Geofizikai Közlemények, XII, 1—2, 3—47

GEOPHYSICAL TRANSACTIONS 1983 Vol. 29. No. 1. pp. 3— 114 GEOTHERMAL CONDITIONS OF HUNGARY Péter DÖVÉNYI*, Ferenc HORVÁTH*, Pál LIEBE**, János GÁLFI**, Imre ERKI* A brief review is given of the history of geothermal investigations in Hungary followed by a summary of the methods of temperature measurement, their reliability and applicability. All available and valuable temperature data are qualified on the basis of conditions and probable accuracy of measurements and are listed in a temperature catalogue. A map of geoisotherms at 1 km depth on a scale of 1 : 1,000,000 and another at 2 km depth (scale 1 : 2,500,000) are constructed for Hungary and adjoining territories. In addition, average temperature vs. depth profiles are presented for different sub-units of the Pannonian basin. All Hungarian measurements are used to obtain the average thermal conductivities of Neo gene sedimentary rocks and of basement rocks. Hungarian heat flow determinations are reviewed and three new heat flow data are also presented. A method is proposed for estimating the heat flow value for boreholes where no actual thermal conductivity measurements were performed but the temperature conditions and the lithology are well known. The heat flow map of the Pannonian basin has been prepared by the use of measured and estimated heat flow data. The Pannonian basin is characterized by high temperature and heat flow values with signifi cant spatial variations. An estimate is made of the temperature disturbance caused by regional water circulation systems. The conclusion is reached that if the influence of water convection is corrected, high undisturbed (purely conductive) temperature and heat flow values are obtained for the Mesozoic carbonate complex of the Transdanubian Central Range. In other parts of the country —apart from local anomalies—water circulation does not influence significantly the conductive temperature field. Finally a review is given on the geodynamic interpretation of the Pannonian basin thermal anomaly and attention is called to the importance of knowledge of the paleogeothermal conditions. d: geothermics, heat flow, geothermal gradient, thermal conductivity, thermal waters, Hungary, Pannonian basin 1. Introduction It might well be useful to avoid any confusion by defining here what we mean by Pannonian basin. It is a fairly big Neogene intramontane depression surrounded by the Alps, Carpathians and Dinarides. It is however not a uniform depression because locally pre-Neogene basement rocks crop out (e.g. Trans danubian Central Range) which separate the Pannonian basin into sub-units. The largest part of the Pannonian basin belongs to Hungary and the peripheral parts lie in the surrounding countries (Austria, Czechoslovakia, USSR, Roma nia and Yugoslavia). Geothermal research in the Pannonian basin and the utilization of geother mal energy have been well in the forefront of scientific interest for more than * Eötvös Loránd University, Department of Geophysics, H-1083 Budapest, Kun В. tér 2. ** water Resources Research Centre, H-1095 Budapest, Kvassay J. u. 1. Manuscript received: 20 October 1982 4 P. Dövényi— F. Horváth— P. Liebe— J. Gálfi— I. Erki a century. Since the beginning of drilling activities it has become clear that thermal waters are available in vast amounts in the Mesozoic carbonates of the Hungarian mountain chain (Transdanubian Central Range, Mátra, Bükk, see Enclosure 2) and in the Neogene sediments of the basins; these thermal waters can be utilized for balneology and also as an energy source. After initial investigations, major development work started in the 1950s. Thermal conduc tivity measurements started at the same time, and they showed that not only the thermal gradient but also the terrestrial heat flow is markedly higher than the world average in most parts of country (Figure 1). This suggests that thermal highs in the Pannonian basin are not local features related to upward migration of thermal waters but are parts of a regional phenomenon which reflect elevated temperatures of the whole lithosphere. Nevertheless, tem peratures vary considerably from place to place and the mapping and under standing of this variation are important tasks. Construction of reliable tem perature maps is however a difficult job because most of the temperature data measured during routine geophysical surveys do not fulfil strict physical require ments. In view of this, temperature data should somehow be corrected and, if necessary, some of them should be dropped. Fig. 1. Frequency diagram of continental 300 ! heat flow data [Jessop et al. 1976] and the average for Hungary 200 I 1. ábra. A kontinentális hőáramsürűség adatok hisztogramja [Jessop et al. 1976] ! A,p.-c, -ijrujary a magyarországi átlagérték feltüntetésével 100 Фи.