Geothermal characteristics of the Krsko basin, Slovenia, based on geophysical research

Transcription

1 Physics and Chemistry of the Earth 28 (2003) Geothermal characteristics of the Krsko basin, Slovenia, based on geophysical research Dusan Rajver a, *, Danilo Ravnik b a Geological Survey of Slovenia, Dimiceva 14, 1000 Ljubljana, Slovenia b Faculty of Natural Sciences and Technology, University of Ljubljana, Askerceva 12, 1000 Ljubljana, Slovenia Received 27 March 2002; received in revised form 29 October 2002; accepted 4 April 2003 Abstract The Krsko basin with its thermal springs is a syncline, filled with low permeable Tertiary and some Quaternary sediments. Their thickness reaches about 1.8 km in its central eastern part. This is perceivable also in geotherms reflecting a conductive temperature field. In the syncline basement, especially in Triassic and Jurassic carbonates, but less in Cretaceous rocks, convective thermal field predominates. The syncline pattern and structure have been determined from the results of gravimetric, seismic and geoelectrical measurements and deep drilling since The most important geothermal anomaly is the Catez field with the greatest concentration of investigations. There, the highest borehole temperatures have been reached, ranging from 50 to 64 C. Geothermal anomalies at other localities have been less investigated, and temperatures do not exceed 36 C. Deep boreholes which were available for geothermal measurements are found mostly along the southern rim of the basin. In much wider area the only source of data for the construction of geotherms were geoelectrical soundings, applied to elaborate geothermal maps and cross-sections. This was enabled by a conversion from resistivity and borehole lithology into temperature data, using one-dimensional simplified solution of LaplaceÕs equation. In such a way an approximative knowledge of geothermal conditions below surface and beyond known geothermal anomalies has been extended. Circulation of meteoric water into few kilometers deep fissured and fractured hot zones is the only heating possibility for thermal water in the Catez field. Water circulation is probably the deepest there than elsewhere in the Krsko basin. Taking into account all information collected until now, we assume that geothermal reservoir could extend at least km deep below the surface. Ó 2003 Elsevier Science Ltd. All rights reserved. Keywords: Geothermal exploration; Geophysics; Geothermal anomalies; Krsko basin; Slovenia 1. Introduction * Corresponding author. Tel.: ; fax: address: dusan.rajver@geo-zs.si (D. Rajver). The Krsko basin, especially its southern part between Smarjeta Spa in the west and Croatian border in the east, belongs to the geothermally best explored areas in Slovenia. It is located in the westernmost margin of the Pannonian basin. In many parts elsewhere in Slovenia some prospective geothermal areas have not been so multiphased investigated. Deep investigations in the Krsko basin have been limited only to depths less than 1 km. While the intention of geothermal research in northeastern Slovenia has been conditioned by oil research, for the Krsko basin it was necessary to find the most successful methods based on joint investigations and intended to geothermics. Besides geology and hydrogeology also other, especially regional geophysical investigations (geoelectrics, seismics, gravimetry), are of great help. Geophysicists usually search for structures suitable for capturing and accumulation of hot fluid, or for anomalies that reflect the properties of hydrothermal fluids and their mutual reactions with surrounding rocks. As a result of this recognition almost all existent geophysical methods are used in geothermal research (Duprat, 1987). The choice of geophysical method mainly depends on local geological conditions that usually define the objective and purpose, either structural or hydrothermal. In the case of sedimentary environment, where the Krsko basin belongs, geophysical investigations will tend to define structures, depths and nature of geothermal resources, and to measure their geothermal parameters. On the basis of the results and assumptions of so far accomplished geological and geothermal investigations /03/$ - see front matter Ó 2003 Elsevier Science Ltd. All rights reserved. doi: /s (03)

