European Geologist Journal 43
Geothermal subregions in Upper Pannonian sediments: A case study from East Hungary
by E. Buday-Bódi, R. W. McIntosh and T. Buday
Local variations in sedimentological, structural and temperature conditions and in physical properties fundamentally determine the exploitation possibilities, supply conditions and thus sustainability of geothermal energy utilisation within geothermal plays. Based on such varying factors geothermal plays can be divided into geothermal subregions; different utilisation types should be aimed for in each region to achieve successful and sustainable geothermal energy utilisation. In this paper, a possible division into geothermal subregions of the Upper Pannonian succession in the study area is presented. Discussion of the subregions is based on the analysis of various maps: thickness, average depth and temperature, tectonic structure, seismic patterns and energy density. Identification of subregions with different conditions could help regional planning to provide environmental and economic sustainability for long-term thermal water utilisation.
Geothermal play types show incredible diversity worldwide, but diversity is also present locally within one play type, where greater complications are not apparent, but affect geothermal utilisation possibilities (Moeck, 2014). This is true for the reservoir-forming sandy and clayey Upper Pannonian sediments in Hungary, which are given in this paper as an example of the conduction-dominated geothermal play type and which is classified as an intracratonic basin type in Moeck’s system (2014). The present paper aims to study the local variability of a study area in East Hungary and to investigate the parameters that could help in the identification of geothermal subregions.
In the present study the term ‘geothermal subregion’ refers to a unit within which geothermal characteristics show a high degree of homogeneity (in structure, heat potential, water content, uncertainty, risk of investment, etc.). Designation and characterisation of the geological conditions are considered the main basis of the division of an area into geothermal subregions. This paper attempts to analyse which parameters should be taken into account when subregions are identified and how to compare subregions. There is also discussion about the scale of the relative differences among subregions and whether they are significant enough to be taken into account in planning. The authors hope that the conclusions of the present paper may prove to be useful for resource assessment, sustainability and renewable energy investments of the region.
Upper Pannonian sediments are widespread in the Carpathian Basin and have been studied for decades in a wide scientific spectrum (e.g. Vakarcs, 1997; Juhász et al., 2006). Their role in geothermal energy utilisation is important due to the depth and temperature of the sandy reservoirs and the relatively uniform geological set-up from which thermal water is extractable. In this paper, we handle this geological unit as a play that can be divided into a group of geothermal subregions and study the processes forming these subregions based on seismic interpretation and focusing on local variabilities.
Demarcation of the study area was made with the aim of studying the local diversity of the well-known and well-studied thermal water bearing Pannonian s.l. (sensu lato) formations, developed near the northeast margin of Lake Pannon at the time of its greatest extension. For this reason, the boundaries of the study area were defined to represent beddings variable in morphology, tectonic settings, petrology and to include relatively large cities with spas and baths. This seems important since major spas and baths are located within the same region and all utilise thermal water from the Pannonian reservoirs (Buday et al., 2015); however, considerable differences may appear among them. Therefore, different ways of managing thermal water utilisation may be needed, from extraction to re-injection or wastewater treatment.
The geodynamic evolution of the Pannonian Basin outlined by Horváth et al. (2015) explains why good geothermal reservoirs can be found in the Miocene to Quaternary basin fill sediments and in karstified Mesozoic carbonates in general. If regional details are studied, differences in exploiting such reservoirs can be revealed. According to Fodor (2010), from the Jurassic until the Pliocene twelve deformation phases occurred with at least two rotation phases. Stress field analyses show that NE-SW, N-S and NW-SE compression alternated with N-S and NW-SE extension and also with extrusion towards E and NE. This complex tectonic history resulted in the formation of a great variety of micro- and macro-tectonic structures that were partly preserved, while later stress fields formed their own resultant structural features, occasionally re-activating the preserved older ones as well. As a result, the type, size and orientation of structural elements (e.g. normal and thrust faults, horizontal and transfer faults, positive and negative flower structures in transpression and transtension zones, folds and multiple superposed folds) can be highly diverse in any study area in the Pannonian Basin. The identification of the basic characteristics and roles of the structural elements is essential when geothermal subregions are typified.
