## 4 Results and Discussion
### Interpretation of the Fracture Network Model  
Three zones could be distinguished based on the fracture density and hydrogeological characteristics of the well. The first lies between 0 and 345 m, the second zone between 345 and 400 m and the third one between 400 and 845 m (Figure 4). Several factors can influence the fracture density of a rock body; the most important are the rock type, the grain size, the prevailing stress field and the tectonic events. In the BAF-3 well, the main factor affecting the properties of the fracture network is lithology; since the well penetrated two different rock types, sandstone and claystone, and the two formations have identical structural evolution. The first zone is the upper part of the well between 0 and 345 m, where sandstone is the predominant rock type. The fracture density based on both measured parameters is significantly higher in this zone. The coinciding hydrogeological properties are better (higher transmissivity and effective porosity) than in the other parts of the well. Although this sandstone body is well cemented, it undeniably has some intergranular porosity, and the pores may interact hydrodynamically and so may play a role in the measured transmissivity. Therefore, the fractured porosity calculated using the DFN model and the measured transmissivity data are upper estimates that also take into account the effects of the intergranular porosity of the sandstone. The fracture density data confirm the previous observation that the sandstone more easily and intensively deforms in a brittle way during the same tectonic events than the claystone does, which has self-sealing properties due to its high clay content [33]. From the hydrodynamic perspective, this upper 300 m interval of the well forms a single HFU more than 300 m thick; fluid flow controlling properties are internally uniform in this zone. Within this HFU, a single zone at \(\sim\)200 m has similar FZI values to those computed for the BCF. This behaviour could indicate that the KSF sandstone is becoming finer grained, increasing the clay content in this zone, or could indicate more intensely cemented zones. Below 343 m, two zones could be distinguished within the BCF based on fracture density and hydrogeological characteristics of the fracture system. The second zone of the well is formed at the boundary between the two formations. This transition zone is located between 343 and 400 m in the claystone body, where both the fracture density and the hydrogeological properties of the BCF differ from the deeper parts. The fracture density in this zone is the highest in the entire claystone body, associated with higher aperture  
Figure 7: **(A)** Poro-perm trends of the BAF–3 well based on the measured permeability and computed effective porosity data. **(B)** The hydraulic flow units of the BAF–3 well are indicated by the flow zone indicator (FZI). Different colours indicate different FZI trends, dashed black lines denote the borders of the HFUs. coefficient and effective porosity values. This zone forms a distinct HFU, which means that the rheological and geological properties controlling fluid flow are different from the other parts of the well. The top few dozens of metres of the BCF differ in mineralogical composition from the typical BCF. No authigenic albite is characteristic, and the proportion of K-feldspar also decreases, whereas the typical clay mineral is illite. This different mineral composition probably could lead to a significant change in the rheological properties of the rock body and could cause unique fracture geometry features. The third unit is below the transition zone (400 m), where the fracture density decreases significantly. The lowest values of fracture density of the whole well were recorded at 450 m. Further down, the fracture density increases continuously, but the aperture coefficient and effective porosity do not change to the same extent.