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.

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

Representative elementary volume (REV) of cementitious materials from three-dimensional pore structure analysis

Statistically well-documented sequences of fossil species are rare, as has been observed, for example, by Eldredge and Gould (1972), Boucot (1978) and Dodson and Hallam (1977). Rapid advances in theoretical paleontology made over the last few years have, however, made it imperative that the many new and exciting theories we now have at our disposal be tested against real data (Maynard Smith, 1981). Although suitable data should be available from several sources, it seems that the most likely area for providing good study material is that of micropaleontology. The present paper summarizes the results of mainly genetically oriented statistical analyses of borehole samples of the Cretaceous bolivinid foraminifer Afrobolivina afra Reyment, a species which has been well studied in several connections (Reyment, 1959, 1966, 1980; Castelain et al., 1962; Campbell and Reyment, 1978; Campbell, 1979; Reyment and Reyment, 1979; Campbell and Reyment, 1980). Although these contributions are of significance for the present account, the results given here are new. The quantitative-genetic analysis of the present data owes much to the research of Lande (1976, 1979, 1980b), who has demonstrated how the principles of Falconer (1960) can, in some cases, be successfully adapted for models framed in terms of the evolution of the average phenotype in a population by natural selection and genetic drift. The intent of the present paper is to attempt to apply quantitative-genetic reasoning to the interpretation of morphologic changes in an evolutionary series and to search for eventual interactions between the average phenotype of the organism and the paleoenvironment. It is shown that phenotypic variation in characters of the foraminiferal test is closely associated with fluctuations in the paleoenvironment, as evidenced in the properties of the fossil substrate. These environmental variations seem to have been caused by epicontinental transgressions and regressions. The analysis points to regionally valid weak selective effects as having brought about the morphological changes in the foraminifer, the influence of which seems to have been heightened by the ecophenotypic reaction of the test to the environment; random genetic drift does not appear to have been of first-order importance. Variation in the size of the proloculus of megalospheric individuals is shown to be connected with population density and certain chemical elements. Obviously, quantitative-genetic principles can only be applied to fossil material in a tentative manner and it is important that the limitations of this approach be recognized. The present study should therefore be seen as one possible interpretation of the observed sequence of morphologic changes, and not the only one.

Phenotypic evolution in a cretaceous foraminifer.

The purpose of this paper is to present an innovative methodology for 3-D soil stratification based on geoelectrical prospection. This methodology is useful in optimal substation grounding system designs by providing an accurate 3-D soil resistivity model. In the present methodology, the area being studied is subdivided into squared subareas and a recently developed geoelectrical prospection method based on the Wenner's method and on a genetic algorithm is applied to each subarea's edge. For each subarea, the soil stratification result consists of the number of layers and of its electrical resistivities and thicknesses. A global 3-D soil stratification solution is then obtained from these data. Results obtained with this new method are presented and discussed.

3-D Soil Stratification Methodology for Geoelectrical Prospection

The complex interplay between depositional facies and diagenesis in dolostones presents numerous challenges for calculating petrophysical properties from wireline logs. Complex pore geometries and mineralogies control rock petrophysical properties, and equations for calculating porosities and saturations must therefore be tailored to specific pore geometry-mineralogy combinations. The complex mineralogy of some dolostone reservoirs, moreover, has profound effects on wireline log measurements. If dolostone reservoirs are divided into petrophysical-mineralogical facies of similar depositional and diagenetic textures and, thus, similar pore geometries and mineralogy, empirical equations that apply specifically to that geologically identified petrophysical-mineralogical facies can be developed so that porosity and water saturation can be calculated accurately. We present some examples from Permian shallow-water dolostone reservoirs of the Permian Basin, southwestern United States, that demonstrate analytical approaches for calculating petrophysical properties in these complex rock types. The four general petrophysical-mineralogical facies that characterize Permian shallow-water dolostone reservoirs are (1) subtidal, muddominated dolostone; (2) subtidal, grain-dominated dolostone; (3) dolomitic and siliciclastic peritidal rocks; and (4) diagenetically altered, subtidal dolostone.

W.H. Fertl

Papers

Geologic well log analysis