European Geologist Journal 47

Use of depleted hydrocarbon reservoirs for geothermal energy and its significance in energy transfer toward a low-carbon economy: a paper factory project in Slatina, Croatia


By Ratko Vasiljević*

ECOINA ltd, Avenija Savezne Republike Njemačke 10, 10020 Zagreb, Croatia



The city of Slatina is placed in the Croatian part of the Pannonian Basin, an oil and gas bearing region with a long tradition of exploitation and exploitation. Due to depletion of hydrocarbon reserves, since the 1980s the possibilities of using geothermal energy from the existing wells have been considered. This paper presents the project of a new paper factory in Slatina. In the first phase, it will use geothermal water directly in the paper factory for heating and in manufacturing processes. A geothermal power plant will be constructed in the second phase. In case of success, this project can serve as a good example of energy transfer toward a low-carbon economy.


The city of Slatina is situated in north-eastern Croatia (Figure 1) in the Croatian part of Pannonian Basin, within an oil and gas (O&G) bearing region with a long tradition of fossil fuel exploitation which resulted from a high degree of exploration. After the peak in 1981, the oil production in Croatia slowly declined. Although depletion of hydrocarbon reserves was still decades away, in the 1980s, the possibilities of using geothermal energy from the existing wells were already being considered. Oil and gas fields and geothermic aquifers are placed in Neogene sandstone and breccia reservoirs screened by marl at depth intervals between 1 to 5 km. Successful conversion of O&G exploration wells in geothermal fields has taken place and significant reserves of geothermal energy have been discovered in the Lunjkovec and Ferdinandovac aquifers or in the surroundings of the Molve and Kalinovac O&G fields (Figure 2). Considering these developments, it was decided to use geothermal energy from the old oil wells in the new paper factory to be built in Slatina (Figure 3).

In the first phase of the project geothermal water will be used as the heating source for water used in manufacturing processes and for the heating of halls and offices. In further phases we plan to install a geothermal power plant in order to supply the factory with electricity and possibly the Slatina City electric grid, as well.

In the area of Slatina, geothermal potential was determined based on investigations carried out in the second half of the last century. The National Croatian Oil Company INA conducted exploratory drilling for the purpose of determining oil and gas reserves from the 1960s through the 1980s. Significant reserves of geothermal water with a temperature range of 130–191°C at depths of 3,000–5,000 metres were found in more than a dozen exploration wells.

The geothermal potential of the area will be used in a new paper factory. In the first phase the paper factory will use thermal energy directly, and in the second phase it is planned to install a geothermal power plant. The idea is not to use geothermal energy for the paper factory only, but to spread it to satisfy part of the energetic needs of Slatina City in the future. During the planned two-phase development of the paper factory, the knowledge obtained is expected to ensure input for the future development of the geothermal infrastructure.

In the first phase, geothermal water will be used directly in the paper factory for heating halls and offices and in manufacturing facilities. The first phase requires a flow rate of 73 litres per second with a temperature between 150 and 200 °C (ECOINA, 2013). Capacities of similar reservoirs are between 100 to 347 litres per second (Dukić, 2012). In the second phase, the planned geothermal power plant will use geothermal water at a temperature of 190 °C and a pressure of about 80 bar. The power of the power plant is 10 MW. The second phase requires a capacity of 83 litres per second (ECOINA, 2013).

Urban planning documentation for the city of Slatina in Virovitičko–Podravska County takes into account the use of geothermal energy in the future and the synergy potential of the new paper factory (Figure 1), with the plant being recognised as the starting point in this process. The required Environmental Impact Assessment (ECOINA, 2013) was approved by a government commission and passed public review with discussion, and finally was accepted by the Ministry of Environment and Energy, Republic of Croatia (MEP 2014).

Figure 1: Location of the project (ECOINA, 2013).


The Republic of Croatia can be roughly divided in two different areas: the Pannonian basin and the Dinarides. In the Dinarides the average geothermal gradient is 0.018 °C/m. In the Pannonian basin, the average geothermal gradient is much higher: 0.049 °C/m (EIHP, 2017). Since the geothermal gradient in the Pannonian area is considerably higher than the European average (0.043 °C/m), in addition to already discovered geothermal fields, it is probable that new ones will be discovered.

