Regional underpressure: a factor of uncertainty in the geothermal exploration of deep carbonates, Gödöllő Region, Hungary

Hungary has a remarkable geothermal potential, and there are ongoing projects investigating the possibilities of thermal water utilization. The regional hydrodynamic assessment of one of the most promising geothermal sites of Hungary, the Gödöllő Hills Region, could provide valuable additional information to them. The aim of this research was not only to carry out a regional study but also to demonstrate the relevance of the hydrodynamic approach in the reconnaissance phase of exploration. Regional evaluation was carried out based on the created database of preproduction wells’ hydraulic, temperature, and geochemical data. Based on the dataset from the depth extending to −2300 m asl, it could be shown that the temperature field (up to 120 °C) and the reservoir conditions are favorable: structurally influenced Triassic Main Dolomite and Dachstein Limestone, Miocene, and Upper Pannonian formations, and fluid flow is basically topography induced. The recharge and discharge zones of the gravity-driven system, the basement elevation, and the temperature distribution show good correlation. The vertical connections between Pannonian and Eocene-Triassic basement reservoirs could be followed through the changes of pressure regimes and by the interaction of “fresh” and “basinal” fluids of the region. It has been revealed that, due to the differentiated uplifting of the region in the Pliocene and the Quaternary, the pore pressures in the aquitards and in the basement reservoir have been shifted toward subhydrostatic, indicating geologically transient pore pressures. The detected subhydrostatic pressure gradients cause downward vertical groundwater flow in the Hills characterized by restricted recharge across low permeability strata. Upward vertical flow appears from the basement carbonate reservoir toward the highest negative pressure increment values in the aquitards. The underpressure in the basement carbonate reservoir causes an uncertainty factor in the exploration of a potential geothermal site. The area was differentiated based on flow and pressure conditions for geothermal exploitation.