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GEOTHERMAL CONCEPTUAL MODEL within the framework of project Screening of the geothermal utilization, evaluation of the thermal groundwater bodies and preparation of the joint aquifer management plan in the Mura-Zala basin


Project co-workers on this report:

Geološki zavod Slovenije (GeoZS)

Magyar Állami Földtani Intézet (MÁFI)

Report prepared by: György Tóth, Ph. D. (MÁFI) Judit Muráti (MÁFI) Dušan Rajver, M. Sc. (GeoZS)

Co-workers on this report: Andrej Lapanje M.Sc. (GeoZS) Annamária Nádor (MÁFI)

Director GeoZS:

Director MÁFI:

Doc. Marko Komac, Ph.D.

Ljubljana, Budapest, 28.2.2011

Tamás Fancsik, Ph.D.


1 Introduction ............................................................................................................................. 1 2 Data background of the geothermal models............................................................................ 3 3 Factors influencing heat flux and thermal field ...................................................................... 5 3.1 Thermal conductivity ................................................................................................. 5 3.2 Radiogenic heat production........................................................................................ 8 3.3 Geothermal gradient ................................................................................................... 9 3.4 Groundwater flow .................................................................................................... 10 3.5 Sedimentation rate/erosion....................................................................................... 12 3.6 Tectonic structures ................................................................................................... 12 3.7 Volcanic and post-volcanic activity ......................................................................... 13 4 Expected results and outputs (of numerical modelling)........................................................ 13 5 Geothermal conditions of the T-JAM project area................................................................ 13 5.1. Summary of the present geothermal direct heat use ..................................................... 14 5.2. Heat flux conditions of the area ............................................................................... 14 5.2.1. Hungary...................................................................................................................... 15 5.2.2. Slovenia................................................................................................................ 20 5.3. Initial temperature distributions ............................................................................... 28 5.4. Heat extraction from the system................................................................................... 32 5.4.1. Natural discharges ...................................................................................................... 32 5.4.2. Heat loss through the surface ..................................................................................... 33 6 Summary ............................................................................................................................... 33 7. References .................................................................................................................... 34



Geothermal models in general aim to provide a firm geoscientific basis for sustainable utilization of all aspects of geothermal energy (power production and direct heat use) as well as thermal water management for balneological purposes. The immense heat of the Earth (whose main source is the decay of radioactive isotopes in the crust) is stored in the different rocks themselves, as well as in the fluids filling their pores and fractures. Therefore geothermal models cover processes of heat transfer due to conduction (heat transfer among rock particles that are in physical contact with each other) and convection (heat transfer from one place to the other due to fluid motion, e.g. groundwater flow). Geothermal models strongly rely on geological models (3D distribution of the different rocks characterized by important parameters, such as porosity, specific heat, heat conductivity, density) and hydrogeological models (groundwater flows may significantly modify the subsurface heat distribution, furthermore due to different densities resulting from the inhomogeneous temperature field, buoyancy itself can initiate groundwater flows, or alter the existing ones). Geothermal models provide information about the initial or actual, quasi steady-state conditions of the temperature regime, therefore they can be used to verify the hydrogeological model. Furthermore, if the amounts of extracted thermal water and depending on the utilization - the reinjected cold water are given, then the possible future evolution of the heat content of the hydrogeothermal reservoir can be predicted (transient models). This helps reservoir engineers and managers to optimize the heat extraction from a given reservoir. Construction of the geothermal model happens in two stages. First, a conceptual model is established (subject of the present report), where necessary data and boundary conditions for the numerical model are determined (e.g. heat-flow density map, temperature distribution maps at different depths, thermal conductivity parameters for the lithostratigraphic units, some characteristic borehole depth/temperature profiles). It also contains a general description on the geothermal conditions of the project area. Coupled numerical flow and heat model in the second stage will help to outline convection systems, refine temperature distribution maps and show calculated isotherms along the geological cross-sections. In general T-JAM focuses on the utilization of naturally heated subsurface waters (above 20 °C) (Hydrogeothermal Utilization). Although one of the main targets of the T-JAM project is to promote heat-pump technology, geothermal models do not consider the shallow subsurface, nor they deal with the potential petrothermal use (Hot Dry Rock, or Enhanced Geothermal Systems). There are two main hydrogeothermal utilization concepts: (a) single well thermal water extraction – typically for balneological purposes, where re-injection is not possible due to contamination and (b) geothermal doublets (reinjection of used thermal water into the same reservoir after energetic utilization). A single well thermal water extraction with subsequent disposal of utilized thermal water to a surface discharge is not sustainable if the amount of abstracted water exceeds the amount that is naturally re-supplied through recharge, as it leads to decrease of pressure and yield in the reservoir. The doublet concept – in theory – keeps the mass- and the pressure balance equalized. Nevertheless, a temperature change in time at the water extraction site (production well) may occur due to the thermal breakthrough of injected cooled water via the injection well.


In general, there are 5 different technical utilization schemes with specified production (θout) and injection temperature levels (θin), operational hours: 1. General (reference scheme): θout > 30°C; θin = 25°C; year-round operational hours [single-well; doublet, multiplex]. 2. Pure Electric Power Generation considering ORC schemes: θout > 90°C; θin = 70°C; specified operational hours [doublet, multiplex]. 3. Combined Electric Power Generation and Local Heating Scheme: θout > 90°C; θin = 30°C; specified operational hours for power generation and heating [doublet, multiplex]. 4. Combined Heating and Balneological Scheme: θout > 50°C; θin = 20°C; specified operational hours for heating and year-round mass extraction for balneological use [doublet, single-well in terms of a mass deficit at the injection well]. 5. Pure Balneological Use: θout > 30°C; θin = 20°C (at a surface discharge) year-round operational hours [single-well] Regarding the present utilization schemes of the T-JAM project area, and the overall geological, hydrogeological and geothermal conditions, balneological (5) and combined heating and balneological (4) utilization schemes are the main targets in our investigations, as high reservoir temperatures (> 90°C) for pure or combined electric power generation (2 and 3) are not (or at least not at an economic level according to our present knowledge) available in the area. On the T-JAM project area the utilization survey (LAPANJE ET AL., 2010) showed, that the majority of thermal waters is used for balneological purposes, especially in Hungary. Although individual space heating is relatively significant in NE-Slovenia, they are mostly linked to thermal spas. The three existing district heating systems (Murska Sobota and Lendava in Slovenia and Vasvár in Hungary) do not have operating re-injection wells either. Therefore the T-JAM project mostly evaluates the single-well scheme, and focus is put on to determine the effects of present transboundary water abstractions, determine the amount of water/heat that naturally recharges, therefore can be abstracted, and at the end of the project make recommendations for sustainable joint transboundary utilization and monitoring. The geothermal model in the T-JAM project provides information on the actual temperature field based on the 3D geological model and groundwater flow patterns, therefore will contribute to verify the numerical hydrogeological models. Model is focusing on the establishment of steady-state models, describing the natural heat flow density and temperature distribution. Due to the lack of sufficient data on production, transient models analysing current thermal abstraction, and scenario models for future exploitation trends will not be examined. Another important goal of geothermal model, also strongly linked to the hydrogeological model is to contribute to the definition of the cross-border thermal water flow. Furthermore results will add to the outline of a joint transboundary thermal groundwater body between Slovenia and Hungary (so far thermal groundwater bodies are not officially delineated in Slovenia, only in Hungary).