\ /. Частотная диаграмма данных о континентальном тепловом потоке 0 : ^ - — n________ ^ [Д жесоп и др. 1976] с указанием средних 0 50 100 150 200 250 значений для Венгрии. q (mw/rrí) The recent energy crisis has upgraded the value of geothermal energy. Intensive research has been initiated to seek low or high enthalpy geothermal reservoirs and to use them as an alternative energy source. There is, however, further value to be obtained from knowledge of geothermal conditions. For example, they are important parameters which constrain geodynamic models of basin formation. Furthermore, it is widely accepted that the generation of liquid hydrocarbons and their migration through permeable rocks are basically thermally controlled processes. The prime goal of this paper is to present all reliable temperature data for Hungary in the form of a data catalogue, maps of geoisotherms and temperature depth diagrams. A new heat flow map of the Pannonian basin has also been constructed. Geothermal conditions of Hungary 5 Quantities generally used in geothermics Symbol Name SI unit Relationships to other units T Temperature К T(°C) = T(K) —273.15 AT Temperature difference К 1 °С = 1 К Q Heat J = kg m/s2 1 cal = 4.187 J G Geothermal gradient mK/m I °C/m= 103 mK/m G ~l Reciprocal gradient m/K 1 m/°C = 1 m/K Ф Heat power w = J/s 1 cal/s = 4.187 w Я Heat flow (thermal flux) w/m2 1 cal/cm2s = 4.187- 104 w/m2 X Thermal conductivity w/Km 1 cal/cm s °C = 4.187 ■ 102 w/Km X Thermal diflusivity m2/s 1 cm2/s= 10“ 4 m2/s C Specific heat J/kgK 1 cal/g °C = 4.187 - 103 J/kgK A Heat generation w/m3 1 cal/cm3 s = 4.187- 106 w/m3 в Thermal resistivitv m2K/w 1 cm2 s °C/cal = 2.388 • 10“ 5 m2 K/w 2. Historical overview Thermal springs have been known in Hungary since Antiquity. The first drilling for thermal water was done in the last century by V. Z sig m o n d y (Table I and II). Further thermal water exploration drillings led to some early under standing of temperature conditions. K. Papp could conclude as early as 1919 that “The considerable warmness of the subsurface in the Great Hungarian Plain is striking”, and the 18 to 22m/°C inverse gradient was significantly different from the 33 m/°C world average. After the First world war, the first petroleum exploratory drillings found many hot water aquifers. It became possible to measure the outflowing water temperatures. In 1929, J. Sümeghy interpreted and published 431 such data. Bottom-hole temperature measurements were also performed. These data unambiguously demonstrated the validity of the anomalously high tem peratures in the Pannonian basin, which had previously been doubted. These temperature measurements showed, furthermore, the inverse relationship bet ween temperature gradient and thermal conductivity of rocks, and the possibil ity of using these data in geological exploration [Sc h m id t 1936]. Temperature measurements in mines were made by Bo l d iz s á r [1944], primarily to solve problems of ventilation. He demonstrated that the geothermal gradient was higher in Hungary than the world average not only in the Neogene, but also in older sequences. Temperature measurements in mines and at shallow depths were made by Steg en a [1952, 1957]. He demonstrated that the morphol ogy of the good conducting basement affected the temperature distribution in the overlying sediments. It was eventually realized that the relationship of geothermics and geology can be understood by measuring both the rock temperatures and conductivities. This led to the beginning of conductivity measurements [Balyi—Papp 1950, 6 P. Dövényi— F. Horváth— P. Liebe— J. Gálfi— I. Erki Boldizsár 1956, Stegena 1958] and resulted in the first reliable heat flow determination, made by Boldizsár [1956] in the twin-shaft of a coal mine. Although at that time no data were available in continental Europe and there were only 57 determinations from other continents, his conclusion about the unusually high heat flow in Hungary proved to be valid. The first temperature maps were based on the data published by Sümeghy [SÜMEGHY 1929, Scheffer—R ántás 1949, Stegena 1958]. Although regular geophysical survey of water exploration wells was suggested long ago, tem perature measurements in water wells became compulsory only in 1956 (for wells deeper than 300 m) and in 1963 (for wells deeper than 200 m) [Bélteky et al. 1965]. Temperature and other relevant data are published regularly by the water Resources Research Centre (VITUKI) in a book series entitled “Thermal wells of Hungary” (edited by Bélteky, Alföldi, Korim, M arcell, Papp, Rémi, Simon, U rbancsek, Liebe, Székely, Pozsgai). Temperature measurements in other boreholes became general in the early 60s, as a part of routine geophysical well logging. As a result, the number of temperature data has increased rapidly and several geothermal analyses have been made. Unfortunately, data from wells attaining thermal equilibrium were very exceptional in view of which published maps and interpretations were often different as a consequence of subjective data selection and problems of data correction.

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