Neogene magmatism caused by the tectonic evolution of the Pannonian Basin resulted in volcanic and volcanic-sedimentary depositions at variable depths in the Transtisza Region. Since volcanic events during the Miocene occurred in several periods, the lithology of these successions is variable. Their presence is explained by subsequent compressive movements of NNW vergence, forming volcanic depressions (Széky-Fux et al., 2007).
The relative vertical movements (subsidence and uplift) played a decisive role in the complexity of Pannonian sedimentation. Based on a comprehensive basin analysis, Juhász et al. (2006) concluded that Late Pannonian geological development was driven by a complex system of intrabasinal tectonic movements and climatic factors, with the greater sedimentary units (3rd order sequences) being affected by tectonic movement and the smaller units (4th order sequences) by climatic factors. They also stated that the water level changes within the basin cannot be correlated with the global sea level changes at the Pannonian stage due to the absence of convincing analogies.
The term Upper Pannonian refers to a sedimentary unit accumulated in Lake Pannon based on facies characterisation represented by delta front and delta plain. Due to relative water level changes, several delta front and delta plain facies units formed within a vertical column, but in some cases erosion modified them. Ages of these facies show slight differences throughout the basin: from the marginal areas towards the basin centre younger delta units occur regarding the stages of infilling.
In the case of Late Pannonian geological development, researchers prefer treating the pre-sedimentary state, geodynamics of the basin and syn- and post-sedimentary development of the research area separately. Syn-sedimentary development is characterised by palaeoenvironmental variability, relative water level changes, and influenced by local morphology and climate controls, while post-sedimentary development is determined by tectonic movements, compaction and subsidence. These post-sedimentary factors may influence the syn-sedimentary stage, as well. The rate of subsidence and rate of crustal heating processes should be also considered, since lower geothermal gradient values are measured in areas with fast and long-term subsidence.
The 75×100 km research area is located in NE Hungary, in the northern central part of the Pannonian Basin (Figure 1). Its Pre-Neogene basement is composed of belts of crystalline rocks, Mesozoic carbonates and siliciclastic rocks and flysch, the depth of which varies between 1000 m and 5500 m (Haas et al., 2010). These basement units are structurally complex (with major faults, nappe boundaries, imbricated zones). The basement is partially covered by Neogene volcanic rocks, the depth of which varies between 500 m and 3000 m (Széky-Fux et al., 2007). The overlying Pannonian sediments have an average thickness between 500 m and 2500 m (Bérczi and Phillips, 1985). The series is covered by 200–400 m thick Quaternary sediments.
This marginal position of the study area and the cyclic relative water level changes in its Pannonian development resulted in the order of the Pannonian formations being somewhat different from what the general concept would suggest. For instance, delta slope and delta front sediments, traditionally named Lower Pannonian and Upper Pannonian sediments, respectively, are interfingered, thus repeatedly occur in a vertical profile. Elsewhere delta front and delta plain sediments are pinching out, whichinfluences geothermal potential and utilisation only moderately. Consequently, the sub-play division to be presented in this paper could be more informative for potential thermal water users than a facies-based division.
Figure 1: Location of the research area with the interpreted seismic sections.
Tools and methods
The analysis was based primarily on seismic sections striking NE-SW and NNW-SSE (Figure 1). In the north-eastern part of research area there are no seismic sections, therefore these areas are not involved in the analysis.
The bottom and top surface of Upper Pannonian units were identified with the interpretation of the seismic sections, while for further study of the area (e.g. for identification of the geological column or geothermal gradient values) data from wells were also applied (Buday et al., 2015).
The following maps (Figure 2) were applied in the analysis, composed using the software Surfer and GeoMedia:
Figure 2: The map layers used for the designation of geothermal subregions.
The bottom surface and top surface of the Upper Pannonian succession were identified on the basis of 2D seismic section interpretations and projected well data. The obtained two-way travel time (TWT) values (data in time dimension) were converted into depth values (data in length dimension) based on regional analogies. For further calculations, data sets were used in the form of grids and block units.
- Average depth and temperature
In this paper, average depth of the reservoir is calculated as the average of the bottom and top surface depths. Average temperature values are calculated using these average depth values and the temperature-depth function characterising the area.
