Review of geothermal conditions of Hungary

Abstract

The heat flow map of Hungary was presented in the Atlas of Geothermal Resources in Europe in 2002 and was last updated in 2005. Since that time several geothermal projects, e.g. TransEnergy (2010-13), assessment of the geothermal potential of the Dráva Basin (2013), Paks-II NPP (2016) and continuous drilling activity in the country have been in progress. Large amount of temperature data became available, which allowed the update of the Geothermal Database of Hungary and the compilation of an updated heat flow map and temperature maps. The heat flow is determined based on the Fourier law using the thermal conductivities of rocks and temperature gradient calculated from temperature observations in boreholes and wells. The thermal conductivity is known from laboratory measurements made on core samples. The thermal conductivities and the temperature data are stored in the Geothermal Database of Hungary. The heat flow is calculated in 2001 boreholes and wells deeper than 1 km using the Bullard-plot technique. The mean heat flow in Hungary is 90 mW/m2, varying between 30 mW/m2 and 120 mW/m2. The high heat flow is due to the Middle Miocene extension of the lithosphere. The high values are found over buried basement highs in the eastern and southern part of the country, while the low values are located in the recharge areas of karstic flow systems. In the sedimentary basins, where the thickness of the Neogene and Quaternary sediments reaches 5-7 km, the heat flow is slightly below the mean value (80-90 mW/m2). It is mainly due to the cooling effect of sedimentation and lesser degree to the heat refraction caused by the low thermal conductivity of sediments. Temperature maps at 500 m, 1 km, 2 km and 3 km depths were also compiled. The mean temperature in these depths is 40 °C, 60 °C, 100 °C and 140 °C, respectively. The decrease of geothermal gradient is due to the increase of thermal conductivity with depth. Similarly to the heat flow, the temperature anomalies strongly reflect the local and regional groundwater flow systems. Groundwater flow in the sedimentary basins mainly influences the heat flow in the upper 1 km, because porous-permeable sediments occur until 2 km depth. Groundwater flow in the karstic basement influences the temperature until the basement depth, that is until 3-4 km depth.