Karst aquifers are important sources of thermal and groundwater in many parts of the world, such as the Alpine–Dinaric–Carpathian region in Europe. The Upper Triassic dolomites are regionally recognized thermal and groundwater aquifers but also hydrocarbon reservoirs. They are characterized by predominantly fractured porosity, but the actual share of depositional and diagenetic porosity is rarely investigated. In this research, we presented the geometric characterization of the measured microporosity of the Upper Triassic dolomites of the Žumberak Mts (Croatia), through thin-section image processing and particle analysis techniques. Pore parameters were analyzed on microphotographs of impregnated thin sections in scale. A total of 2267 pores were isolated and analyzed. The following parameters were analyzed: pore area, pore perimeter, circularity, aspect ratio (AR), roundness, solidity, Feret AR, compactness, and fractal dimension. Furthermore, porosity was calculated based on the pore portion in each image. The effective porosity on rock samples was determined using saturation and buoyancy techniques as an accompanying research method. We analyzed distributions of each parameter, their correlation, and most of the parameters are characterized by an asymmetric or asymmetric normal distribution. Parameters that quantify pore irregularities have similar distributions, and their values indicate the high complexity of the pore geometry, which can significantly impact permeability.

Fractured aquifers, especially dolomites, are important hydrocarbon reservoirs and sources of thermal and groundwater in many parts of the world, especially in the Alpine-Dinaric-Carpathian region. The most dominant porosity type is fracture porosity, which acts as the preferential fluid pathway in the subsurface, thus strongly controlling fluid flow. Outcrops provide valuable information for the characterization of fracture networks. Dolomite rock properties and structural and diagenetic processes result in fractured systems that can be considered fractals. The fracture network was analyzed on 14 vertical outcrops in 35 digitized photographs. The values of the fractal dimensions varied slightly by the software and method used, but the trends were consistent, which confirms that all methods are valid. Small values of fractal dimension indicate the dominance of a few small or large fractures, and high values of fractal dimension result from a combination of large numbers of small fractures accompanied by a few large fractures. The mean value of the fractal dimension for analyzed fracture networks was 1.648. The results indicate that the fracture network of the Upper Triassic dolomites can be approximated by fractal distribution and can be considered a natural fractal, and values can be extrapolated to higher and lower scales (1D and 3D).

This study presents a structured evaluation of key factors influencing the design of shallow geothermal (SG) hybrid heating and cooling systems in the Danube Region. As part of the Danube GeoHeCo project, a criteria catalogue was developed using the Analytic Hierarchy Process (AHP) to support the creation of an IT decision support tool. Through expert-based pairwise comparisons and eigenvalue analysis, the relative weights of technical, geological, socioeconomic, and environmental criteria were derived. Results show that thermal demand, drilling costs, and hydrogeological site characteristics are the most influential parameters. The final ranked list of twenty sub-criteria provides a basis for prioritizing input data and optimizing early-stage system design. This framework supports more efficient project planning, targeted data collection, and the development of robust shallow geothermal hybrid solutions aligned with regional energy transition goals.

The paper is devoted to the derivation, by linearization, of simplified (fully homogenized) homogenized models of an immiscible incompressible two-phase flow in double porosity media in the case of thin fissures. In a simplified dual porosity model derived previously by the authors the matrix-fracture source term is approximated by a convolution type source term. This approach enables to exclude the cell problem, in form of the imbibition equation, from the global double porosity model. In this paper we propose a new linear version of the imbibition equation which leads to a new simplified dual porosity model. We also present numerical simulations which show that the matrix-fracture exchange term based on this new linearization procedure gives a better approximation of the exact one than the corresponding exchange term obtained earlier by the authors.

Gamma-ray parameters are widely used to screen organic-rich intervals, yet the influence of lithofacies heterogeneity on the reliability of gamma-ray-based TOC predictions remains incompletely evaluated. This study aims to (1) assess the relationships between gamma-ray parameters and TOC, (2) evaluate how lithofacies heterogeneity influences predictive reliability, and (3) establish practical screening criteria for identifying organic-rich intervals in two Middle Miocene marine successions from the Croatian part of the Pannonian Basin System. Outcrop gamma-ray spectrometry measurements (K, U, Th) and dose rate (DR) were paired with co-located laboratory TOC analyses (Voćin n = 45; Podsused n = 96). Pearson correlation, multiple and simple linear regression, logistic regression for TOC ≥ 1 wt.%, and threshold analysis were applied to evaluate relationships between gamma-ray parameters and organic matter enrichment. U and DR show moderate positive relationships with TOC in both sections, whereas K and Th are not statistically significant predictors. Regression and threshold analyses identify U as the primary predictor of TOC and show more stable and systematic predictive behaviour in the homogeneous Podsused succession than in the heterogeneous Voćin section. The results indicate that lithofacies heterogeneity primarily influences predictive stability rather than the existence of TOC–gamma-ray relationships. The study demonstrates that lithofacies heterogeneity is an important factor influencing the reliability of gamma-ray-based TOC screening and provides a framework for assessing the suitability of uranium and dose-rate proxies in different sedimentary settings.