## 2 Geological Setting  
In the Szeghalom area, the basement of the Pannonian Basin comprises Variscan metamorphic rocks (Figure 1; [16]). A detailed petrological analysis has revealed that three blocks with varying metamorphic evolutions can be distinguished, which are separated by wide (\(\sim\)10 to 20 m) post-metamorphic structural boundaries [19]. Through evaluations of seismic profiles, a complex system of low- and high-angle fault zones has become evident [20]. The deepest structural level includes a block dominated by homogeneous orthogneiss (OG; [21]), above which lies a polymetamorphic garnet- and sillimanite-bearing paraphesis block (SG block). The topmost block is dominated by amphibolite and amphibole gneiss (AG; Figure 2). [PERSON] et al. [19] evaluated composite well-logs in the area and distinguished the above lithologies from the rocks of the tectonised major fault zones. Through further refinement of the well-log evaluation algorithm, the most essential tectonic rock types, such as cataclasite, fault breccia and fault gauge, can be classified. Based on an interpretation of the well-logs from numerous distinct wellbores, the spatial extension of the fault zones between the SG and AG blocks can be determined to be at a low angle (8\({}^{\circ}\)-13\({}^{\circ}\)) on top of the N-NW thrust sheets between them cut by younger normal faults, which is in harmony with seismic interpretations (Figure 2). The structural boundary between the OG and SG blocks exhibits similar characteristics, but its presence is less evident.  
The fracture intensity and geometric parameters of the fracture network substantially differ for the three lithological realms. By far, the most intense brittle deformation is typical in the topmost AG block. Previous models have suggested a communicating fracture system for the massive amphibite bodies, while the fracture networks of both the schistose SG and gneissic OG blocks usually appear to be disconnected [17].  
Nevertheless, the vein-filling minerals are identical throughout the area, independent of the host rock, and are characterised by the following sequence: pyrite \(\rightarrow\) chlorite/kaolinite \(\rightarrow\) calcite1 \(\rightarrow\) quartz \(\rightarrow\) calcite2 \(\rightarrow\) laumontite (Figure 3). [PERSON] et al. [22] discovered that the first five phases precipitated during a period of continuous cooling in which the basement uplifted to the surface. This was later confirmed by [PERSON] et al. [23], who described plant fragments and pollen inclusions in the calcite2 phase, representing a Miocene terrestrial flora. [PERSON] et al. [18] also analysed hydrocarbon-bearing fluid inclusions in vein-filling quartz crystals and determined that the formation temperatures ranged between 120 \({}^{\circ}\)C and 135 \({}^{\circ}\)C (Figure 3). Moreover, they found oil inclusions with various compositions in different segments of the reservoir, indicating its temporally and/or spatially compartmentalised  
Figure 1: Sketch of the study area. **(a)** Position of the Pannonian Basin in Europe. **(b)** Subsurface topographic map of the SE part of the basement of the Pannonian Basin. Isolines denote depth (in m) below the present surface (m). **(c)** Topographic map of the Szeghalom metamorphic dome. Dots represent wells reaching the basement of the Pannonian Basin.  
Figure 2: N–S geological section across the highest point of the Szeghalom metamorphic dome (modified from [19]).  
behaviour when these ancient hydrocarbons migrated into the fracture system. The final phase in the veins was laumontite, which is assumed to have formed under recent reservoir conditions [22].  
The well that was studied in this project (A-1) was drilled during the 1980s in the northern part of the area (Figure 1c) and penetrated the metamorphic basement with \(\sim\)100% core recovery rate between 2050 and 2080 m below the present surface. The rock material of the total bore core, as well as the above set of well-logs, is available for complex evaluation.