- Tectonic structure
Various tectonic structures occur in the study area. Moreover, the possibility of the presence of vertical movements of non-tectonic origin cannot be excluded. Normal faults, thrust faults, backthrusts (?) and positive flower structures (?) are the identified tectonic structures.
- Main patterns on seismic sections
The most distinctive seismic patterns were also detected and seem to be area-specific. Each provides different characteristics to the area, such as a pattern of sinusoidal layers, slightly wavy layers, parallel layers, interfingering successions, pinching outs, incised submarine canyons and valleys or an inclining pattern of prograding delta slope.
- Energy density
Several potential calculations exist for its estimation; defining the energy content of a volumetric unit can be the basis of geothermal potential calculations (Muffler and Cataldi, 1978). For reservoir characterisation, parameters such as thickness, temperature distribution (depending on depths of the reservoir and geothermal gradient), porosity, volumetric heat capacity of rock matrix and also of fluids have to be determined. While thickness has a geometrical nature, the other parameters can be estimated by approximate depth-dependent functions. As a consequence, more accurate heat content determinations are strongly based on the vertical density of available data related to the reservoir. For the analysis the energy density was determined as the energy content per unit area of the total vertical unit of the certain Upper Pannonian sediments. According to our research, less detailed calculations are also sufficient to identify subregions in this area, which in the case of depth-dependent parameters are based on the probable values at average depth. This can be a useful approach if a parameter is slightly or linearly dependent on depth.
- Geothermal subregions
Based on the energy density and pattern types, there is an opportunity to design spatial geothermal units, so called subregions that occur both vertically and horizontally within similar characterisation. Boundaries were determined by the spatial extent of interfingering and the highest gradient values of energy density. In addition, the identified tectonic elements were also considered in the designation process.
The above-mentioned aspects strongly depend on the position, quality and quantity of data, which generates a certain degree of uncertainty in the geothermal subregion map.
The study area is divided into 8 geothermal subregions, presented in Table 1, Table 2 and Figure 3. Areas lacking data are also identified. It is important to note that the boundaries are belts or transitional zones rather than exact lines and the defined characteristics show similarities at some points. The following statements can be made, apart from the raw data.
Table 1: Characterisation of geothermal subregion: characteristic depth (CD), maximum depth (Dmax), thickness, characteristic temperature (CT), maximum temperature (Tmax) and geothermal gradient (GG).
|River Tisza “valley”
Table 2: Characterising patterns, detected tectonic elements and theoretical geothermal potential of geothermal subregion.
|Theoretical geothermal potential
|prograding succession of delta slope facies occurs at bottom of the unit
|normal faults as a subregion border to Subregion II, intra-Pannonian tectonic elements not identified, lack of data on N subregion border
|not uniform, relatively high values in the SW part due to subsidence
|parallel, slightly wavy units
|normal faults along the subregion border to Subregion I, intra-Pannonian faults also occur
|parallel, slightly wavy units, pinching outs
|flower structure, normal fault, thrust fault
|interfingering successions, incised submarine canyon, valley
|lack of notable seismic elements
|interfingering successions, inclining pattern of prograding delta slope
|normal fault, thrust fault, flower structure fault set(?), non tectonic vertical elements (?)
|moderate to high values
|interfingering successions, parallel units
|normal fault, Ebes Thrust, fault set related to nappe
|moderate to high values
|probable interfingering successions, pinching outs, subsiding trough
|normal fault, nappe (bedrock)
Figure 3: Geothermal subregions of the studied area.
Subregion I is the least uniform due to the increasing thickness of Pannonian sediments towards the southwest. In the northern part of this subregion the basement clearly influences the geometry of the reservoir and the supply of thermal water, while in the southern part this effect is less significant, resulting in more favourable extraction possibilities (Figure 4). In most of the subregion wide utilisation spectrum is expected.
Subregion II is structurally one of the most complex subregions; several faults are identified, many of which are related to the strongly fragmented basement. The thickness and continuity level of Upper Pannonian sandy units are variable here. This significantly influences water supply (Figure 4); however, in most parts of the subregion a wide utilisation spectrum is expected.
