## 5 Discussion
One part of the measured samples originates from the fractured part of the formation [60], so the measured higher permeability values do not deliver information about matrix permeability. Figure 14: Porosity-depth trends of the main Late Miocene and Pliocene lithostrattigraphic units to describe the mechanical compactions of different sediments: (**a**) proposed trends that rely on well log-based and measured porosity data; (**b**) [PERSON]’s compaction trends for marls, siltstones, and sandstones. Two conceptual groups have been formed relating to the overpressure generation mechanisms in case of the Pannonian Basin. The first group explains the abnormal pressure conditions with the combined result of the hydrocarbon generation and the disequilibrium compaction [10; 92]. The hydrocarbon generation has been identified as the main mechanism, because evaluation of the pore pressure-temperature plots suggest that the overpressure is present above the 120-125 \({}^{\circ}\)C temperature in case of the Bekes Basin. This interval is the lower temperature of the gas generation window, so [PERSON] et al. [10] have identified this mechanism. According to [PERSON] [92], the under-compaction has participated in the forming of overpressure, while the minimal depth of the abnormal pressure has been identified at 2000 m in the Bekes Basin. Chemical compaction has been mentioned, but the applied dataset was not enough to prove the contribution of the mechanism [10]. Evaluated well log data and the numerical simulations suggest that the under-compaction is the main overpressure generation mechanism, which partly fits with the first explanation group. Basin models suggest that the hydrocarbon generation and the cracking can generate local 0.5 MPa overpressure, which has only a local impact on the subsurface pressure regime. Source rocks were identified in the Totkomlos Member and the Middle Miocene series in the Mako and the uppermost part of Hajduszoboszlo Formation (Figure 3). Pliocene biogenic hydrocarbon systems have not been simulated, because overpressured Pliocene sediments are unknown in the Hungarian Great Plain. Moreover, the 120-125 \({}^{\circ}\)C temperature interval is around 2000-2500 m depth, considering the average geothermic gradient of the Pannonian Basin [93]. The evaluated V\({}_{\text{P}}\)-depth plots and the log signatures have proved that the disequilibrium compaction is present below this depth. Studies from analogue basins, where Tertiary programg shale-rich delta systems controlled the sedimentation, have proved that the hydrocarbon generation was not able to affect the subsurface pressure regime [94]. According to the study from the Bohaiwan Basin (China), the main overpressure generation mechanism is the under-compaction, but the hydrocarbon generation had a minor role, too [95]. Three main parameters have been published from the Williston Basin (Bakken Shale) that controls the abnormal pressure generation in case of hydrocarbon generation and cracking [96], which are the regional distribution, the thickness, and the TOC. Source rocks of the Pannonian Basin meet the regional distribution and the thickness requirements, but the TOC values are generally one order of magnitude lower compared with the Bakken Shale [88; 89; 90]. The quality deficiency of the source rocks within the Pannonian Basin could be the bottleneck from the overpressure generation point of view during the source rock maturation, whose conclusion is supported by the numerical simulations, too. Chemical compaction has been considered, but the simulations have not suggested more than 3% overpressure contribution from this mechanism. The second explanation group is assuming the tectonic compression as the main overpressure generation mechanism [11; 12], which relies on the highest relative overpressure being present above the basement highs. However, the disequilibrium compaction should be present in the deepest subbasins [12].