2 444 D. Rajver, D. Ravnik / Physics and Chemistry of the Earth 28 (2003) a procedure has been ascertained that might enable an assessment of the most likely shape of the geothermal system at the base of the basin also in places where the borehole data are insufficient or are absent at all. Exactly there, vertical electrical soundings (VES) give the only chance to acquire some approximations of the data required. The procedure is composed of two parts: 1. Direct investigations Chiefly in boreholes (data acquisition related to lithology, hydrology, geothermics). On the surface (geophysical and geological methods). In the laboratory (thermal and electrical properties of the rock cores from boreholes). 2. Indirect calculations of formation temperature and heat flow density (HFD) based on Fourier and Laplace equations. Moreover, for deep temperature determinations also geothermal gradient with its extrapolation to different depths is advisable. The following maps have been constructed from the acquired data: Map of the base of the Tertiary basin, i.e. map of isobaths of the contact Tertiary or Cretaceous/Triassic or Jurassic (farther in the text Tertiary Mesozoic). Map of the formation temperature at this contact. HFD map down to this contact. These maps cover mostly the southern half of the basin between the Croatian border in the east and the area of Kostanjevica in the west. Much less data are available for the northwestern part of the basin. The thermal water outflow from the Catez geothermal system is possible to estimate solely from the hydrogeological investigations, i.e. from tests of productive boreholes and systematic observations during pumping tests. 2. Geologic and tectonic setting, hydrogeological conditions The Krsko basin forms a part of the Sava folds, and lies in the transition zone between the External and the Internal Dinarides or between the Dinaric carbonate platform and the deep-sea sediments of the so-called Vardar sea (Placer, 1998; Rajver, 2001). Dinaric structures are characterized by folds, thrusts and successory longitudinal faults in the NW-SE direction, and they are of Cretaceous-Paleogene age. They are typical for Mesozoic rocks of the Krsko hills and the Gorjanci Mountains. The base of Tertiary sediments in the basin is presumably constituted in the same way. Folds, thrusts and longitudinal faults in the E-W to the SW-NE direction in the eastern part (Balaton structures) are characteristic for the Alpine structures of Neogene age. Fault plane solutions of earthquakes at Kostanjevica express strike-slip displacement that can be related to southwestward present movement of the supposed tectonic block of the Krsko basin along some faults of the SW-NE direction (Poljak and Zivcic, 1995). According to the seismotectonic model of the region by Prelogovic et al. (1992), the greatest concentration of shallow earthquake foci is inside the Zumberak Mountain (SE of the Gorjanci Mountains) that is linked with a zone of reverse faults Kostanjevica Stubica (NW Croatia). At the bottom of the Krsko syncline there is a supposedly several hundred meters thick sequence of Middle and Upper Triassic dolomite and partly also Jurassic limestone (Poljak, pers. commun.). This sequence is in places strongly fractured and crushed owing to tectonics. In the dolomite beds that are fractured enough, geothermal aquifers may occur. Dolomite and limestone outcrops in the periphery of the Krsko syncline (Fig. 1) form recharge areas for these aquifers. In the central part of the syncline dolomite beds are covered with chiefly hydro- and thermoisolated cover of Tertiary and Cretaceous sediments (marl, clayey marl, sandstone, marly limestone, sandy marl, clayey silt, limy breccia) with thickness of more than 1 km, even around 2 km in some places. Within the marly section, several tens of meters thick layer of Miocene lithotamnion sandstone and limestone exist. It is in some places locally confined aquifer, known especially in the Catez field (Verbovsek, 1990). Its porosity is of karstic type with greater caverns, which holds also for Cretaceous limestones. The cold groundwater flow through these layers influences the local cooling of the already heated groundwater within the dolomitic aquifer (Petauer et al., 1993). Triassic dolomites, Jurassic limestones and dolomites and few Cretaceous sediments represent abundant aquifers that are of regional importance due to their extension and mutual connection (Verbovsek, 1990). They are recharged by meteoric water circulating from surrounding areas of the basin into the deeply lying fractured hot zones. Meteoric origin has been proved from ratios of deuterium and oxygen isotopes in thermal water samples from three boreholes (L-1/86, V-15/88, SI-1/86; Fig. 1) that all lie on the CraigÕs meteoric water line. Due to the forced and partly free convection (especially in the Catez field) this water appears at numerous thermal and subthermal springs along the southern and southwestern rim of the basin. In the thermal spring areas water of higher temperature and quantity is captured with exploitation wells in the Catez field and at Smarjeta Spa (12 km WNW of Kostanjevica, not visible in Fig. 1; Table 1). There are three main presumptions for the heating source of thermal water, namely magmatic intrusion, deep sedimentary basin and water circulation into few kilometers deep faulted and fractured zones. Only the

