Geothermal Atlas represents the concise textual and graphic review of all previous knowledge about geothermal characteristics of the Earth crust in the territory of the Republic of Srpska.
Regarding very complex geological-tectonic relations of the territory, the Atlas emphasizes that previous explorations did not solved many questions important for more complex understanding and use of the resource. However, the results obtained during the period longer than a half of a century obviously point out to the prospective and validity of further investments into the geothermal explorations in the territory of the Republic of Srpska.
An important characteristic of the geothermal energy is a fact that it represents clear and renewable energy source, and it differs this kind of energy from the fossil flues (coal, oil, gas, bituminous rocks) and nuclear resources (uranium, thorium) whose reserves are limited and non-renewable.
The Geothermal Atlas is conceived through the following chapters:
- The role of geothermal energy in the complete energy production;
- Common geological and hydrogeological characteristics of the territory of the Republic of Srpska;
- The origin of geothermal energy;
- Types of geothermal potentials and geothermal characteristics of the territory of the Republic of Srpska;
- Basic geothermal parameters, prognoses and supposed isotherms in the depth of 500, 1000 and 2000 m under the surface;
- The possibility of use and utilization of geothermal resources in the territory of the Republic of Srpska;
- The occurrences of thermal water in the territory of the Republic of Srpska, which were not the object of the detail hydrogeological explorations and the priority zones for further geothermal explorations.
The Atlas also contains detail characteristic of geothermal (rather hydrothermal) exploitation fields in textual and graphic form, basic characteristics of exploitation objects, natural occurrences of thermal water, perspective regions and zones for further geothermal researches, analyzed hydrochemical characteristics of water, the prognoses of possible temperatures in the primary aquifers where they have not been exactly defined until now etc.
The importance of further geothermal researches is emphasized especially in the area of Banja Luka and Semberija, because of prospective resources, potential users and the possibility of significant reduction of the fossil fuel use. The first edition of the Atlas, especially the part related to the given prognoses, should be verified through the results of further geothermal researches.
PRIORITY ZONES FOR FURTHER GEOTHERMAL RESEARCHES IN THE TERRITORY OF THE REPUBLIC OF SRPSKA
The previous results of complex geothermal researches in the territory of the Republic of Srpska indicates the possibility of defining priority regions for further geothermal researches, primarily in the aim of determination of hydrothermal potentials (all regions given in the Figure 3). According to the priorities they are:
1. Large artesian basins in the north part of the Republic of Srpska
2. Central ophiolite belt
3. Zone of the Jurassic-Cretaceous flysch
4. Una-Sana Paleozoic
5. Durmitor block in the south-east part of the Republic of Srpska
6. Paleozoic in the east part of the RS with massive intermediary vulcanite and the following tuffs.
According to the priority several subregions could be separated within the above-mentioned region for further researches.
The zone of artesian basins represents a zone with proven low and middle-enthalpy hydrothermal systems in the area of Semberija (Dvorovi, Slobomir, Bijeljina) and Posavina (Domaljevac, Federation of BiH). The area of Semberia is the most prospective for further geothermal researches and expected temperatures of fluids could be higher than 100⁰C in the depths about 2500 m, especially in the area of Bijeljina. Similar systems in the area of Posavina also could be explored. This possibility exists based on the research results in the area of Domaljevac (Federation BiH). The current level of knowledge (especially the data related to the boreholes OB-1 in Obudovac and BR-1 in Brvnik) shows that there is a need to be careful. The prospective zone is the area from the borehole OB-1, towards BR-1 and further to the north. Hydrothermal systems within the Cretaceous and Triassic sediments in the areas to the west of the line Obudovac-Domaljevac have not been detected. The other parts of this region have not been the object of the detailed geothermal researches yet (a certain potentiality could be just supposed in the remaining parts of the large artesian basins of the Republic of Srpska).
Within the Central ophiolite belt there are numerous thermal and thermomineral springs (springs in Sočkovac, Teslić, Kulaši, Slatina, Lješljani etc), and the boreholes (maximal depth 672 m in Lješljani, Novi Grad) in the convective drain zones cover more significant amounts of water with higher temperatures than the temperatures of the springs are (the highest temperature difference between a borehole and a spring is obtained in Lješljani and it is 11⁰C). The primary research task within this zone is the drilling of deep boreholes (deeper than 1000 meters) to the aquifer (the assumption is that the primary aquifers had been formed in the Upper and Middle Triassic limestones) with significantly higher water temperatures (probably 80⁰C) that those registered in the boreholes and springs in this zone (the higher determined temperature is 40⁰C in the boreholes in a park in Slatina, Laktaši municipality). An ophiolite massif near Banja Luka with its Tertiary basin and the Jurassic-Cretaceous flysch and wider area of Prijedor – Prijedor depression are very important.