Data background of the geothermal models

The most important information necessary to establish a geothermal model are the following: 

Reservoir temperature

Reservoir pressure

Thermal and hydraulic characteristics of the reservoir

Geometry and type of reservoir (porous, fractured), especially the geometrical distribution of pores, which governs the heat exchange between the solid matrix and the circulating pore-fluid

The characteristics of the pore-fluids: gas content, mineralization.

Geothermal models rely on harmonized data-sets. The most crucial data source is represented by boreholes and wells situated at the project area whose data have been collected and filled into a joint multi-lingual database (output of WP2, part of which is public and is available on the project website: Furthermore, most of the needed reservoir parameters are already provided by the previously accomplished geological and hydrological models. As regard to legitimate areas of Pomurje and Podravje in Slovenia the project area is circumscribed well enough with the localities of boreholes with geothermal and other hydrogeological data. In the west these boreholes are in Maribor and at Ptujska gora, in the south in Ptuj, at Bukovci and Ormož. In the southeast the boreholes are at Lendava and Petišovci, which is almost on the border with Croatia and Hungary, in the east there are few boreholes almost on the border with Hungary. In the north there are boreholes in Goričko, although not quite on the border with Austria, with an exception of those at Strukovci and Nuskova. At the T-JAM part of Zala and Vas counties there are several operating thermal wells with relatively suitable information for geothermal and hydrodynamic evaluations for the Tertiary porous aquifer systems. Along or close to the Slovenian-Hungarian border these wells are in Lenti, Szécsisziget, Bázakerettye and Letenye. Because of the large, (3-4-5 km) depth, not any existing operating or monitoring wells opened the basement aquifer here. Detailed geothermal information (temperature-log and heat conductivity measurements on core samples) were made only in the Bárszentmihályfa-I, (Bm-I) borehole. Here the top of the Pretertiary (Pre-Miocene) basement is at the depth of 3070 m, and the borehole penetrated Mesozoic complex down to the depth of 5075 m. (3070-4460 m: Triassic, mainly dolomite, 4460-5075 m: Jurassic, mainly limestone). Data needs for geothermal evaluation originate from different archives and cadastres. These are: data base of drilled cold and thermal water wells; boreholes for hydrocarbon exploration and a few geological core boreholes. The most useful geothermal and hydraulic information comes from the cadastre of thermal wells. The following input parameters are needed in order to establish the aimed steady-state geothermal models: Thermal Conditions  Borehole Temperatures  Thermal conductivity and specific heat capacity: rock matrix, fluid (anisotropy)  Mean annual surface temperature (measured and interpolated)  Surface heat-flow densities (modelled, averaged) 3

Most of these data are available from the database or from literature studies to establish the geothermal model. There are measured temperature data from 154 boreholes on the Slovenian side and 369 measurements from 284 boreholes on the Hungarian side of the project area. The purpose of these boreholes was different, but the hydrocarbon wells prevail, with exploitation geothermal wells following, while the number of structural-exploration, hydrogeological and other boreholes is lower. Both countries used five different modes of temperature measurements, and their final aim is the acquisition of formation temperatures. These measurement modes carried out are the following with showing relevant data from NE Slovenia Bottom hole temperature During the drilling or immediately after, especially of the hydrocarbon wells, the so called Bottom Hole Temperature (BHT) has been measured in 83 wells. Since it was measured in thermally nonequilibrated borehole, a correction is needed. But this hasn't been done because some important data needed for correction are missing. Fortunately, later on during the last decade steady-state temperature and pressure measurements were done in numerous (but not all) hydrocarbon wells. Continuous temperature logging Temperature has been measured continuously in 28 boreholes in the framework of the logging operations. Also some geothermal production boreholes are included where it was measured in only few points. Using this mode it was measured also in some boreholes after longer stand-still time, especially in the last five years; these are already better quality measurements. Temperature measured in the frame of testing a certain borehole section – DST (drill stem test) In the frame of testing the certain sections of oil wells for potential repository investigations (for oil or gas storage) temperature has been measured in 10 boreholes in the framework of DST measurements. When DST is reliably accomplished, such measured temperature is of better quality than the extrapolated BHT. Point- by-point temperatures Temperature has been measured in 28 boreholes using the point-by-point manner of uniform depth steps with step of usually 5 or 10 m. These temperatures are of best quality because they were measured mostly after longer borehole stand-still time. Temperature measured in individual points In 89 boreholes temperature has been measured in individual depth points. Also such boreholes are included where temperature has been measured in several depth points with greater interval, for example every 25, 50, 100 m or more, but not all along the whole borehole column. These temperature values are also considered of good quality. The BHT values are of low quality because they are uncorrected values. In most cases BHT has been measured and registered only once at a certain depth, therefore it is impossible to extrapolate it to real formation temperature. Temperatures from DST are of higher quality. The continuously logged temperature is of good quality from some boreholes, especially the recent measurements with new logging equipment. Data from the earlier period of temperature data collection are stored in our database and their interpretation is described by RAVNIK (1991), while later enlarged database is delineated by RAJVER & RAVNIK (2002). The Nafta Geoterm Co. (Lendava) began with systematic improvement of oil wells 12 years ago. Their temperature measurements in the last 11 years in steady-state condition gave improved temperature gradients in selected boreholes. However, there are still many


boreholes and oil wells with no recent temperature measurements yet done. Some of them are located in the areas with no other boreholes around. The collected geothermal data show that we have reliable data from many deep oil wells in NE-Slovenia, and also from some shallow boreholes in other parts of northern Slovenia (Maribor). Therefore also HFD data are available from some shallow boreholes if geothermal measurements have been good there. Temperatures in our boreholes have been measured mostly after longer stand-still time. In Hungary 3 major datasets are available for geothermal evaluations, based on the above mentioned 5 measurement modes. The first, and till now, the largest compilation of these kind of data was in 1983 (DÖVÉNYI ET AL. 1983). This data catalogue contains 288 boreholes in the T-JAM area. The catalogue is based on the files of the former National Oil and Gas Trust (OKGT), Water Research Centre (VITUKI), and Geological Institute of Hungary, (MÁFI). The temperatures of these wells are (1) measured under steady state conditions; (2) measured during production tests (DST-tests) both in water and hydrocarbon exploration wells; and (3) calculated from out-flowing water temperatures and yields using an empirical formula. The second data-sets contains 149 drilled water wells with measured bottom hole and outflowing temperature data. The third data set contains 15 wells with detailed point-by-point temperature measurements made by GEOLOG Kft. Five of these wells have been measured in the frame of T-JAM project.

3 3.1

Factors influencing heat flux and thermal field Thermal conductivity

Thermal conductivity governs the temperature distribution at a subsurface volume at a given heat-flow density (HFD) assuming heat transfer by pure conduction. Thermal conductivity of rocks is determined either directly from thermal logs, or indirectly derived from other geophysical logs, or based on the lithological composition of formations using literature data. Thermal conductivity has been measured on 129 rock samples cored from 24 boreholes in NE Slovenia, such as hydrocarbon wells, exploitation geothermal wells or from exploration boreholes. Therefore, these direct measurements were done using a transient hot wire method with Slovenian made instruments (Faculty of Physics and Mathematics of Ljubljana University; see RAVNIK, 1991; RAVNIK ET AL., 1995), only the samples from one borehole were measured with a new noncontact optical scanning method. Thermal conductivity data from Hungary are based on hundreds of laboratory measurements at the Department of Geophysics of the Lorand Eötvös University (LENKEY, 1999). The original water content of core samples was preserved by waxing, or samples were saturated by vacuum. The measurements were repeated three times.