3 D. Rajver, D. Ravnik / Physics and Chemistry of the Earth 28 (2003) Fig. 1. Simplified geology of the Krsko basin with Bouguer anomalies and locations of principal geophysical investigations (modified from Gosar, 1998). Table 1 Temperature and discharge of thermal springs and exploitation boreholes No. Temperature ( C) Average discharge (l/s) Depth (m) to top of the geothermal reservoir Thermal springs Subthermal springs Boreholes Catez area, Dobova Smarjeta Spa last one seems to be reasonable (Rajver, 2001) according to results of geothermal and geophysical research. For the hydrogeothermal system of the Catez field some conclusions may be drawn: 1. There are no signs for Quaternary magmatism (igneous magmatic activity) in the wider basin area. 2. Piezometric gradient as a result of altitude difference between recharge areas and thermal springs can maintain thermal water outflow of the springs. There is also no need for thermal buoyancy above eventual heat source. 3. The groundwater flow from rather shallow depths, at least km, can maintain the temperatures that are observed in the boreholes, especially in the Catez geothermal anomaly. 3. Review of geothermal research and geophysical investigations A complex research treated the whole discussed region in a period (Premru et al., 1974; Lapajne

4 446 D. Rajver, D. Ravnik / Physics and Chemistry of the Earth 28 (2003) et al., 1975, 1976, 1979). Later on regional geophysical, hydrogeological and tectonic investigations were also carried out (Verbovsek, 1990). In the late 1960s and in the 1970s all existent thermal springs were inventoried, the aquifer types were hydrogeologically classified, hydrogeochemical interpretation was done, and structure of the region was interpreted with geophysical methods and geotectonic study. The localities of Catez Spa and Toplicnik near Kostanjevica were investigated in more detail and described in numerous internal reports (Rajver, 2001). Many exploration and exploitation boreholes were drilled there. The Catez field was more precisely elaborated (Nosan, 1959; Ivankovic and Nosan, 1973). In both areas investigations continued in with additional geophysical measurements (Rajver, 2001) and drilling of three exploitation boreholes with depths of 400 and 704 m in the Catez field and of 800 m near Kostanjevica. Geothermal research continued in with deep geoelectrical soundings in a wider region between Krsko, Globoko, Brezice and Kostanjevica and temperature gradient survey in shallow boreholes in the area between Brezice and Krsko (Rajver et al., 1996). Another exploitation borehole was drilled at Dobova in 1995 with a depth of 700 m. The review of all geophysical investigations for various purposes has been compiled by Rajver (2001) following a previous one by Brezigar et al. (1993). The methods used are: gravity, seismics (longer reflection and shorter refraction profiles), geoelectrical sounding and geothermics. Geothermal methods as the main investigation technique included temperature measurements in boreholes, thermal conductivity determinations on cored rock samples and to a lesser extent of radiogenic heat production of the same samples. Seismic profiles were measured mostly perpendicularly to the syncline axis that runs in the WSW-ENE direction (Fig. 1). For the support and additional enlargement of geothermal research geoelectrical measurements were carried out. VES are necessarily concentrated in the wider Catez field area and near Kostanjevica. Even more concentrated are both exploration hydrogeologic and exploitation boreholes in the Catez field, near Kostanjevica and at Smarjeta Spa. Gravity has confirmed very well the shape and orientation of the syncline. Fig. 1 shows the pattern of Bouguer anomalies as a result of gravity measurements for the purposes of structure delineation. At Drnovo, roughly in the middle of the basin, a threshold in the base of Tertiary beds that separates eastern and western part of the syncline has been determined. The homogeneous synclinal structure of the basin was delineated from reflection seismics in several parallel profiles in the N-S to NNW-SSE direction across the whole region. They were recorded by Geofizika Co. (Zagreb) in 1959 in analog technique for the