Figure 2:  Project area and significant geothermic aquifers in Croatia suitable for use in geothermic binary process power plants (modified after Geoslatina, 2010).

The Croatian part of the Pannonian Basin is divided into four depressions: the Drava Depression, Mura Depression, Sava Depression and Slavonija-Srijem Depression. Oil and gas reservoirs along with source rocks are placed in a neogene complex, which is divided into formations. Since the geothermal water is connected with oil and gas reservoirs, this division is applicable in the exploration of geothermal energy. Each depression has its own division (Table 1). Dominated lithology members are sandstones and marls, with oil and gas fields, formed in sandstone reservoirs from Lower Pannonian to Upper Pontian, while some reservoirs are also placed in eroded rocks of a pre-Tertiary complex. Reservoirs are screened lithologically, with marls, or tectonically, by faulting. Slatina is placed in the Croatian part of the Pannonian Basin in the Drava Depression (Figure 2).

Table 1: Lithostratigraphic division of the Croatian part of the Panonnian Basin (after INA – Naftaplin, 1988, simplified).







Slavonsko        – Srijemska Depression

Stratigraphy unit






Vuka Formation

Upper and Middle Pliocene and Quaternary

Široko Polje




Vera Formation

Upper Pontian

Kloštar–Ivanić Formation

Lower Pontian

Kloštar– Ivanić Formation







Ivanić–Grad Formation

Upper Pannonian





Murska Sobota Formation



Lower Pannonian







Pre-Badenian, Badenian, Sarmatian

Pre-tertiary rocks, Mesozoic carbonates, Paleozoic magmatite and metamorphic complex


In the thirty years between 1959 and 1989 thirteen oil wells were drilled around Slatina City (Figure 3). The depth of boreholes ranged from 800 m to more than 5,000 m with an average geothermal gradient of 0.04 °C/m (Table 2).

Figure 3: Oil boreholes drilled around Slatina City (INA, from Geoslatina, 2010).

Table 2: Oil boreholes drilled around Slatina City (INA, modified according to Geoslatina, 2010 and Brajko, 2014).



Drilling period

Final depth


Measured temperature

Calculated geothermal gradient (°C/m)


Podravska Slatina 1 (PS-1)

May to Aug. 1959


110 °C



Podravska Slatina 1 (PS-5)

Jul.1984 to Apr. 1985


183 °C



Ćeralije (ćer 1)

Dec. 1962 to Feb. 1963


69 °C



Podravska Slatina 2 (PS-2)

Aug. to Nov. 1959


138 °C



Podravska Slatina 3 (PS-3)

Dec. 1961 to Feb. 1962


99 °C



Podravska Slatina 4 (PS-4)

Jun. to Nov. 1962


No data


Ćeralije 2 (ćer-2)

Mar. 1963


No data


Gornje Viljevo (GV-1)

Jul. 1988 to Jan.1989


172 °C at 3,943 m



Gornje Viljevo 1alfa (GV-1a)

Feb. to May 1989


191 °C at 4,500 m



Bakić 1 (BAK-1)

Jun. to Oct. 1984


178 °C at 4,872 m



Donja Bukovica 1 (DB-1)

Jun. to Nov. 1978


178 °C



Bukovica 1 (BKC-1)

Nov. 1988 to Mar. 1989


170 °C



Donja Bukovica 2 (DB-2)

Jun. to Sept. 1984


174 °C








According  to  available  temperature  logs  from  the  wells (ISOR/EFLA, 2011), at  depths  of  4–5  km the temperature reached 190 °C, i.e. the resulting measured geothermal gradient is approximately 45 °C/km (Figure 4). The temperature at a depth of 800 meters is constant and is about 30 °C, after which it grows approximately linear up to a depth of 2,800 m where it reaches a value of 90–100 °C, so the geothermal gradient in that interval is between 30 and 35 °C/km. At depths of 2,800–2,900 meters there is a temperature leap of 20 °C to 110 °C and from this depth to 4,500 m there is a mostly linear increase in temperature to 190 °C (gradient of 50 °C/km).  In some wells, the jump rates in intervals between 3,300 to 3,400 m were from 150 to 185 °C and in intervals between 3,600 to 3,700 m leaps in measured temperatures from 140 to 180 °C were found.