According to LAND & PAULL (2001) the thermal conductivity of rocks (KT) varies between 0.7 and 3.0 W/m/°C. According to STONESTROM & BLASCH (2003) these values in saturated porous media is 2.2 W/m/°C for sands, 1.4 W/m/°C for clays, and 2.9 W/m/°C for soils and clays. Thermal conductivity of some most important rock types on Slovenian side of project area can be presented as value ranges for the following rock groups (in W/(m·K)): a) Sand, loose sandstone: 1.4 to 2.96 (mean: 1.93) b) Sandstone (compact), siltstone (sandy, calcareous), breccias of sandstone and marl: 1.49 to 4.44 (mean: 2.78) c) Clay, marl, claystone, marly and clayey siltstone: 0.92 to 3.21 (mean: 2.09) d) Metamorphic: greenschist, micaschist, gneiss, phylonite, eclogite: 2.09 to 4.6 (mean: 3.21) e) Carbonate rocks (dolomite, limestone): 2.01 to 3.66 (mean: 2.88) In Hungary thermal conductivities of the measured Neogene ‘sandstone’, (sand) and ‘shales’, (clay, claystone, silt and siltstone) were plotted as a function of depth (DÖVÉNYI & HORVÁTH, 1988). They concluded the thermal conductivity of Neogene sediments mainly depends on their porosity, within the major lithological types (sandstone or shales). The porosity-depth trends and their combination of the measured heat conductivity data of Neogene sediments is shown in the Table 1. Table 1. Characteristic thermal conductivity of Neogene sands and shales (After Dövényi & Horváth, 1988)

Origin of the core sample

Porosity (%), according to lithology of the cores

Heat conductivity of cores (W/(m·K)

Vertically alternating sand and shale layer’s averaged heat conductivity (W/(m·K) for four rate (% ) of the sandstone layers in the compound formation

*Depth (m Shales Sandstones Shales Sandstones 5 20 40 80 bgl) 500 48 43 1.3 2.0 1.3 1.4 1.6 1.9 1000 32 38 1.7 2.4 1.7 1.8 1.9 2.2 1500 19 30 2.1 2.7 2.1 2.2 2.3 2.6 2000 11 21 2.4 2.9 2.4 2.5 2.6 2.8 2500 5.0 14 2.6 3.2 2.6 2.7 2.8 3.1 3000 3.1 8 2.7 3.5 2.7 2.9 3.0 3.3 3500 2.9 6 2.8 3.8 2.8 3.0 3.2 3.6 4000 2.8 4 2.8 3.9 2.9 3.0 3.2 3.7 4500 2.7 3 2.8 4.1 2.9 3.1 3.3 3.8 5000 2.6 2 2.8 4.1 2.9 3.1 3.3 3.8 *Depth means the deepest value during continuous sedimentation of the basin, without supposing erosion. In the T-JAM area the inferred erosion values vary between 0 and 1000 meters. With a help of the time-slices outlined on the seismic profiles some depth corrections should be made on thermal conductivity data before geothermal evaluations or modeling start.


Based on the laboratory measurements in Hungary some additional thermal conductivity values for other rocks are: Mesozoic limestones: 2,7-3,1 W/(m·K), dolomites: 4,4 W/(m·K); shales: 2,8 W/(m·K); Paleozoic sandstones: 2,7 W/(m·K); metamorphic rocks: 3,1 W/(m·K). If dolomites prevail as carbonate rocks, their mean thermal conductivity may be slightly higher. Thermal conductivity of rocks from the boreholes of NE Slovenia as regard to lithology is presented in Figure 1.

Fig.1. Thermal conductivity of rocks from the boreholes of the NE Slovenia grouped according to lithology (T-JAM project area)


It is worth mentioning here that permeability of porous media can vary in wide, several magnitudes of order ranges unlike its thermal conductivity which is rather constant. The reason is that heat is transferred directly in the entire media (including rock grains and the pores in-between filled by fluids), while fluid flow can happen only along interconnected pores. The fluid flow is the result of transmissivity and hydraulic potential gradient (Darcy law), while similarly conductive heat flux is the result of thermal conductivity and thermal gradient (Fourier law) (Fig. 2). Heat can be used as a tracer for calibration of different flow models.

Fig. 2. In saturated porous media transmissivity (blue band) is largely depending on the sediment structure (grain size, pores), while thermal conductivity (brown band) is practically independent of that. The width of the bands also refers to the value ranges (CONSTANTZ & STONESTORM 2003).


Radiogenic heat production

Diffuse heat production is derived from the decay of radioactive elements in the Earth’s crust which produces energy. Radiogenic heat production in the rocks has been determined in 53 rock samples from 14 boreholes in NE Slovenia. The contents of radioactive elements U, Th and K40 have been determined at the Institute JoŞef Stefan in Ljubljana, and the rock density at our geomechanical laboratory (RAVNIK, 1991; RAVNIK ET AL., 1995). This parameter has a certain smaller influence on the temperature calculation in greater depths (usually >3-5 km?), where the rocks with higher concentration of radioactive elements are anticipated. It is important for temperature modeling in the areas with no reliable borehole data. No such measurements are available from Hungary.



Geothermal gradient

The temperature distribution towards the depth of a homogenous media without groundwater flow can be characterized by the geothermal gradient (Fig. 3).

Fig. 3. Geothermal gradient

In regional models T0 is the annual mean surface temperature of the area, T1 is the temperature at a certain depth, λ is thermal conductivity, q is the heat flux, and z is depth. Correlation of thermal conductivity and geothermal gradient is clearly indicated as gradient is higher in younger sediments with lower thermal conductivities and vice versa. Lower thermal conductivity is due to increased clay and perhaps also marl content. Geothermal gradient is the lowest in the dolomite section with characteristically highest thermal conductivity among the rocks in the area. Geothermal data on Slovenian side, i.e. measured thermal conductivities as well as qualitative temperature measurements, have been arranged in a proper way and presented as characteristic values for individual lithostratigraphic and lithologic units. For each lithological description within the individual stratigraphic unit a characteristic range of measured thermal conductivity values and their mean value are cited in Fig.4. Also geothermal gradient ranges and their mean value have been drawn on the same graph. There are greater geothermal gradient ranges for the youngest unconsolidated sediments and for poorly consolidated sediments while narrower ranges hold true for hard compacted sedimentary rocks of Špilje and Haloze formation and for pre-Tertiary rocks. From Fig. 4 the following geothermal gradient ranges may be deduced for layers of different age: Pliocene: 39-77, Miocene: 28-103, Mesozoic: 22-34, and Paleozoic: 30-48 °C/km.


Fig. 4. Range and mean values of measured thermal conductivities on rocks of lithological units and range with mean values of calculated geothermal gradients in NE Slovenia .

Such detailed analysis of geothermal gradients for different lithologies is not available for the Hungarian part of the project area. However the mean geothermal gradients in different depths of the T-JAM area are quite similar in Slovenia and Hungary: Hungarian part: 500 m: 49,8 °C/km; 1000 m: 47,4 °C/km; 2000 m: 45,6 °C/km; 4000 m: 42,4 °C/km. Slovenian part: 500 m: 50,1 °C/km; 1000 m: 46,8 °C/km; 2000 m: 42,8 °C/km; 4000 m: 38,1 °C/km. 3.4

Groundwater flow

In addition to conductive heat flux, vertical and horizontal groundwater flows can modify the heat flux. Fractured, especially karstified rocks have large permeability which allows easy infiltration of the precipitation which is cooling its environment in the recharge area. At a depth of 3-4 km, the water is warming up and upwelling and discharges at the surface as thermal springs. Upwelling to the surface is restricted to small areas in contrast to the cooling effect in the recharge areas, therefore heated areas are much smaller than the cooled ones. As a result, open karst areas are characterized by low subsurface temperatures and heat flux.