purpose of oil research. More recently, since , seven reflection seismic profiles were recorded across the basin for the earthquake hazard assessment at the Krsko nuclear power plant (NPP) site and for better structural model of the basin. Their interpretation was verified by 2D modeling of gravity data (Roach, 1993). Fig. 2 exhibits a 2D gravity model along the longest profile of 13 km running in a N-S direction (Gosar, 1998). The relief of Pre-Tertiary basement and other horizons within Tertiary beds have been adopted from the interpretation of seismic profile, only density in single beds has been slightly changing, so that good agreement between observed and calculated curves of Bouguer anomalies has been shown. However, on the basis of seismic data it is difficult to decide whether Cretaceous rocks (flysch or carbonates) or Triassic carbonates form the Tertiary base. About 300 VES have been measured in the Krsko basin since 1959 using the method of apparent electrical resistivity with the Schlumberger electrode configuration. The current electrodes (AB) have been several hundred to several thousand meters apart, maximally about 20 km. Measurements have been performed without any great troubles (industrial noise, etc.) and data are of good quality. Using conventionally applied interpretation methods the depth-dependence of resistivities is acquired. Examples are the E-4/78 sounding (Krakovo forest), showing low permeable high resistive carbonate rocks, and the E-37/71 sounding ( Catez field), indicating on the contrary permeable low resistive carbonate rocks at the base, likely with thermal water (Fig. 3). Lower values of electrical resistivity under the Catez field in Triassic rocks can be assigned most likely to greater fracturing of the dolomite massif, conditioning the occurrence of geothermal aquifer. Temperature has been measured in 56 boreholes, in 33 among them in thermally stable condition what we always strive for. Three different temperature sensors have been used: the CS-3000 Ohm 30% (at 25 C) thermistor with negative temperature coefficient (in a range C: )2.9%) in a period , the AD- 590 (Analog Devices) integrated sensor with precision of 0.1 C until 1990, and since 1985 the electrical resistance thermometer with Pt-100 sensor with precision of 0.01 C and relative accuracy of 0.05 C. Temperature was usually measured point by point at intervals of 5 10 m, depending on variations in the lithology. In the deepest borehole DRN-1 (1252 m deep, Fig. 1) it was measured in stable condition only down to a depth of 640 m. In the most significant geothermal anomaly ( Catez field) geothermal measurements have been done in 10 deep boreholes. We have distributed geotherms according to the relation between increasing temperatures in Tertiary beds and much higher and constant ones in Mesozoic complex. The increasing (dt =dz > 0) and the isothermal

5 D. Rajver, D. Ravnik / Physics and Chemistry of the Earth 28 (2003) Fig. 2. A 2D gravity model along the seismic profile P-4/95 and P-3/94-95 (Gosar, 1996). borehole is situated at top of the fractured convectiondominated area. An example of such geotherm is represented from the AFP-1/95 borehole at Dobova (Fig. 4a) that has reached a thermal aquifer with 63 C. An example of completely isothermal geotherm is from the V-2/69 borehole at Catez Spa (Fig. 4b). Geothermal gradient there is almost zero due to measurements in artesian condition. An example of geotherm with lower geothermal gradient in Tertiary beds and lower temperature in geothermal aquifer is from the SI-1/86 borehole (uncased below 635 m) near Kostanjevica (Fig. 4c). In very low permeable Tertiary beds temperature increases constantly down to the contact with Jurassic limestone, while in the aquifer section it remains constant. 4. Geotherms constructed with assistance of VES Fig. 3. VES E-4/78 (Krakovo forest) and E-37/71 ( Catez field). (dt =dz ¼ 0) types of geotherm have been obtained in many boreholes. At the contact between Tertiary and Mesozoic sediments the isothermal type predominates, especially in the Catez field area. Its main characteristic is high temperature, which is maybe a sign that a Data needed for the most studied geothermal system ( Catez field) are temperatures at different depths and depth to Mesozoic carbonate base of the syncline where thermal water discharges into younger sediments. The missing geothermal information in poorly explored areas can be at least partly obtained with a help of VES curves. Depths of the syncline could be given also by seismic reflection profiles and partly by gravity, but the coverage of the profiles was not sufficient all over the basin.