In Subregion III the thickness of the sediment series is similar to that in Subregion II, but here they have more of a parallel geometry, and can be characterised by slightly lower average temperature values. Conditions for thermal water extraction are favourable; the delta front and delta plain sediments have high transmissibility and the available well head temperatures are as high as 75 °C, ensuring a wide utilisation spectrum.
Subregion IV is characterised by low thickness of Upper Pannonian sediments and small depth of the reservoir, which can be explained by slow syn- and post-depositional subsidence (Figure 5). Instead of fracturing, folds are the overrepresented tectonic elements here, which is considered more favourable from the aspect of the continuity of sand bodies. However, the reservoirs of this subregion are the least explored in terms of geothermal properties. Due to the low reservoir temperature the main geothermal utilisation could be balneology and certain types of agricultural utilisation.
Reservoirs of Subregion V are found at greater depth due to intense syn- and post-depositional subsidence. Geothermal gradient, however, is small due to this intense subsidence. As a result, geothermal gradient is moderate despite the relatively great depth. Within the study area, the accuracy of the determination of geothermal gradient is the most uncertain in this subregion. Due to the interfingering facies, only sandy parts of the sediment series can be exploited, which decreases the possibilities of geothermal energy utilisation.
Subregion VI, similarly to Subregion V, is part of the interfingering zone and the prograding delta slope facies are thick, making the identification of the bottom of the Upper Pannonian sediments doubtful. A relatively small number of exploitable strata can be identified, thus utilisations with low or medium water demand are expected. However, temperature values are higher than in the previous subregions.
Subregion VII is strongly dominated by interfingering facies and tectonic effects that decrease the exploitation possibilities of thermal water, since thick clay bodies vertically divide Upper Pannonian sand layers, decreasing transmissivity. Despite this, the most intensively exploited thermal water reservoirs of the research area are located in this subregion (Buday et al., 2015). In addition, the vertically disconnected sand bodies allow advanced thermal water management (e.g. different yield from different reservoirs with different temperatures and higher levels of medical water reservoir protection).
In Subregion VIII there is a deep trench in which thick sand bodies occur. Certain strata dip towards the axis of the trench (Figure 5). Although thermal water can be extracted from the sand bodies, their supply area is either limited horizontally or cooler water moves in the inclined sand bodies of the flank. This results in lower geothermal potential compared to adjacent areas to the north.
Figure 4: Interpretation of seismic section PO-81.
Figure 5: Interpretation of seismic sections DE-9 and NY-6.
It can be stated that palaeoenvironmental variability, local morphology and subsidence are factors influencing Pannonian geothermal subregions. These factors affect the thickness and geothermal gradient values of the study area. Subsidence and an originally deeper accumulation space result in sedimentary succession in greater thickness, while geothermal gradient values are less favourable; therefore, thickness alone cannot guarantee excellent geothermal potential values.
Further geothermal subregion identifying factors are the following sedimentary features: interfingering strata, pinching outs, incised canyons and valleys, and thickening prograding delta slope strata. These result in various lithologies and the presence of clayey strata, which may decrease the transmissivity and limit thermal water recharge, thus significantly affecting subregion parameters. Overlying sediments are preformed by basement structures so strongly that most subregion boundaries can be identified along them, thus the boundaries notably follow the SW-NE strike.
Intra-Pannonian tectonic features alone do not seem to be subregion forming factors, however, within a subregion they play a decisive role in subregion characterisation, and it would be worth studying their effects on exploitation, as they definitely seem to be a factor affecting geothermal potential and utilisation possibilities.
The presented variability of the Pannonian conditions significantly influences geothermal conditions and the utilisation of geothermal energy, even in a relatively small area compared to the whole basin. Diverse basin morphology appears in the whole basin and can form geothermal subregions, but its effect is more characteristic on the margin of the basin, where the thickness of the basin fill sediments is significantly lower than that in deep basins. The effect of this is important primarily in the centres where thermal water is currently being extracted intensively.
Spa development and increasing utilisation of thermal water for energy in the near future will result in an increasing density of wells and increasing load on thermal water reservoirs. Identification of subregions of different conditions and possibilities may help regional planning and design to ensure the environmental and economic sustainability of long-term thermal water utilisation.
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This article has been published in European Geologist Journal 43 – Geothermal – The Energy of the Future
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