In basin areas where porous sediments are quasi-horizontally deposited, the vertical flow velocity of groundwater is several magnitudes smaller compared to the horizontal one; therefore it does not really influence the heat flux, as opposed to karstic areas. Fig. 5 summarizes the main components of groundwater flow systems, where those ones with having an influence on the heat flux are shown by red color.

relief and drainage lines altitude and subsurface depth of (unconfined) groundwater table horizontal outlining underlying layer of the unconfined aquifer 3D distribution of the saturated zone thickness and underlying layers of the different aquifers horizontal outlining porous double porosity / fractured Hydrogeological homogenous / inhomogeneous character isotropic / anisotropic Hydrogeological bedded / unbedded characteristics of cavernous the unsaturated flow: pF, K(w) zone Hydrogeological transport: dispersivity parameters for rock-water interaction: models mineralogical composition, adsorption properties porous double porosity / fractured Hydrogeological homogenous / inhomogeneous character isotropic / anisotropic Hydrogeological bedded / unbedded characteristics of cavernous the saturated flow: k, n, S, anisotropic zone coefficient (位) Hydrogeological transport: dispersivity parameters for rock-water interaction: models mineralogical composition, adsorption properties regional model water budget base-flow measurements, archive Natural recharge (infiltration) data time series on water levels (simulation of runoff, evaporation) water quality Artificial recharge (water supply) seepage from waste water sinkholes and sewage system, irrigation Present and predictable changes (best-worst and most probable scenarios), e.g. weather, vegetation cover, human activities springs (discharge and Natural discharge water level) 3D distribution of the unsaturated zone

3D distribution of hydrostratigraphic units

Internal factors controlling the system Hydrogeological parameters of hydrostratigraphic units

External factors controlling the water and mass-budget of the system (natural and artificial)




streams (discharge, water level) evaporating stagnant water bodies: marshes (water level) evaporating groundwater) Artificial discharge wells, waterworks drainage, melioration sewage systems Present and predictable changes (best-worst and most probable scenarios), e.g. decreasing evaporation of groundwater due to climatic effects quality of rainwater Material supply of natural and artificial components, tracers: Cl, NO3, K, freon, transport processes heavy metals, isotopes dissolved material from contaminations potential conditions of the soil moisture Potentials unconfined groundwater table confined groundwater table zone of soil moisture Hydrogeochemical, isotopic data, timeunsaturated zone Data characterizing the series environmental status of saturated zone the system (for model surface waters calibration and Geothermal properties Subsurface temperature data, verification) temperature time-series data of springs alkaline soils Different changes in the soils and rocks accumulation due to groundwater flow, transport precipitation processes and rock-water interactions absorption Fig. 5 Main components of groundwater flow systems


Sedimentation rate/erosion

The thermal effect of sedimentation also has to be considered, especially in areas (such as the Pannonian basin), where several thousand meters thick sedimentary successions deposited with various thermal properties. In the Drava basin adjoining the project area from the SE, the thickness of Neogene and Quaternary sediments is more than 4000 m, therefore the heat-flux deficit due to the thick sedimentary cover is around 20 mW/m2 (LENKEY, 1999). This is less relevant in the Zala basin, furthermore a significant uplift and erosion removed several hundreds of meters of sediments after the infilling of the basin, which increased heat-flux. This will be considered during numerical modeling. 3.6

Tectonic structures

Different tectonic structures (e.g. faults) may be acting as forced flow-paths for thermal groundwater. The most important are those nearly vertical, several hundred meters long conduits, where thermal waters can upwell with little cooling gradient. This is called a „thermal lift�, which has an important role in the formation of free-, or partly gravity-driven convections. The Benedikt site in NE Slovenia may belong to this category. The convection zone in metamorphic rocks was revealed when the exploration-production Be-2 well was finished in 2004 (KRALJIĆ ET AL., 2005). The possibility of existence of such a zone was


indicated already earlier with borehole BS-2 when high temperature gradient (82 mK/m) was measured in Tertiary rocks. There is obvious deep thermal water circulation, perhaps as much as 2 km deep, which is enough for water temperature to reach some 82-86°C. There is no need for existence of young deep magmatic body as a heat source. In Hungary at Zalaegerszeg, Nagylengyel and Zalakaros region several geothermal anomalies can be connected with vertical or subvertical high permeability conduits, inferred fault zones. The low salinity of the waters indicates forced convection (eg. Zalaegerszeg), the high one indicates free convection cells (eg. Zalakaros). 3.7

Volcanic and post-volcanic activity

Volcanic activity can result in high heat-flux, but the size of the older volcanoes of T-JAM area (inactive during the last ~10 million years) do not cause an increased heat flux, as the magma chambers cooled down during such a long time. (LENKEY 1999). This factor therefore can be neglected in the T-JAM area.


Expected results and outputs (of numerical modelling)

The final outputs of geothermal models which will derive from the joint numerical hydrogeological and geothermal models, performed by ModFlow (fluid flow modelling) and FeFlow softwares (fluid flow and coupled heat transport modelling) will be different types of maps, such as  Surface heat flow density map  Anomaly map showing areas with significant (positive or negative) conductive heat flux, combined with a specified classification of hydrogeothermal reservoir types (e.g following HOCHSTEIN, 1988).  Temperature and isopache schemes -

Contour map series showing the isopaches of some temperature levels (e.g. 30°C, 50°C, 70°C and 90°C isothermal surface).

During the evaluation of the results of the numerical models, tables and graph as notes to maps will be compiled. The major outputs of the regional flow and heat numerical models will contribute to a better understanding of the regional fluid and heat flow systems. Based on these achievements better predictions of the effects of different geothermal utilization scenarios (e.g. impacts of fluid abstractions, or geothermal abstraction-injection doublets, or multiplets) will be possible.


Geothermal conditions of the T-JAM project area

The Pannonian basin – and within that the investigated Mura-Zala basin – has one of the best geothermal properties in Europe, due to the thin (60–100 km) lithosphere. The crust is also thinner than the average, it is about 24-26 km, so about 10 km thinner compared to the neighboring areas. This is all the result of the extensional basin formation during the Middle Miocene, when the hot astenosphere got closer to the surface (ROYDEN ET AL., 1983). Regional geothermal picture of the project area has been presented in the 1990’s as


geothermal maps for example within the framework of Geothermal Atlas of Europe (HURTIG ET AL., 1992). 5.1. Summary of the present geothermal direct heat use On the T-JAM project area the heat extraction is happening mostly via thermal wells, which has been summarized in the survey of utilization aspects (LAPANJE ET AL. 2010). In the Hungarian part there are only two categories of direct heat use of geothermal energy, while in the Slovenian part there are five, and in near future at least one or two more are in preparation (snow melting, fish farming). Probably similar future plans exist also in south-western Hungary to expand the direct heat use to some other categories. Data on geothermal direct heat use have been available from 13 users with 25 production wells in NE Slovenia (without Benedikt). For SW Hungary data have been on disposal from 29 thermal water utilization sites (users) with 42 production wells and one strong spring in Hévíz. Geothermal heat pump (GHP) units are at the moment more spread in NE Slovenia than in SW Hungary. The installed capacity of all five categories in NE Slovenia (individual space heating, district heating, air conditioning, greenhouse heating, bathing & swimming with balneology) of direct heat use is 38.83 MWt, and together with GHP units it is 48.83 MWt. In SW Hungary the installed capacity for the two categories (district heating, bathing & swimming with balneology) is 71.17 MWt, with no data for capacity of GHP units. The annual energy use in NE Slovenia amounts for the five categories to 382 TJ/yr, and together with GHP units to ca 432 TJ/yr, while in SW Hungary the annual energy use for the two categories is ca 660 TJ/yr, and together with GHP units it is ca 678 TJ/yr. While in NE Slovenia the open loop water source GHP units prevail out of at least 600 units, in SW Hungary there are mainly closed loop vertical ground coupled installations out of 98 GHP units. 5.2.