In the area of the Una-Sana Paleozoic there are no determined thermal and thermomineral waters, just springs with higher temperature, e.g. a spring 4 km to the south of Oštra Luka whose temperature is 17.4 ⁰C. There are thermal water springs (Т=25.3⁰C) in Kozačka Ilidža and in Sanska Ilidža (Т=32⁰C) near the entity border in the Federation of BiH. The Una-Sana Paleozoic, especially a part near the entity border, has a certain potential for further geothermal researches in the territory of the Republic of Srpska.
Within the Durmitor block in the east part of the Republic of Srpska there are several springs with higher water temperature from the Devon limestone (15⁰C in Čeljadinići, the Federation of BiH, near the entity border). There are no springs whose water temperature is more than 20⁰C on the surface. In high depths it could be expected to find higher temperature waters than the one registered in Čeljadinići.
In the zone of the Jurassic-Cretaceous flysch there is only one registered thermal water spring (precisely thermomineral) and it is located in the contact zone with the Ophiolite belt to the south of Banja Luka (the area of Šeher – Toplice). The temperature of the springs which occurs near the Vrbas River is approximately 30⁰C. The same temperature is determined in the boreholes, but with significantly higher amounts of energy (large quantities of thermomineral water). In other parts of this flysch there are no thermal springs registered on the surface, but it is supposed that the accumulations of thermal and thermomineral waters could be found in the flysch sublayer, but in significantly higher depths.
The canyon of the Drinjača River in the east part of the Republic of Srpska is also prospective for geothermal energy researches, especially a part between Perin Grad and Raševo (Milići and Šekovići municipalities). This zone and the Višegrad thermal anomaly are the only areas in the east part of the Republic of Srpska where the spring water temperature is higher than 20⁰C. High-temperature waters (14-18⁰C) occur in the Knežina River watershed near Sokolac and in the area of Rogatica and Rudo, so they are also prospective for further researches.
According to N. Miošić (1986) in the area of large artesian basins and the Jurassic-Cretaceous flysch in the depths greater than 3500 m there could be found even thermo geopressed zones. The same author considered that the petro geothermal potentials could occur within the volcanite of the east part of the Republic of Srpska (5a), the Neocene volcanite in the Ophiolite belt and intrusives of the Prosara and the Motajica Mts. Their depths and other characteristic are not known.
This Geothermal Atlas represents brief textual and graphical overview of the current knowledge about geothermal characteristics of the Earth crust in the territory of the Republic of Srpska.
It is more than obvious enhancement in consumption of the all types of energy resources in the world during years. During the last few decades, significant attention has been directed to production of the geothermal energy, especially for electricity production, as a kind of the relatively clear and renewable energy. It is the reason why The International Energy Agency (IEA) insists on enhancement of this one type of the energy resource.
The focus in these papers is on the hydrogeothermal systems. It is the only type of the geothermal resources in use in the Republic of Srpska, in the world as well (if we exclude some smaller projects of use of the geothermal energy from hot dry rocks). The knowledge obtained during researches in the last forty years of the 20th Century and during the 21st Century obviously indicates the justification of the further investments in the research of this kind of energy resource.
The territory of the Republic of Srpska is characterised with very complex geological and tectonic setting. Its territory comprises the central part of the Dinarides orogene system and in the north the smaller part of the south edge of the Pannonian Basin. In the hydrogeological sense it is possible to define a few different regions. Each of these regions characterises with specific geothermal characteristics. The most important defined structures in geothermal sense are the large artesian basins in the north and the central-ophiolitic zone in the middle part of the Republic of Srpska. Only massif of the carbonate Mesozoic rocks (eastern Herzegovina and smaller west parts of the Republic of Srpska) is without any practical importance regarding to the exploration of the geothermal resources (figure 3).
The geothermal gradient, as one of the main geothermal parameters, generally increases from the south to the north of the territory of the Republic of Srpska. The biggest value of geothermal gradient is registered north of Bijeljina (about 50⁰C/1 km). The maximum convective value is higher. The heat flow shows similar spatial distribution like the geothermal gradient. The largest value is more than 110 mW/m2 (north of Bijeljina as well).
The most important proved hydrothermal resources are situated within the large artesian basins in the north part of the Republic of Srpska. The results of the deep drillings in Semberija and Posavina (mostly primary drilled for oil purposes) indicate that rocks of the Mesozoic age (the Cretaceous and the Triassic limestones) represent very important aquifers of thermal (Semberija) and thermo-mineral water (Domaljevac, the Federation of Bosnia and Herzegovina, near the entity border). The temperature of water on the mouth of wells is between 75⁰C (Dvorovi) and 86⁰C (Domaljevac). Expected temperature beneath Bijeljina town is more than 110⁰C (on 2500 m depth). This temperature allows electricity production in binary plants.