The temperature generally increases in depth, however at some intervals, the growth is unexpectedly sudden, and in some locations a decrease in temperature was recorded. Unfortunately, there are no detailed data on the measuring conditions so interpretation is limited by this factor. Most oil wells in the observed area were drilled between the 1960s and 1980s. At that time geothermal energy, with some exceptions, was generally not considered as a significant energetic source. Temperature loggings were made immediately after drilling, when infiltrated drilling mud was present in surrounding rocks of a well channel, which could affect the results of measurement. This could be an explanation for the drop in temperature in Well PS-5 in the interval between 3,900 to 4,500 m.

Considering the measured values in petroleum wells, a geothermic reservoir is classified as medium-temperature (100–200 °C), and geothermal energy from these reservoirs can be exploited for space heating, in various technological processes and for the production of electricity by the binary process.

Figure 4: Measured temperatures in oil wells in the Slatina area (ISOR/EFLA, 2011).

In order to determine the possibility of using geothermal water in the production process, the Icelandic company ISOR/EFLA reviewed the potential of the surrounding area for the exploitation of geothermal water.

Based on preliminary results, additional exploration activities on geothermal energy reservoirs were carried out in 2011. The studies included magnetic telemetry (MT) measurements on 61 probes. The principle of MT measurement is based on the detection of natural changes in the Earth’s magnetic field flux due to changes in the lithology in the underground, where the depth zones of different resistance are located, which in this case indicated the existence of geothermal fluids (ISOR/EFLA, 2011). MT measurements have established the location of fault zones, which would be the most appropriate goal for possible additional drilling to obtain greater yield.

The drilling target is at depths of about 3,000–4,000 m. At a depth of 3,000 m the geothermal water temperature is lower, but still significantly above 100 °C, whereas at depths of 4,000 m it would be realistic to expect temperatures between 150 and 200 °C.

UNFC-2009 classification

The reserves were classified in compliance with the UNFC- 2009 classification (Tables 3 and 4) regarding the criteria of economic and social viability (E), the maturity of studies (F) and geological knowledge (G). Concerning the criteria of economic and social viability (E), present research results show sustained discharge for use of heat in a production process. The required capacity of 73 l/s is proven by previous well testing, and the minimum required temperature of 150°C was proven by well logging, so in part for direct use it can be considered as a commercial project.

The future power plant was classified as E2 (a future thermal power plant); according to data from similar projects in Croatia, its expected capacity is at least 2 to 5 times greater than required (Dukić, 2012), but to prove this, additional measurements of capacity are necessary.

Considering the maturity of studies (F), feasibility of extraction for direct use has been confirmed, but for the further phase, the development of the thermal power plant, additional research should confirm whether the required geothermal resources are present.

A rating of G2 was given for geological knowledge (G) – quantities associated with  a  known  deposit  that  can  be  estimated with  a  moderate level  of  confidence. G categories  may  be  used  discretely,  particularly  when  classifying  solid  minerals and quantities in  place, or in  cumulative form (e.g.  G1+G2), as is commonly applied for recoverable fluids (UNFC, 2009). Although the quantity is sufficient for the first phase, it is necessary to perform further investigations in order to define reserves of thermal water and to define the impact on the thermal water reservoir.

Table 3: Classification of the project regarding economic and social viability (E).


UNFC-2009 definition

Reasoning for classification


Extraction and sale has been confirmed to be economically viable

Present research results show sustained discharge for use of heat in a production process.

Required flow of 73 l/s is proven by previous well testing, minimum required temperature of 150 °C was proven by well logging.

Synthesis of conclusions and economic feasibility was confirmed in the Environmental Impact Assessment (ECOINA, 2013).


The classification of E1.1 only applies to the heat for direct use in production.


UNFC-2009 definition


Extraction and sale is economical on the basis of current market conditions and realistic assumptions of future market conditions


UNFC-2009 definition

Reasoning for classification


Extraction and sale are expected to become

economically viable in the foreseeable future

The classification of E2 refers to future thermal power plant. According to data from similar projects in Croatia, expected capacity is at least 2 to 5 times greater than required (Dukić, 2012).

