighbourhood and moving average), with special emphasis on kriging. The interpolated maps are different. Based on a comparison of isoporosity, lines, shapes, and cross-validation results, kriging was evaluated as the most appropriate method for porosity interpolation.

Facies and Facies Architecture of the Ivanić Grad Formation (Upper Pannonian) - Sava Depression, NW Croatia

Four facies were distinguished within the Ivanić Grad Formation of the Sava Depression: massive marl facies (F1), thick-bedded to massive sandstone facies (F2), thin-bedded sandstone facies (F3) and thin bedded sandstone, siltite and marl facies (F4). Interpretation of the depositional mechanisms confirmed the presence of two basic sedimentary processes. Hemipelagic deposits are represented by fine grained detritus. The lithification of these produced a massive marl(facies - F1). Sand detritus was transported into the depression by turbidite currents (facies - F2- F4), and formed a narrow elongated sedimentary body. By comparing the facies defined on the basis of core samples, the spontaneous-potential curve (SP) and the resistivity curve (Ra) of identical intervals, four facies associations were defined on well logdiagrams: channel filling (FA), depositional lobe (FB), lateral and distal turbidites (FC) and massive marls (FD).

Chronohorizons Based on Resistivity Curve Variations - Upper Miocene Sediments of the Ivanić Grad Formation in the Sava Depression (NW Croatia)

Regional definition of chronohorizons represents a fundamental andmost difficult problem in geology. Adequate precision of dating canonly be obtained from biological evolution, radioactive decay andfrom the key surfaces of the sequence stratigraphic concept (MFS,TSE, and LSE). It is concluded in this paper that the variation of formation resistivity values (Ra curve), as measured in wells located inthe Croatian part of the Pannonian Basin System, is also a criterion for the stratigraphic definition of concurrently deposited sediments. This enables the construction of a sequence of lithogenetic units related to very small time-intervals. The morphology of the resistivity curve measured in well sections composed of massive marls (that were deposited in a quiet environment of moderate water depth), is regionally exceptionally well defined. This pattern consists of a succession of peaks of lower and higher resistivity values that are in the same relative position regionally, which results in similar form of the resistivity curves of the wells in the study area. On the curve, one peak of higher values together with the next (overlying) peak of lower ones composes a single cycle(SC). Marls of a single cycle were regionally deposited at the same time, thus enabling the points of both the maximal and minimal resistivity values of each SC to be considered as time-markers (chronohorizons). Variation of the resistivity reflects fine changes of petrographicand granulometric composition which can be associated with cyclic water-level fluctuations (Milankovitch cycles). For future exploration of the remaining, potential hydrocarbon accumulations in the sandstone reservoirs of the Ivanić Grad Formationin the Sava Depression, spatial modelling of the distribution, geometry, facies and palaeocurrent structure of separated depositional episodes and genetic units is suggested. This should be done with special emphasis on the spatial definition of channel sandstones thatare the most important hydrocarbon reservoirs (according to current interpretations).