Heat flux conditions of the area

The heat-flux map of the project area and its surroundings is shown on Fig. 6. The map is based on 27 heat-flux measurements and about 1500 estimations in Hungary, while outside of Hungary content is based on the „Geothermal Atlas of Europe” (HURTER & SCHELLSCHMIDT 2003). The preciseness of the contour lines is ± 15 %. Since the edition of this map (2005), no new heat-flux measurements have been performed in Hungary, furthermore new temperature measurements in boreholes are in accordance with the previous ones, so the heat-flux map was not modified. From the SW corner at Ptuj with around 60-70 mW/m2 the surface HFDs increase towards the Hungarian border. The elevated HFDs, above 120 mW/m2, are found on the Murska Sobota high, from Lenart to Moravske Toplice and Pečarovci-Dankovci area, which may be explained with shallow pre-Tertiary basement there and convection zones in the pre-Tertiary rocks, that is proved at Benedikt, and is possible beneath Murska Sobota and Moravske Toplice. Smaller anomaly, above 110 mW/m2, is located at Lendava, but its range is to be connected with situation in SW Hungary. The Hungarian part is characterized by a wider range of surface HFD. The lowest values occur in the Keszthely mountains at the ENE, where infiltrating cold karstic waters cool down the environment. Values show a gradual increase towards the SW and may reach 90-100 mW/m2 close to the Slovenian border.


Fig. 6. Heat-flux map of the project area and its surroundings (HORVĂ TH ET AL. 2005)

5.2.1. Hungary The convective heat-flux of the area can be estimated from archive temperature distribution measured in boreholes (Table 2, Fig. 7) and from the geological-hydrogeological knowledge of the area. Furthermore 5 new measurements were performed by a subcontractor (Geo-Log) in the autumn of 2010. (Table 3, Fig 8, Appendices I-V). In order to determine the temperature distribution, continuous temperature logging was performed in these 5 wells. To verify the well structure, borehole diameter was also measured, as well as natural gamma. These values will be added to the numerical heat transport models to precise heat flux.


Table 2. Temperature values and other heat parameters measured earlier in the Szombathely II borehole. Thermal Corrected thermal Depth Thickness Temperature Heat flux conductivity conductivity (m) (m) T (°C) (mW/m2) (W/mK) (W/mK) 0 - 1003

ksh = 1.83 kst = 2.6

" "

821 182

1003 - 1062

k = 2.54



1062 - 1810

ksh = 1.99 kst = 3.28

" 2.93

728 20

1010 - 1913

k = 2.66



1913 - 2064

k = 3.61



2064 - 2085

k = 2.84



2085 - 2150

k = 3.42



0 - 2150







sh – shale st - sandstone Table 3. Basic data of the measured wells within the project T-JAM Name of well


NG-1 MÁFI observation well

Nádasd N-2



























year of construction






Bottom (m)






Top of screen (m)






Bottom of screen (m)






Number of screens






Bottom hole temperature (°C)






Outflow water temperature (°C)






Static water level (m)






max. discharge (l/min)







Fig. 7. Geological log of the Szombathely II borehole with measured thermal conductivity, temperature and thermal gradients


Fig. 8. Location of the measured wells in Hungary

In the Szombathely Fürdő-1 well (Appendix 1A-B) temperature and natural gamma logging was performed between 3,3–758,8 m. The crossed strata are Upper ‘Pannonian’ sandy, aleuritic-clayey layers (Tihany F.). The nearby Szombathely Fürdő-3 well has the same geological section, where temperature logging was performed between 2,7–664,4 m and natural gamma logging between 2,7–662,9 m. The borehole final depth is 1498,6 m and the BHT measured at the bottom is 83,5 °C. The temperature-depth (T-z) profile measured in stand still conditions shows in the upper part the isothermal values around 14,20°C down to 59 m depth (we don’t know the reason for that), then mostly constant temperature increase from 58 m to the final depth of measurements, 758,8 m with temperature of 42,89°C. The only exception is where the measured temperatures show that thermal water enters the borehole from the lower water horizons (deeper than 770 m), rises up the borehole and escapes from the borehole into the rock formation in the filter section of 600-625 m The average temperature gradient in depth section 59-758,8 m reaches 41 °C/km. Considering the temperature of 10,5°C at depth of 20 m as the default value for the mean annual surface temperature, then the interpolated temperature trend line (in green) from the deepest measured logging temperature is presented with a gradient of 43,8 °C/km. It suggests us that the measured T-z profile in its most upper part is not yet stabilized. No special lithological dependence is visible. The Nagygörbő NG-1 well (Appendix 2A-B) crosses the Upper ‘Pannonian’ strata (Újfalu and Hanság F) underlain by upper Miocene Tinnye and Szilágy Clay Marl and Rákos Limestone F. Below that Middle Miocene (Baden Clay, Tari Dacitic Tuff, Tekeres Schlier) and Lower Miocene (Budafa, Szászvár, Gyulakeszi Rhyolitic Tuff) formations are found. The


lowermost part of the borehole exposes Oligocene Csatka Formation. Temperature logging was performed between 1,6–1106,9 m, natural gamma logging between 1,6–1105,4 m. The final borehole depth is 1517 m with no BHT measurement at the bottom. The T-z profile measured in equilibrated condition shows the isothermal values in the most upper part (to a depth of ca 42 m), then increasing trend with an average temperature gradient of 17,6 °C/km in depth section 42,5-1106,9 m. The maximal temperature 30,17°C was measured at a depth of 1106,8 m. The profile shows some variations in gradient which are probably in correlation with lithological changes. Down to a depth of 380 m the cold meteoric water infiltration is pronounced in the T-z profile with temperature gradient much lower there and higher in deeper part with less permeable lithological section. The interpolated temperature trend line shows the gradient of 18,1 °C/km. The T-z profile was measured in almost stand still conditions. The Hegyháthodász Nádasd N-2 (Appendix 3A-B) well crosses the Upper ‘Pannonian’ sandy, clayey, aleuritic deposits (Tihany F) till 1146,0 m. It is underlain by the sandy, marly, aleuritic sequence of the Algyő F. till 1322,0 m. Between 1322,0-1832,0 m clayey marls, sandy marls, sand and clayey sand layers of the Szolnok Formation occur. Loggings were performed down to a depth of 1832,3 m. The final borehole depth is 2395 m and the BHT reached 73,9 °C at a depth of 1536 m. During the temperature logging the maximal measured temperature was 91,8°C at 1832,2 m depth. The T-z profile does not show any peculiarities except slight variations due to mostly lithological changes and/or weak water migrations. In depth section of 11-1832,2 m the average temperature gradient is 44,1 °C/km. The interpolated temperature trend line shows the gradient of 44,9 °C/km. This tells us that the T-z profile was measured in practically stand still conditions. The Kehidakustány, Kd-3 well (Appendix 4A-B) crossed the Upper ‘Pannonian’ sandy, aleuritic layers of the Somló Formation below a thin Quaternary cover. Temperature logging was performed between 3,0–209,3 m, natural gamma logging between 3,0–207,8 m. The final borehole depth is 3212,3 m. When the drilling reached a depth of 1498,6 m the outflow water temperature was 44°C. The T-z profile shows slight lithological variations and/or probable lateral water movements at depth section of 110-125 m, and also at a depth of ca 155 m. The maximal temperature in the logged section was 53,83°C at only 209,2 m depth. Consequently the average temperature gradient in a depth section 20-209,2 m reaches extremely high value of 190 °C/km. The interpolated temperature trend line shows the gradient of 229 °C/km. It tells us that the T-z profile is not yet stabilized, at least not in the upper 120 m. Such high temperature gradient suggests that the borehole very probably reached the thermal water horizons in greater depths (maybe deeper than 1500 m?) from which thermal water rises upwards and influences on the T-z profile in the upper 210 m. The Zalaegerszeg, ZG-1 (Appendix 5A-B) well crosses Upper ‘Pannonian’ sandy, aleuritic layers below a thin Quaternary cover. Temperature logging was performed between 3,0–937,6 m, natural gamma logging between 3,0–936,1 m. The borehole reached the final depth of 940 m. The BHT was measured to 46 °C, probably at the hole bottom. During the logging the maximal measured temperature was 44,8°C at a depth of 937,6 m. The T-z profile shows great variations in the upper 200 m, which can be correlated to lithology, because in more sandy layers cooler water advection influences the measured temperature. Besides, the T-z profile was not measured in completely stand still conditions. This is obvious from the interpolated temperature trend line that shows the temperature gradient of 37,4 °C/km, while the temperature gradient from the measured T-z