6 448 D. Rajver, D. Ravnik / Physics and Chemistry of the Earth 28 (2003) Fig. 4. Geotherms from the three boreholes. (a) AFP-1/95 (Dobova; measured in steady-state condition). (b) V-2/69 ( Catez field; measured in flowing condition). (c) SI-1/86 (Kostanjevica; measured in steady-state condition). Legend for geology: S stratigraphy, Q Quaternary, Pl Pliocene, M Miocene, M 3 Upper Miocene, M 2 Middle Miocene, K Cretaceous, J Jurassic, T Triassic; L lithology, b breccia, bl limestone breccia, c clay, d dolomite, l limestone, ld dolomitized limestone, m marl, mc clayey marl, ms sandy marl, s sand, st sandstone, sq quartz sandstone; H hydrogeology (hydro permeability), (1) very good, (2) good, (3) bad, (4) very bad. Legend for the geotherms: points represent temperature; continuous line represents temperature gradient; BHT bottom hole temperature. The emphasis of geothermal research was in the southern part of the Krsko basin, while in the other areas it was restricted mostly to shallow boreholes. For the study of geothermics of the whole syncline, data are

7 D. Rajver, D. Ravnik / Physics and Chemistry of the Earth 28 (2003) Fig. 5. An example of transformations: geoelectrical data (VES) of KBT-9/96! lithology! thermal conductivities! formation temperatures in depth (T z ). required from greater depths. Only in the southern area it was possible to get such information, while this was excluded in most poorly investigated area without the help of geoelectrical sounding. The results of VES were used as a supplement to the study of formation temperatures at the Triassic Jurassic base. They rendered possible indirect transformation of available electrical resistivities into lithological data, to which thermal conductivities were assigned. Geoelectrical sounding gave also the depths of most important geological beds. Lastly, formation temperatures versus depth (synthetic geotherms) were calculated assuming steady-state heat conduction by relation of Laplace (Eq. (1)): T ðzþ ¼T 0 þ q 0 X n i¼1 Dz i k i for n layers ð1þ where z is the depth (m); T 0, the average annual surface temperature ( C); Dz i, the thickness of the layer (m); q 0, the surface HFD (W/m 2 ); k i, the thermal conductivity of the ith layer (W/m K); Dz i =k i, the thermal resistance (m 2 K/W). More sensitive was the transformation of electrical resistivities into lithological data, and the assignement of thermal conductivities to the latter. It was favourable in this procedure that some VES were measured next to some deep boreholes, for instance SI-1/86, DRN-1/89 and L-1/86 (Fig. 1), where rock cores were obtained and their thermal conductivity determined in laboratory. The lithological composition of individual beds was known from the boreholes. Therefore, we first used data from deep boreholes where nearby a deep VES has been measured. It was possible to compare the ascertained thicknesses and resistivities of individual beds to appropriate geothermal parameters from the boreholes. From geoelectrical model characteristic values of resistivities have been attributed to individual lithological units and typical thermal conductivities (numerous internal technical reports, see: Rajver, 2001) have been ascribed to them (Fig. 5). For the VES without boreholes nearby, suitable thermal conductivities have been comparatively adopted for lithological beds from the model of VES. For boreholes that have not reached Triassic Jurassic base the lithologic composition and especially depths to the base were acquired from the VES data. Thus using only geoelectrically determined parameters, construction of the isobaths of the high resistive base and its temperature has been made feasible also in wider regions where no deep boreholes are available. The insight on geothermal circumstances beyond already known thermal anomalies is therefore enlarged although with a rough approximation. At some locations we did the whole procedure on the boreholes and on the VES next to them. The obtained results match satisfactorily well. We present two examples of boreholes, where near them suitable VES were measured, from which geoelectrical models are available, and one VES with geoelectrical model without a borehole in close vicinity (Table 2). Results are presented for some locations, i.e. temperatures and depths at the contact Tertiary Mesozoic. Example (a) shows the adopted thermal conductivities for the V-15/88 lithology that are transformed from the geoelectrical model only 100 m away. Example (b) presents the adopted thermal conductivities for the location of borehole B Z-1/87 that are transformed from the geoelectrical model of VES (B-3/87) close to the borehole. Example (c) shows the adopted thermal conductivities

