## 5 Discussion  
### Characterisation of the Fracture Network  
According to the well-log interpretation, the total interval of the metamorphic basement penetrated by the A-1 well is lithologically homogeneous. Petrographic data confirm that the well crosscut is exclusively amphibite and amphibole gneiss, which are typical rock types of the AG block. Although the presence of the SG block can be assumed below this block [16; 19], the shear zone that separates the two lithological realms throughout the study area is not reached by the well. Nevertheless, the well-log interpretation indicates three depth zones, with the host rock being intensively deformed at each zone. Two of these (\(\sim\)2052 and \(\sim\)2075 m) were also dominated by brecciated amphibolite (Figure 8). The parameter weights in the discriminant function _F_(_1-2_) define the main petrophysical differences between the undeformed host rock and the tectonsised zones. The most significant factor in the separation is the high natural gamma-ray value, which indicates an elevation in the amount of clay minerals caused by the intensive weathering and alteration of the host rock along the fault zone. At the same time, the relatively low electrical resistivity of the tectonics reflects considerable fluid infiltration into these zones. The lower densities probably relate to the higher frequency of fractures in the tectonised zones, coupled with porosity enhancement. According to the function \(F\)3, higher levels of deformation are manifested inside the brecciated layers due to an increase in the compensated neutron porosity values and a further decrease in electrical resistivity. In the function \(F\)4, higher density and lower gamma-ray values are characteristic. These petrophysical features indicate that the intensely deformed fault cores (cataclasites and fault gouges) can be characterised by low densities and resistivities, along with elevated natural gamma activity and compensated neutron porosity values. This is consistent with the observations of both [47] and [48]. In case of well A-1, the applications of \(F\)3 and \(F\)4 functions indicate that the two thick deformed intervals at \(\sim\)2052 and 2075 m depths are mainly composed of coarse fault breccias. Furthermore, based on the \(F\)3 and \(F\)4 discriminant functions (Figure 4b), these zones are characterised by higher density and resistivity values, along with lower gamma-ray and neutron-porosity scores within the fault rocks. These values suggest that weaker disaggregated zones can maintain a significantly higher porosity than commanded fault cores. Previous studies on the structure of fault zones [49; 50] have clarified that similar fault breccia layers are typically related to a weakly disaggregated, densely fractured \"damage zone\", which is generally associated with higher conductivity and permeability relative to the undeformed protolith or the clay-rich fault core. The thin deformed layer between 2062 and 2065 m has lower _F_(_1-2_) values, indicating that compared to the brecciated zones at \(\sim\)2055 and 2075 m, this zone is affected by weaker deformation not intense enough to trigger fault breccia formation. Nevertheless, this process is presumably so pervasive that it forms an interconnected fracture network, also able to act as a migration pathway. The brittle deformed character of all these zones is also demonstrated by the core material. The zone at a depth of \(\sim\)2075 m is a thick (\(\sim\)5 m) fault breccia zone, while the layer at \(\sim\)2065 m is not brecciated but falls into numerous thin layers separated by an intensively fractured host rock. The behaviour of the zone at \(\sim\)2052 m is more contradictory. Based on the well-log evaluation, the zone appears as a brecciated layer, while the cores indicate an intensively fracturedhost rock. A comparison of this well-log pattern (Figure 8) with those identified in the neighbouring wells (not presented) reveals a similar appearance in all wells North of A-1.