profile in the depth section of 200-937,6 m is only 30,4 °C/km. The T-z profile measured in a depth section of 764-937 m during the pumping testing with outflow of 150 l/min exhibits a filter zone (840-857 m) where water enters in the borehole. 5.2.2. Slovenia

The average HFD is calculated for the sites of 27 boreholes, because the measured thermal conductivities on rock samples are available from 24 boreholes, and also 3 boreholes with good temperature-depth profiles have been added, for them thermal conductivity has been adopted from the nearby boreholes with the same or very similar geological composition. At certain sites it was not possible to extrapolate, so we resorted to theoretical calculation of temperature-depth profile instead after RYBACH (1981). In the Slovenian area few points have been added with this calculation for 1-D modeling in conductive regime just to improve the maps in those parts with no good borehole data. The calculated surface HFD values range between 66 and 155 mW/m2, the first is from Jan-1/04 borehole at Janežovci and the latter value being determined for the site of borehole Pg-9/89 at Petišovci near Lendava. The values mainly fall between 90 and 130 mW/m2. Benedikt (NE Slovenia) is a prominent site with convective heat-flow density beside the conductive one. In the older borehole BS-2 (788 m deep) temperature logging in 1976 down to 635 m depth showed increased temperature gradient in Tertiary layers, 82 °C/km, due to convection zone in the metamorphic rocks just beneath which was only touched by this first borehole in a depth interval of 31 m (RAVNIK ET AL., 1987). This convection zone was revealed much later when the production Be-2 borehole just 877 m away was finished in 2004 (Fig. 9, data from KRALJIĆ ET AL., 2005). Geothermal data from the BS-2 borehole were used for the HFD determination, and for the interval HFD calculation the mean geothermal gradients were used for depth section ±20 m at each depth with thermal conductivity determination (Table 4). The mean HFD value at BS-2 site is determined to 145 mW/m2. The Be-2 borehole drilled through Tertiary sediments (clay, sandy clay, marl, silt, limestone breccias, sandstone) and reached Paleozoic metamorphic rocks at 760 m depth. Metamorphic series of the greenschist facies (phyllite) were drilled down to 810 m and muscovite biotite schists alternating with dolomite marble, amphibolite and quartzite to the bottom. Temperature gradient in the Tertiary sequence is pretty high, reaching 85 °C/km. At 800 m depth the maximal temperature 86°C has been measured. Table 4. Temperature values and other heat parameters from the older borehole BS-2 at Benedikt. Depth (m)


geothermal gradient (mK/m)

155 230 405 420 465 772 781 Mean q

sandstone silty marl sandy marl sandy marl sandstone greenschist micaschist tuff

70 100 70 70 70 40* 40*

Thermal conductivity K (W/m·K) 3.00 1.59 1.90 1.59 2.77 2.41 2.77

Temperature measured (°C) 27.6 33.8 50.1 51.2 54.4

Interval q (mW/m2) 210 159 133 111 194 96 111 145

* geothermal gradients at depths 772 and 781 m were estimated with assumption of constant q throughout the whole well with no significant water migration.


With the aim of improving the temperature–depth distribution the point-by-point temperature (typically in 100 m or 200 m step) and pressure measurements were performed in many hydrocarbon wells that have been closed for a longer time. Some characteristic boreholes are presented herein from different parts of northeastern Slovenia with more or less recently measured temperatures that are useful enough for geothermal interpretation. There are many other boreholes and production geothermal wells with temperature measurements, but mostly BHT, DST and single point values, together with or only continuous logging in thermally not equilibrated conditions. Such wells are, for example, located in Murska Sobota, Gabrnik, Radenci, Dankovci, Filovci and many other sites.

Fig. 9. Measured temperatures in the Be-2 borehole in Benedikt (data from KRALJIĆ ET AL., 2005).

In the deepest Slovenian well Ljut-1 at Ljutomer, drilled for hydrocarbon research, temperature was measured few times. Soon after the drilling the DST measurements were done in 3 different depths. First temperature point-by-point logging (every 5-10 m) was done in Oct. 1992 in depth section 10-1965 m after 4 years of stand-still time. The second temperature point-by-point logging (every 100 m) was performed in June 1997 in depth section 0-4026m after more than 2 years of stand-still. At 4026 m depth maximal temperature of 173,4°C has been measured. No peculiarities are seen in the T-z profiles (Fig. 10 and 11), only the first profile shows some influence of the last ice age. The well drilled through


Pleistocene and Pliocene sediments, then through thick Miocene sedimentary sequence (Upper Pontian to Karpatian) with mainly marls, sandstones and siltstones in alternation, then Upper Triassic silicified brecciated dolomitic limestone and finished in pre-Ordovician gneiss. The mean temperature gradient in Tertiary layers is 40,3 째C/km, and the HFD determined from this gradient and thermal conductivities amounts to 116 mW/m2.

Fig. 10. Simplified geological log of the Ljut-1 well with measured temperatures, thermal conductivity, radiogenic heat production and interval HFDs (from GeoZS geothermal database).


Fig. 11. Measured temperatures in the Ljut-1/88 well.

The Mt-2 Rimska čarda well, with a purpose of oil research, drilled through the sedimentary sequence from Pliocene to Badenian with clay, sand, sandy clay, and deeper sandstone with marl, and finished in Paleozoic low grade metamorphic rocks (schist similar to phyllite). First temperature point-by-point logging was done in Nov. 1985 in depth section 10-810 m (every 10 m). Measurements were done after long stand-still time, and the T-z profile don’t show any curiosities as water migration zones and similar, in fact they are quite linear (Fig. 12). From the temperature gradient (61 °C/km in Tertiary rocks) and adopted thermal conductivity values from nearby borehole at Moravske Toplice with almost identical lithology the HFD is determined to 120 mW/m2.


Fig. 12. Simplified geological log of the Mt-2/61 Rimska čarda well with measured temperatures, deduced temperature gradient (for the 1st logging) and calculated interval HFDs.

