| Abstract |
Hungary is in a favorable geothermal position because of its geological conditions. The reason for this is the doming of the crust and the lithosphere, i.e. the tectosphere, located under the Pannonian basin. As a result, the heat flow density and geothermal gradient are higher than the world average. The Basin areas of the country are filled with Neogene sediments with low thermal conductivity. They insulate relatively well the base of the basin, which consists of higher thermal conductivity Precambrian-Paleozoic-Mesozoic rocks underlying them at depths, in some places, of 7000 m. At the boundary of the two formations the temperature increases sharply. The temperature is not the same in the same depth. The temperature and the geothermal gradient depend mainly on the depth of the tectosphere, the local heat flows, and the thermal conductivity of the rocks. It is a characteristic of the thermal conditions of the base of basin that the geothermal gradient is higher in the more outlying areas than under the deep basin. Where the rocks of the base of basin have appropriate productivity and permeability, it is possible to achieve wet steam production even from a relatively shallow depth. In such reservoirs the formation pressure can be more than twice the hydrostatic pressure. Such reservoirs were mainly formed in karstified carbonate rocks or brittle rocks by tectonic and hydraulic breccia formation. The breccia reservoirs are connected to the main tectonic feeder with wrench faults of the Pannonian Basin in the direction of the NE-SW strike or local reverse fault zones. Three such reservoirs have been identified by deep drilling but, on the basis of theoretical considerations, there can be several others. The deep wells which discovered these reservoirs are described below. On the basis of magnetotelluric and SiO2 temperature measurements, it appears that the high temperature fluids flow into the reservoirs from deeper formations than those explored by the boreholes. |