Bruno Saftić

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.

The Neogene evolution and hydrocarbon potential of the Pannonian Basin in Croatia

Sedimentary basin fill of the Central Paratethys in Croatia consists of variable deposits. The oldest are the Egerian and Eggenburgian paralic and marine sediments that were determined only in the western marginal part of the studied area. The Neogene sedimentary sequence in major subsidences - the Sava and Drava Basins starts with Ottnangian-Karpatian transitional deposits that are covered by Badenian marine and Pannonian-Pontian brackish to freshwater formations. Pliocene and Quaternary terrestrial deposits cover major parts of the former depositional basins. Three stages of structural development of the SW Pannonian Basin are differentiated: initial structural changes between Oligocene and Early Miocene, main extensional processes that prevailed in Early and Middle Miocene and transpression during Pliocene and Quaternary. The newly formed structures are essential for hydrocarbon accumulation. The main source rocks are Lower to Middle Miocene marly limestones and limy marls. Hydrocarbon accumulations are found in the uplifted and fractured parts of the Neogene basement and in the Lower to Middle Miocene coarse-grained clastics, but the most important reservoir rocks are the Upper Miocene sandstones.

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.

Geological Storage Options in Peri-Mediterranean Countries - Plans for Case Studies

Regional estimates of geological storage capacity included integration of knowledge on the subsurface with a unified way of calculating the capacity in HC fields, regional saline aquifers (basin-wide sedimentary bodies saturated with brine) and coal beds. One way to calculate the effective storage capacity was agreed upon as an attempt to accommodate for highly differentiated geological conditions and assumptions that had to be taken due to scarce data. In the main sedimentary basins of Spain there is a high potential for CO2 storage and another opportunity is seen in ECBM development in the NW coal basins. In Italian sedimentary basins with different geological composition country-wide estimates still have to be made. However, depleted oil fields offer an opportunity to plan pilot injections. In Slovenia, potential storage locations in aquifers have been found, some with significant estimated storage potential but with very few reliable subsurface data. Potential storage in unmineable coal layers deserves therefore additional attention. As for Croatia, a history of O&G industry and resulting data, combined with relatively favourable natural conditions enabled estimates of significant capacity in the SW part of the Pannonian basin, and likely also in the Northern Adriatic off-shore. An example is given from each country.