engleski

Characteristics and utilization of geothermal aquifers of Bjelovar subdepression

North-western part of Croatia represents the southwestern margin of the Pannonian Basin System (PBS), and it is characterized by high average geothermal gradient (49 °C/km) and surface heat flow (76 mW/m2) and Bjelovar Subdepression is situated in this part of Croatia. It is a part of Drava depression and extends towards Sava depretin, margined by mountains on the northwest Kalnik Mt., Bilogora Mt. on the northeast, Papuk, Ravna gora and Psunj Mts. on the east and southeast and on the south is Moslavačka Mt. Geothermal springs occur at the east – southeast edge of the subdepression. Thanks to intensive hydrocarbon explorations in 70ties, 80ties and 90ties, numerus geothermal reservoirs were discovered. Two main types and four subtypes of geothermal reservoirs were determined and delineated in the Bjelovar Subdepresion that presents different types of porosity. First type includes basin reservoirs which are formed within siliciclastic formations and divided in two subtypes: i) Upper Pannonian sediments – delta front and shelf delta plain formation where water temperatures ranges from ‘’cold’’ until 30°C ; ii) Lower Pannonian sediments – turbidity formation where water temperatures ranges from 20 until 40°C. The water ranges from Ca-HCO3, Na-HCO3 until Na-Cl and it contains high amount of dissolved organic matter, ammonium, iron and hydrogen sulphide. The second and the most important type, includes reservoirs in basement sediments which can be build up from i) fractured/karst carbonate sediments (dolomite – mainly Triassic age and limestones – Triassic/Cretaceous age) and/or ii) fractured/fissured crystalline metamorphic rocks of Paleozoic age. With these reservoirs are associate the Miocene sandstones and carbonate sediments often overlying on Triassic carbonate sediments. Within these reservoirs water temperatures ranges from 40 until 170°C. The water ranges from CaMg-HCO3 until Na-HCO3 and water from deep wells contains high amount of CO2. Now, only basement reservoirs are utilized. Geothermal waters which have temperature from 45 until 50°C are used for balneological and recreation purposes in the town Daruvar while water of 170°C in Velika Ciglena near the town Bjelovar is used for electricity production. There are also plans for the town Križevci – district heating of three schools, gymnasium with swimming pool and greenhouses as part of the agriculture department of the High school and College of agriculture. During hydrocarbon exploration, several locations had been found where basement reservoirs have temperatures higher than 70°C (Letičani, Pavljani, Velika Trnovitica) but they need to be investigating in more details. Three counties (Bjelovarsko-bilogorska, Koprivničko-križevačka and Zagrebačka) have territory in this subdepression and in the area of these three counties, agricultural production and small and medium enterprises are developed. So, in the future, utilization these geothermal aquifers would help to ensure sustainable goods production and protection of environment (for example reduction of CO2 emission).

Geothermal aquifers ; Basin fill type ; Sustainable management ; Bjelovar Subdepression

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