Within the Central ophiolite zone, opposite to the large artesian basins, numerous occurrences of thermal and thermomineral waters are registered at the surface of the terrain. The temperatures of the waters are various and significantly lower than the temperatures of the waters from the boreholes in Semberija and Posavina. In the locations with occurrences of water with temperatures more than 20⁰C there are usually drilled boreholes, deep up to 300 m (just in Lješljani location in the N-W part of the Republic of Srpska the borehole SB-1 is 672 m deep). Using the boreholes, significantly larger amounts of energy are obtained (much more water with higher temperatures). The hottest water within zone of the ophiolite has been drilled in Slatina (location “Park”), municipality Laktaši (43⁰C). The highest difference between spring and borehole (on the mouth) water temperatures has been obtained in Lješljani (Novi Grad municipality) and it is 11⁰C (spring 20⁰C, on the SB-1 well mouth 31⁰C). The locations with currently active geothermal fields in the territory of Republic of Srpska are given in the table bellow and also in figure 9 (with the temperatures of water from wells).
The boreholes with thermal and thermomineral water in use in the RS currently give about 44 MWth (and about 3 MWth on springs), but it is possible to obtain much more energy from hydrothermal systems. For example, under Bijeljina city it is realistic to expect that the water temperature would be more than 110⁰C in the depth higher than 2.500 m. Also, it is realistic to suppose (based on the results of previously researches) that it is possible to obtain about 20 l/s per well. It means that more than 10 MWth per well should be obtained.
It indicates that much more energy from hydrothermal resources can be obtained thereby new wells in the territory of the Republic of Srpska. Also, in some areas (e.g. Slatina, Laktaši municipality) it is possible to obtain much more energy in the existing wells. The accounted available energy of 9.6 MWth in Slatina (four wells), at the moment, is limited just with the pumps characteristics, but it is available to obtain, (not less than) additionally 4 MWth just by using the appropriate pumps (with higher pumping capacity). Similar situation is in the other locations given in the left table. A significant data is that neither one borehole within the central ophiolite zone reached the primary aquifer of thermal or thermomineral water (the assumption is that primary aquifers are situated in the Middle and the Upper Triassic limestone). In the zone of central ophiolite, thermal and thermo-mineral waters are captured exclusively in the zones of the deep faults which represent ascendant drains of the thermal waters from the primary aquifer. Geochemical thermometers indicate that the temperatures in the primary aquifer are up to 80⁰C. It is necessary to verify the fact by drilling of deep boreholes (deeper than 1000 m). The deepest borehole within the Central ophiolite zone is above-mentioned borehole SB-1 in Lješljani, the municipality of Novi Grad (672 m). This borehole is with artesian level and produces about 7 l/s (the nearest spring is with T=20⁰C and discharge just 0.3 l/s) with the temperature of 30⁰C on the mouth. Unfortunately, this borehole has not been finished on the planned depth and in the primary aquifer (finished in the Jurassic “diabases-hornstone” formation). It is very interesting that in thermal waters of Lješljani there are detected higher homologues of carbon which are older than the Jurassic age. This is one of possible indicators of oil in this area.
Other potential regions for geothermal explorations are following: the zone of the Jurassic-Cretaceous flysch, the Una-Sana Palaeozoic, the Durmitor block of eastern part of the Republic of Srpska and some zones of the Tertiary magmatism (Srebrenica, Prosara, Motajica etc.).
The map of expected temperatures in 500, 1000 and 2000 meters (figure 6, 7 and 8) was based on the data from the deep boreholes in the territory of the Republic of Srpska, also the Federation of Bosnia and Herzegovina and neighbouring countries (especially Croatia). The last mentioned map (expected temperatures on the 2000 m beneath surface) indicates that expected temperature of the rocks and fluids in the Eastern Herzegovina is just about 30⁰C. But in Semberija and Posavina there are some areas with expected temperatures more than 100⁰C (on 2000 m depth).
Currently use of thermal and thermo-mineral water is modest in comparison with available reserves. Mostly for spa and just in few spa (Slatina, Dvorovi, Laktaši) water, beside balneology, uses for heating (with heating pumps or directly), at in the other locations use is mostly for balneology and recreation. Relatively modern spa centres based on thermal or thermo-mineral water are located in: Teslić, Laktaši, Slatina, Višegrad and Dvorovi. Just geothermal field Sočkovac, municipality Petrovo, has facility for capturing of the CO2 and other technical gases. These days are actual considerations about using of hydrothermal systems for district heating of Banja Luka and Bijeljina.