## 4 Results
at the deepest part of the subbasins. However, the hydrostatic pressure regime is dominant in the delta slope sediments above the basement highs and the margin of the basin system. Generation of the overpressure in the delta slope sediments is controlled by the burial, because undercompaction could trigger the forming of overpressure, where the slope sediments could reach the 2100-2400 m depth. The prodelta turbidite system has been divided into two lithofacies, a shaly siltstone and a silty sandstone. Compaction properties and the porosity-permeability relationships are significantly differing in case of the two identified facies. The shaly siltstone lithofacies could be able to cause an overpressure increase in the formation due to its low permeability. However, the permeable layers could lead away the pressure surplus due to the lateral transfer. The generated overpressure in the deepest part of the subbasins can disintegrate into the direction of the margins within the sandstone bodies. The generation mechanism has a dual role in the subsurface pressure regime; it can generate overpressure above the basement highs and at the margin of the subbasins, but it can reduce the overpressure of the deepest parts of the subbasins. The effectivity of lateral transfer depends on the reservoir parameters, the connectivity, and the horizontal distribution of the sandstone bodies [8]. According to the numerical simulations, the magnitude of the overpressure increases in the pelagic clay marl, marl, and calcareous marl series (Endrod Fm., Totkomlos Mmb. ), which is controlled by the fast burial and the isolation of pores. The pore fluid is not able to leave the isolated pores due to the limited permeability that causes the continuous increases of overpressure due to the dis-equilibrium compaction. The fluid expansion, the unloading, the chemical compaction, and the hydrocarbon generation have been taken into consideration during the numerical simulations. According to the basin modeling and the well log evaluation, the fluid expansion mechanisms and the unloading contribution to the overpressure are negligible in the study area. Chemical compaction has been simulated in case of Middle Miocene shales and marls and the Late Miocene pelagic marks, calcareous marls [59]. Simulations suggest that the chemical compaction could generate some overpressure at the middle of subbasins, but the absolute overpressure surplus is less than 1.8 MPa or 3%. Source rocks have been identified in the Middle Miocene, Late Miocene pelagic series [87] that have 2 wt % total organic matter (TOC) and 300 mg/g hydrogen index (HI) based on published data [88; 89; 90]. Numerical simulation has not proved the contribution of the hydrocarbon generation and cracking in the overpressure generation, because the models suggest only 0.5 MPa locally extra overpressure. Figure 11: Development of the subsurface pore pressure system during the last 8.3 Ma. According to the simulation, the most significant pressure anomaly could form in the deepest part of the subbasins. The heterogeneous Middle Miocene series generally starts with breccias and conglomerates that are overlaid by sandstones or limestones, depending on the depositional environment. The pelagic shales and marks have deposited only in the deepest parts of the paleo-basins or the deepest point of the subbasins, which were overprinted by a Quaternary uplift. The lateral transfer is the main overpressure generation mechanism that controls the pressure system within the Middle Miocene series, because the basal breccias and conglomerates have regional distribution. The generated overpressure in the subbasin centres are scattered in the direction of basement highs. This scattering mechanism generates vertical pore pressure jumps, when the Middle Miocene series are penetrated at the basin margins. According to the basin models, the generated extra overpressure from the lateral transfer could be 4 MPa.