8 450 D. Rajver, D. Ravnik / Physics and Chemistry of the Earth 28 (2003) Table 2 Procedure examples for temperature calculations at the Triassic (Jurassic) base from VES Depth (m) (W/m K) Lithology Age Resistivity (Xm) Geoelectrical model from VES Panel A Sand, gravel Quaternary E-28/71, Sandstone Miocene-M 2 27?, m NW from the borehole Marl, sandstone Miocene-M 2 P Limestone, marl Cretaceous-K Dolomite Triassic 950 T calculated (352 m) ¼ 65 C T measured (352 m) ¼ 62 C Panel B Gravel, marl Quaternary 54 B-3/ Marl Miocene-M Marl, sandy marl Miocene? 20 Close to the borehole > Dolomite, limestone Triassic >160 BZ-1/87 T calculated (610 m) ¼ 62 C Panel C Gravel, sand Quaternary-Q 3000 KBT-9/ Clay, sand Q, Plio-Miocene 24 Skopice Lithotamn. limestone Miocene-M Marl,... Miocene? 18 > Dolomite Triassic? 50 T calculated (850 m) ¼ 40 C Panel A: Borehole V-15/88, Catez field, depth ¼ 400 m. Depth sections are taken from the borehole lithology. HFD is calculated for the upper 200 m with adopted thermal conductivities: q ¼ 296 mw/m 2. Panel B: Borehole BZ-1/87, Brezice (northern part of the Catez field), depth ¼ 100 m. Depth sections are according to the borehole data and expected lithology to a depth of 610 m, taken from a map of isobaths of Triassic (Jurassic) base rocks. HFD is calculated for the upper 100 m with adopted thermal conductivity: q ¼ 151 mw/m 2. Data for depths over 100 m are from the geoelectrical model. Panel C: Values of thermal conductivity are transformed from the geoelectrical model of VES. Depth sections are according to the expected lithology from the geoelectrical model of VES. q ¼ 60 mw/m 2 (from the HFD map of the basin, a little lower than in Ravnik et al., 1995). for the location of VES (KBT-9/96), Skopice (N of Krska vas, W of Brezice), with no borehole nearby. For greater part of the borehole and VES locations thermal conductivities were adopted from the measured conductivities from six boreholes in the Krsko basin. For each location we mention the nearest VES that provides the geoelectrical model. A certain correlation had to be made between resistivities from the VES interpretation and thermal conductivities for individual characteristic layers. However, mistakes are possible that can be diminished only with repeated temperature measurements in the boreholes with previous low quality measurements and with some new better located VES. 5. Results of geophysical research For the isobath map of high resistive base Rajver et al. (1996) chose 102 geoelectrical soundings regarding their location and opinion that they reached the Mesozoic carbonate base. The map (Fig. 6) presents mostly depths to Middle and Upper Triassic, somewhere also Jurassic carbonates. In the north-western part of the studied basin we separated an area where very likely only Cretaceous base is determined with VES. We found out that resolution of Pre-Tertiary beds from VES was of lower quality than from seismics, especially in cases of greater depth where Cretaceous sediments overlie Triassic carbonates. In the same map (Fig. 6) temperatures are drawn on top of Triassic (Jurassic) carbonatic rocks. Isotherms indicate a thermal anomaly in the south-eastern part of the basin. High temperatures are calculated also north of the main anomaly of the Catez field that are even higher than those measured in boreholes of the Catez field owing to the rapidly deepening of the Triassic base towards north. If temperatures are presented at a constant depth (i.e. 300 m), the contour of thermal water outflow from deeply lying convective system into Tertiary sediments is better visualized (Fig. 7). The isotherm of 50 C, for example, confines the zone that roughly covers the continuation of the fractured and faulted zone of thermal aquifer from the Catez field to the northeast (Ivankovic and Nosan, 1973). Geothermal anomaly in the broader Catez field area is well evident in the HFD map (Fig. 8) drawn from calculations down to the Triassic (Jurassic) base. A weaker anomaly is detected also near Kostanjevica. The reason for both is thermal water convection in Triassic

9 D. Rajver, D. Ravnik / Physics and Chemistry of the Earth 28 (2003) Fig. 6. Temperature at top of the Triassic (Jurassic) carbonatic rocks with depths to Middle and Upper Triassic and in places also Jurassic rocks. Fig. 7. Temperature at 300 m depth in the Krsko basin with the same depth pattern as in Fig. 6. (Jurassic) rocks. The Catez field geothermal anomaly is well emphasized in the north-south running crosssection (Fig. 9). Isotherms are there closer to the surface due to thermal water convection along the fractured

10 452 D. Rajver, D. Ravnik / Physics and Chemistry of the Earth 28 (2003) Fig. 8. HFD down to the Mesozoic base in the Krsko basin with the same depth pattern as in Fig. 6. Fig. 9. Geothermal cross-section from Globoko to the Catez field (N-S direction). zones. Northern part of the basin does not exhibit any signs of geothermal anomaly according to the research to date. Northwards from the Kostanjevica area circumstances are still not clear enough.