The Pg-7 Petišovci hydrocarbon well drilled through the sedimentary sequence from Pleistocene to Karpatian, with mostly sandy clay and little coal in the Pontian part, and marl, sandstone and siltstone in the greater column section. The complete logging was performed by INA Co. (Zagreb). The first temperature logging was done in Oct. 1988 in a depth section 50-1790 m (in step every 50 m) when the well was still in drilling phase. The second temperature logging was done in Dec. 1988 (6 days after drilling had stopped) in a depth section 2500-2890 m (every 20 m), so the T-z profile is probably more thermally equilibrated (Fig. 13). The mean temperature gradient in Tertiary rocks is 48 °C/km and the HFD determination is 137 mW/m2. At depth of 2782 m the maximal temperature 146°C has been measured.

Fig. 13. Simplified geological log of the Pg-7/88 Petišovci well with measured temperatures, thermal conductivity, radiogenic heat production and calculated interval HFD values.


The Peč-1 Pečarovci well drilled through the Pliocene and thick sequence of Miocene sediments, mostly clay, sand, coal in the upper part and sandstone, marl, siltstone, conglomerate in the greater column part. Near the bottom the well drilled through the 115 m thick section of Mesozoic dolomite and dolomite breccia, and finished in the Paleozoic metamorphic rocks (mostly phyllite). First temperature logging was performed during drilling in Feb. 1991 in depth section 300-1388 m (not presented here). It showed some weak water circulation or infiltration zones, most notably at depths of 690-690 m, 740-750 m and 10601070 m. In a period 1991-2001 four individual temperatures were measured in four depths from 1862 to 2098 m, two among them after deepening at depths of 2001 and 2098 m. The maximal temperature in this well, 104°C is measured at 2001 m depth (Fig. 14). They were used for the temperature gradient determination in Tertiary rocks (45 °C/km) and together with thermal conductivity values the HFD amounts to 111 mW/m2.

Fig. 14. Simplified geological log of the Peč-1/91 Pečarovci well with measured temperatures (only those after Feb. 1991), thermal conductivity, radiogenic heat production and calculated interval HFD values.

The Mg-6 Murski gozd well drilled through Pleistocene (clay, sand) and Miocene sediments (clay, sand, marl, marly clay, sandstone, marly sandstone) from Upper Pontian to Karpatian down to 3732 m, and deeper it was finished in the Triassic (maybe also Permian) dolomitic breccia with shale. Temperatures were measured between February and March 1985 several times in deeper section of the well as BHT and during DST with maximal temperature ever measured in Slovenia, 202 °C at 3739 m depth (within DST). In April 2002 after long standstill time the point-by-point temperatures were measured in a depth section 100-1570 m. They are all presented in Fig. 15. The equilibrated temperatures together with DST values were used for the temperature gradient determination in Tertiary rocks (51 °C/km) and together with thermal conductivity values the HFD amounts to 124 mW/m2.


Fig. 15. Simplified geological log of the Mg-6/85 Murski gozd well with measured temperatures, thermal conductivity, radiogenic heat production and calculated interval HFD values.

The Be-1 Benica well drilled through the Miocene sediments from Pliocene to Badenian age with clay and sand in the most upper part, and deeper alternating marl and sandstone. Temperature point-by-point measurements in March 2001 in a depth section 200-2000 m (in irregular intervals, step of 200-500 m) showed higher geothermal gradient than the BHTs during and after the drilling in 1960 (Fig. 16). The temperature gradient is 49 °C/km in Tertiary sequence. At a depth of 2755 m the maximal temperature as BHT has been 124°C with no other curiosities. The HFD hasn’t been determined on this site.

Fig. 16. Measured temperatures in the Be-1 well with simplified stratigraphical column.


The MB-1 Maribor well drilled through the Miocene sequence of sediments, mostly marl, sandy marl and sandstone, and deeper from 639 m it was drilled in the Paleozoic metamorphic rocks, mainly gneiss, less amphibolites and eclogite. Temperatures were measured several times as point-by-point mode; the first was done on 21.11.1990 in a depth interval 10-657 m when the well was 834 m deep. After deepening to the final depth of 1331 m it was measured in the interval 200-1020 m, and BHT of 56°C was measured at 1330 m depth immediately after drilling has finished. On 1.10.1991 the next point-by-point measurements were performed in the interval 40-1000 m, then on 18.2.1992 the temperature continuous logging was done in the interval 0-970 m, and finally the best point-by-point temperature measurements were done on 17.9.1992 in the interval 40-1330 m. The maximal temperature of 60,4°C was measured at 1330 m depth. The T-z profile (Fig. 17) shows some sections with zero temperature gradient (620-650 m, 680-740 m, 760-800 m) and weaker sections deeper (870-880 m, 930-950 m) that are related to several screen (filter) intervals that enable some sort of weak water circulation. The mean temperature gradient in the Tertiary layers is 46 °C/km, and it is lower in the metamorphic rocks, just 30,4 °C/km. Together with thermal conductivity values the surface HFD at this site is determined to 112 mW/m2.

Fig. 17. Simplified geological log of the MB-1/90 Maribor well with the last point-by-point temperatures, deduced temperature gradient, thermal conductivity, radiogenic heat production and calculated interval HFD values.

The SOB-2 geothermal production well in Murska Sobota was drilled in 1988 completely in the Pliocene (clay, sand, silt, silty and sandy clay) and Upper Pontian sediments (sand, silt, sandy marl, loose sandstone) down to 887 m. Temperatures were measured several times. The first point measurements (not all along the well column) were done in the midst of drilling on 17.-18.4.1988 when the well was 580 m deep. The BHT measurements were performed during the drilling, showing at 580 m: 46,8°C, and 840 m: 57°C. The second point-by-point measurements were performed immediately after reaching the final depth on 16.-17.5.1988 in a section of 450-840 m. Then the well was tested until20.6.1988 and the third point-by-point measurements were done on 23.-24.9.1988, after about 3 months of stand-still time. The most pronounced feature of the last T-z profile (Fig. 18) is the section of 420-760 with very obvious water migration from the lower permeable sandy and silty layers around 750 m upwards to depths around 450 m where thermal water stops migrating to the surface. The


maximal temperature 60,75째C was measured at 870 m depth. The mean temperature gradient along the whole borehole is ca 57 째C/km, and the surface HFD at this site of 100 mW/m2is determined.

Fig. 18. Simplified geological log of the SOB-2/88 Murska Sobota well with the last point-by-point temperatures, deduced temperature gradient, thermal conductivity, radiogenic heat production and calculated interval HFD values.


Initial temperature distributions

Temperature data collected, corrected and put into the expert borehole database were used to extrapolate to some chosen depth levels to show the 3D pattern of the subsurface temperatures. According to the agreement between Slovenia and Hungarian partners, 4 depth surfaces were chosen: in 500, 1000, 2000 and 4000 m below the ground surface. For each surface the nearest measured temperature data were selected, and by the help of the computed gradient data along the same vertical profile, extrapolation was made to the given surface. These datasets served as the basis of interpolation for each surface. 3D temperature distribution will be given for the certain model layers, which will be interpreted from the measured data. Figs 19-22 demonstrate the temperature distribution at certain depths.


Fig. 19. Temperature distribution at a depth of 500 m below the surface

Temperature at 500 m depth East of the line Maribor-Ptuj temperatures are practically everywhere higher than 30 °C. The highest temperatures are found in area of the Murska Sobota high, from Lenart to Moravske Toplice, at Benedikt and Radenci even over 46 °C is expected, but these high temperature zones may be narrower than presented on temperature maps. Slightly elevated temperatures exist also at Lendava and Murski gozd as regard to close surrounding areas as along the Ljutomer trough. High temperatures on the Murska Sobota high are due to shallow preTertiary basement and convection zones in the metamorphic rocks that is proved at Benedikt, while further to the northeast below Murska Sobota and Moravske Toplice such zones may probably be expected. In the Hungarian side there are some areas where the temperature is higher than 45 °C: near Hévíz-lake and Sümeg region indicating upwelling thermal karst water. In the porous basin area there are also some positive temperature anomalies, just above the western part of the Nagylengyel oil-field region, at Szilágy and Csonkahegyhát. Probably the anomaly is resulted by the large anomaly at a depth of 1800-2100 m in the karstic basement, caused by upwelling branches of a regional forced convection system. Detailed, smaller scale evaluation needs to clarify this phenomenon.