The factory will use electricity for its own purpose, and according to urban planning documentation there is an interest to use electricity from geothermal power plant in the city of Slatina.

Additional measurements of capacity and temperature will be done in the first phase.


Table 4: Classification of the project regarding the maturity of studies (F) and geological knowledge (G).


UNFC-2009 definition

 Reasoning for classification


Feasibility of extraction by a defined development project or mining operation has been confirmed

For the first phase of the project (direct use of heat), exploration, well testing and simulation are complete.


UNFC-2009 definition


Sufficiently detailed studies have been completed to demonstrate the feasibility of extraction by implementing a defined development project or mining operation.


Project activities are ongoing to justify

development in the foreseeable future.


Preliminary studies and experience from similar projects indicate the feasibility of development of a geothermal power plant, which is boosted by interest of a local community. Additional research should confirm whether the required geothermal resources are present.


Quantities associated with a known deposit that

can be estimated with a moderate level of confidence.



Contribution to transfer to a low-carbon economy

In order to prevent dangerous climate change, in October 2014 the leaders of Member States adopted the Climate and Energy Policy Framework of the EU for the period from 2020 to 2030,  which includes a binding EU target of at least 40% lower greenhouse gas emissions by 2030 compared to 1990 (UNFC, 2009).

For example, the expected electrical efficiency for a natural gas electric generator is approximately 50% (GE, 2015), that means for each MWh of produced electric energy it is necessary to combust 2 MWh of natural gas. The power of the planned geothermal power plant is 10 MW, which theoretically means 240 MWh of produced electrical energy per day or 87,600 MWh of produced electrical energy per year. In gas consumption that means a savings of 480 MWh (1.73 TJ) of natural gas per day, or 175,200 MWh (630.72 TJ) of natural gas per year.

Rising energy prices and supply instability have led to a serious increase in interest in developing geothermal resources for electric power generation, and turned to a completely new way of understanding country’s geothermal potential  (Kolbah et al., 2015). The average energy capacity of geothermal direct heat consumption in Croatia is 3–4 MWt and it is expected that there is another 1,500 to 2,000 MWt, which could generate as much heat as 600 million m3 of natural gas per year (Kolbah et al., 2015).

Each terajoule obtained by combustion of methane emits 54.9 tons of CO2, that means the emission factor of Methane is 54.9 tCO2/TJ (Commission Regulation 601/2012), which means a decrease in greenhouse gas emissions by 95 tCO2 /day, or 34,627 tCO2 / year. These annual emissions of CO2 are relatively low, and an installation with these emissions would be classified as category A, but obtained experience is expected to enable development of further capacities in the future. Another stimulus can be found in the EUETS Directive 2003/87/EC, where it was prescribed that a significant part of the revenues generated from the auctioning of allowances should be used, among others, to develop renewable energies to meet the commitment of the EU to renewable energies (Article 10(3)). No free allocation shall be made in respect of any electricity production (from combustion) by new entrants (Article 10a(7)) and some allowances will be available to stimulate the construction and operation of projects that aim at the environmentally safe capture and geological storage (‘CCS’) of CO2, as well as innovative renewable energy and energy storage technologies (Article 10a(8)). That actually means destimulation of the production of electricity from fossil fuels and encouraging production from renewable sources.  


A new paper factory planned in the Slatina area of Croatia will use geothermal energy. The Environmental Impact Assessment (ECOINA, 2013) proposed two phases of project development. In the first phase detailed monitoring of geothermal reservoir will be performed, and if it is feasible, a geothermal power plant will be installed in the second phase.

The first  phase  in  the  paper  factory  requires a capacity  of  73  l/s  with  a temperature between 150 and 200 °C. According to data from the Croatian national oil company INA (ISOR/EFLA, 2011), all existing wells are capable of ensuring the required capacity. Two wells of up to 4,000 meters deep are planned, one for exploitation and one for injection, at the factory location.

Since the original measurements were performed primarily for oil exploitation, it will be necessary to perform additional measurements along with the re-evaluation of previously performed measurements in  the older boreholes to  precisely define temperatures and capacities during  the  first  phase  of  geothermal energy exploitation in  order  to determine the feasibility of the electric power plant that is planned in the second phase.


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This article has been published in European Geologist Journal 47 – Geology and the energy transition.