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Previously drilled boreholes of a host rock for a potential nuclear waste repository in Hungary revealed a highly fractured claystone rock body. A crucial step for characterizing the hydrodynamic behavior of such a fractured reservoir is fracture identification and accurate calculation of the fracture density. Although acoustic borehole televiewers provide a reliable base for determining the fracture density, older boreholes usually lack such data. However, conventional borehole geophysical measurements are often accessible in such cases. The aim of this study was to identify any correlations between well log data and fracture density. Multiple linear regression analysis was performed on data from two boreholes penetrating the Boda Claystone Formation in southwest Hungary. The upper section of the BAF-4 borehole was used for training, where the fracture density was estimated with a fit of R2 = 0.767. The computed regression function predicted the fracture density with high accuracy in both boreholes for all intervals with typical lithological features. However, in some sections where anomalous well log data indicated changes in the lithology, the prediction accuracy decreased. For example, the function underestimated the fracture density in sandy intervals.

Quartz is among the most common minerals in the Earth’s crust and is stable within a wide range of temperature and pressure conditions. As its microstructure is sensitive to different deformation mechanisms, quartz may present information about the structural evolution of many different rock types. The Szentlőrinc-1 well with its drill cuttings brought to the surface from approximately 2 km depth provides an exclusive chance to investigate the shear zone beneath. The drill cutting collection includes only a few small rock grains, and more than 80 % of the material consists of tiny (

The presence of anomalous terrestrial radioisotopes must be suspected in black shale-hosted manganese ore deposits, based on high organic matter content, which is useful tool in genetic, among them paleoenvironmental investigations. Our work aims at the characterization of the paleoredox conditions of the Urkut Manganese Formation based on comparison of terrestrial radioisotopes and selected element ratios. Paleoredox indicators were estimated on U/Th, δU, Ni/Co, V/Cr, V/(V + Ni) and Ce*. The results of paleoredox indicator element ratios show, that these methods must be used with caution (microbial selective element enrichments, mobilizations), and the complex interpretation using mineralogy and microtexture can be recommended.

In the present approach, clast geometric parameters—particle size distribution (PSD), clast complexity $$(D^{R})$$ , aspect ratio (AR), circularity (Circ), convexity (Conv) and clast orientation (Angle)—were examined in order to separate the diverse fault rock types (fault breccias, cataclasites and fault gouges), which typically own extremely different hydraulic and petrophysical properties. The available borecore samples were studied in a case of a thrust fault zone from the metamorphic basement of the Pannonian Basin, Hungary. Multivariate statistical methods were applied in order to find the geometric parameters that define the tectonites. The calculated discriminant functions emphasized the importance of the PSD, Angle, AR, and Circ parameters, in descending order. We defined proper combinations of geometric parameters which can clearly separate the fault rock groups, both pair-wise and jointly for the three groups. The discriminant functions also pointed out the relatively similar geometric features of fault breccias and cataclasites in contrast to the significantly different characteristics of fault gouges. The multidimensional scaling demonstrated a three-phase evolution of the analysed brittle tectonites, where the initial deformation is coupled with chaotic fabric giving a weakly disaggregated fault breccia texture. The transitional stage can be characterised by cataclastic flow, while in the most deformed fault gouge samples the strong fragmentation, clast-rounding and oriented texture dominate. These results possible provide constraints on the development and overall behaviour of the fault zone.

There are four main rock types along the ideal rock column of the Kiskunhalas-NE field; in order from the bottom upwards, orthogneiss, orthogneiss mylonite, graphitic gneiss mylonite and graphitic carbonate phyllite. These main rock types are characterized by significantly different reservoir features, as was proved by previous rock mechanical investigation. The geophysical information of well-logs (gamma, resistivity, neutron, density and acoustic logs) allows an understanding of the spatial extension of the good reservoir blocks. In the course of the examination conventionally applied plots, MN plots and discriminant function analysis were used. Three rock types were successfully identified along the wells, the graphitic carbonate phyllite, the mylonite and the orthogneiss. On the basis of the results, lithological boundaries could be estimated in numerous wells. These boundaries were presented along geological sections. Taking also the independent hydraulic regimes of the reservoir into account, a series of south-dipping normal fault-bounded blocks are assumed. Inside each block shallow-dipping mylonite/gneiss boundaries with a north-northeast dip direction are typical. The existence of this low-angled (less than 5°) mylonitized zone refers to a presence of a one-time detachment fault linked to the formation of a metamorphic core complex.

Modeling fractured rocks with numerical methods requires some derived parameters, among which the fracture network connectivity and the size of the representative elementary volume (REV) are both of crucial importance. Percolation and REV analyses were made by the RepSim code. The program uses input parameters such as fractal dimension of the fracture midpoints (Dc), length exponent (E) and relative dip (αr) data. For percolation analysis, the relative sizes of the largest percolation clusters have been calculated by stochastic realizations of the simulated fracture networks with different parameter triplets. Furthermore, fracture networks can be classified into three major types on the basis of their (E,Dc,αr) parameters. For the REV calculations, the porosity of the generated fracture network was calculated. The derived REV size of a fracture network depends essentially on input parameters and shows a decreasing tendency with increasing D and E and vice versa. The method mentioned above was tested on both metamorphic samples of the Pannonian Basin and Variscan granitoid rocks of the Moragy Complex. Percolation values predicted for the Moragy granite are highly sensitive to alterations in the input parameters. The amphibolite bodies displayed a modeled fracture network with 80 to 90% of all fractures being interconnected, while the largest achievable percolation cluster size of gneiss is less than 10%. The REV size of the amphibolite is about 20 m as a result of connected fractures filling the whole body, while gneiss has lower porosity and higher REV (approximately 70 m).

In the Villany Mts of southern Hungary, ocelli-bearing porphyritic lamprophyre dykes and sills of Upper Cretaceous age occur sporadically, intruding Mesozoic carbonate rocks. They at places contain metasomatised mantle xenoliths and quartz xenocrysts of crustal origin. They are moderately fractionated with significant LILE and LREE enrichments and a notable Nb–Ta negative anomaly. Trace elements indicate that they formed in an intraplate environment by very low degree partial melting of a metasomatised garnet lherzolite mantle source that was enriched by earlier subduction. Based on petrography, geochemistry and age constraints, they differ from other Mesozoic basic rocks of the Tisza block (Mecsek Mts and Slavonian basalts); however, they show a significant geochemical similarity to the Upper Cretaceous lamprophyre dyke swarm from NE Transdanubia (northwestern Hungary) situated on the Alcapa microplate. Thus we suggest that lamprophyres from the Villany Mts and NE Transdanubia could have originated from the same or similar enriched asthenospheric mantle sources.