Fig. 20. Temperature distribution at a depth of 1000 m below the surface

Temperature at 1000 m depth Temperatures over 46 °C are expected east of the line Maribor-Ptuj and the pattern is almost the same as for the 500 m depth. The highest anomaly exists in the area from Lenart via Benedikt to Moravske Toplice with values over 66 °C that is so far confirmed or at least almost reliably proved with temperature measurements in the boreholes at Benedikt, Murska Sobota and Moravske Toplice. Over 58 °C is expected from Lendava to the southeast (Murski gozd). The anomaly at Benedikt, and probably at Murska Sobota and Moravske Toplice is perhaps narrower than shown on map, in fact it is not certain how and in which direction it is elongated. It exists owing to some deep fracturing in the metamorphic rocks (at Benedikt at least 1 km deep), and this enables heat to be transferred by convection from depths over 1,9 km through the fractures towards Tertiary layers. The heat probably generates from a weak cooling of some basaltic or other intrusive magmatic bodies beneath the metamorphic rock complex. Temperature above 65 °C indicates positive and below 45 °C indicates negative anomaly. The positive anomaly can be connected to the deep convective flow system in the basement, as mentioned at the 500 meter isotherm map. The Pusztaszentlászló anomaly can be connected to basement high. There is no reasonable explanation for the Bajcsa and Szécsi-sziget anomaly at this moment. The negative anomalies in western, northern and northeastern direction from the mentioned Nagylengyel-W positive anomaly can be explained by downward groundwater movement in the deeper karst systems below 1800 meter.


Fig. 21. Temperature distribution at a depth of 2000 m below the surface

Temperature at 2000 m depth East of the line Maribor-Ptuj temperatures are almost everywhere higher than 80 °C. The anomaly at Benedikt is not much evident, because thermal water circulation causes there very high geothermal gradient in the Tertiary sedimentary rocks only. We may just assume that the convection zone there is not deeper than 2 km, but we don’t have so deep temperature measurements in the boreholes from Benedikt via Murska Sobota to the northeast, the boreholes at Pečarovci and Dankovci are excluded as they are located laterally from this direction. Over 100 °C may be expected at Murska Sobota, from there towards northeast to Slovenian-Hungarian border, at Veržej and at wider Lendava area. Actually lower temperatures are found in the Ljutomer-Ptuj depression (sinform) than around. In Hungary the temperature anomalies (>100 °C) at Nagylengyel-West and ZalaegerszegNorth are the consequences of the regional convection of the thermal karst. Similarly, the negative anomalies (< 75 °C) at Zalalövő, and between Nagylengyel and Zalaegerszeg are also connected to the convective karst systems, but indicate downward water movement there.


Fig. 22. Temperature distribution at a depth of 4000 m below the surface

Temperature at 4000 m depth Temperatures over 140 °C are found almost everywhere east of the line Maribor-Ptuj. In the area of Murska Sobota, Moravske Toplice, Veržej and Ljutomer they are around or over 160 °C, closer to the Slovenian-Hungarian border temperatures around 170 °C are expected, and in the wider Lendava area even over 185 °C. There at Murski gozd the maximal temperature has been measured in the Mg-6 well (202 °C at 3739 m depth). It would be interesting to repeat such deep measurements in few deepest wells there. The reason for such an anomaly at Lendava and around may be looked for in the faulted zone there that may render possible some kind of deep thermal convection in the pre-Tertiary basement. Temperatures > 160 °C are found everywhere in the southern part of Hungarian side of the TJAM area. The highest value is above 200° C at Budafa-region. There are no measured data near to this depth at the eastern part of the Hungarian side.


Heat extraction from the system

5.4.1. Natural discharges The largest natural point-like heat discharge of the area is the Hévíz Lake. The annual heat discharge of the spring-lake can be estimated, from water discharge and the outflowing temperature (T2 = 37,95 ºC) The mean annual surface temperature is T1 = 10,5 ºC, so ∆T = 27,45 ºC. The discharge of the lake is 400 l/s, so the total heat discharge is 46 MW. This refers to an annual amount of 1,45 PJ energy. Considering that groundwater circulating on the covered basement collected heat from 60 mW/m2 heat flux one can estimate a 770 km2 of heat-collecting area. A more exact solution can be provided by the regional numerical heat-


transport model, and then it should be compared to the flow system estimated by other, hydrogeochemical and hydrogeological considerations. 5.4.2. Heat loss through the surface Conductive heat flux, modified by convective groundwater flows and other natural and artificial discharges (see above) is finally directed from the Earth’s interior towards the surface. Calculating with the average – 85 mW/m2 – heat flux and the size of the Hungarian project area (ca 6800 km2), it represents 580 MW. Considering the size of the Slovenian project area (ca 2800 km2) and using the average heat-flow density of 103 mW/m2, this conductive heat flux amounts to 288 MW for Slovenian side.



The presented geothermal conceptual model provides the picture of the actual temperature field which is reflected from the 3D geological distribution and some locally confined hydrogeological effects. In both countries necessary data have been collected from boreholes and water wells, such as temperatures and temperature gradients, thermal conductivities of lithologically different rocks, and calculated heat-flow densities. The data have been jointly interpreted to render feasible the drawing of temperature maps. The values of thermal conductivity of rocks are comparable enough, those from Slovenian boreholes are in the range 0,92 to 4,6 W/(m·K) with mean values of 1,93 to 3,21 W/(m·K), while those in the Hungarian part fall in the range 1,3 to 4,4 W/(m·K). For the construction of temperature maps slightly different temperature data sets have been used in Hungary and in Slovenia, since generally a bit more BHT values are available from Hungarian boreholes and a little bit more steady-state values are available from Slovenian boreholes. Datasets have been improving on both sides recently with steady-state temperature measurements. The mean geothermal gradients in the Slovenian side are in the range 27 (Mesozoic) to 75 °C/km (Miocene), showing especially large range in younger sediments, which is similar also in Hungarian part. However, the mean geothermal gradients are very similar in different specific depths (500, 1000, 2000 and 4000 m) on both sides; for Hungarian part roughly 42 to 50 °C/km, and for Slovenian part 38 to 50 °C/km. At the deepest level (4 km) the geothermal gradients are low owing to possible Mesozoic-Paleozoic rocks at this depth in Slovenian side and/or highly compacted tertiary rocks with thermal conductivity almost as high as for Mesozoic rocks on both sides. The results are presented as temperature distribution maps at different depths (500 to 4000 m) and the heat-flow density map. Common characteristics of temperature maps are few discernible anomalies at shallow depths (500 to 2000 m) connected with local phenomenon, such as convection zone in the faulted metamorphic basement (Slovenia) and upwelling of thermal karstic water (Hungary). In the mid depths (2 to 4 km) the reasons for anomalies may be thermal convection along the deep faults in the pre-Tertiary basement and regional forced convection in the thermal karst. The results of geothermal model present the input parameters for numerical modeling of heat and flow of underground water, the latter results being described and discussed in a separate report.




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ENG Geothermal Conceptual Model