11 D. Rajver, D. Ravnik / Physics and Chemistry of the Earth 28 (2003) Discussion The results, especially from the Catez field, show the temperature increase of thermal water from south to north, but in the same direction also Tertiary low permeable sediments thickens (Fig. 6). A similar situation, although with lower temperatures, is determined north of Kostanjevica. Below Tertiary sediments Mesozoic carbonates (mostly dolomites) form primary aquifers with thermal water. Within them there is up to 100 m thick sequence of mostly low permeable Upper Cretaceous beds ( scaglia ). The highest temperature (64 C) was recorded in a borehole in the middle of the Catez field within Catez Spa. There is at least one deep water flow from the Gorjanci Mountains below the Catez field area further to the north that is heated in depth and somewhere at fault zones it partly turns and flows southward immediately below the Tertiary (Cretaceous) low permeable cover. This flow must be quite intense what is proved by high porosity that is a consequence of chemical solution of rocks by thermal water at the Tertiary Mesozoic contact (Verbovsek et al., 1986) Maximum temperature and circulation depth Surface meteoric waters flow into depth and get warmer due to elevated or just average geothermal gradient. If they are heated enough, they flow, as becoming lighter, to the surface through open fractures or through fissured permeable zones and emerge as thermal springs on the surface. Such circulation can be created also due to forced convection without the need for very heated water. Ordinarily it is possible to get water of higher temperature than the one at thermal spring if it is captured with a borehole below the zone of mixing with colder groundwater. Even if the influence of subsurface water cooling is isolated, it is questionable whether we have attained the maximum possible temperature. Fournier and Truesdell (1970) tried to answer this question creating the graphs of the concentration of quartz (SiO 2 ) in thermal waters as a function of water temperature for equilibrium conditions. If our chemical analyses in early 1970s are reliable, the optimum temperatures in some narrower confined geothermal reservoirs in the Krsko basin would be those given in Table 3 (Premru et al., 1974). Reliability of this method depends, of course, on the fulfilled conditions for validity of the chemical geothermometers. The necessary circulation depth for meteoric water to reach temperatures of C in the Catez field may be estimated from the geothermal gradient. In the boreholes around the Catez field anomaly gradients are in the range mk/m and almost on the border of geothermal anomaly they are about 37 mk/m. Assuming temperature T 0 to be 11 C, such gradients would Table 3 Optimum temperatures in some geothermal reservoirs in the Krsko basin after the diagram by Fournier and Truesdell (1970) Area place the circulation depth of the Catez thermal water with at least 60 C in the range of 2 3 km. Similar depths were estimated by Andrews et al. (1982) for thermal waters of the Bath thermal spring ( km) in England and by Verdoya et al. (1999) for the Tertiary Piedmont Basin geothermal system ( km) in Italy Thermal water age The analysis of tritium content from the borehole V-15/88 ( Catez field) have shown somehow a doubtful result of 0 T.U. (Verbovsek et al., 1988). Residence time of thermal water may be estimated from simple hydraulic model using data of borehole pumping tests. Filtraton water velocity v f can be determined from hydraulic pressure gradient grad h and hydraulic conductivity k f (Haenel et al., 1988): v f ¼ k f gradðhþ ð2þ Pumping tests from three boreholes, L-1 and V-15 in the Catez field, and SI-1 near Kostanjevica, yielded an average hydraulic conductivity of m/s. Considering hydraulic pressure gradient, expressed in waterlevel difference, as referred to a horizontal distance, to be about (500 m/20 km), filtration or Darcy velocity of water is m/s, and its residence time in a subsurface system gets a value of about 1500 years. If less tortuous flowing path of 12 km is assumed, the result is 540 years, and with only 8 km about 240 years. On the other hand, a minimum upward groundwater velocity, that will maintain the discharge of 600 l/s (Petauer et al., 1993) in the area of 3 km 2 that encloses the highest thermal anomaly of the Catez field, is m/s. We assume that maybe even 100 years is too long a period for residence time of thermal water in the Catez field due to large outflow there, but no unambiguous estimate can presently be made Dimensional analysis Optimum temperature ( C) Catez field Buseca vas Kostanjevica Smarjeta Spa Measured temperature ( C) It is interesting to apply dimensional analysis for the assessment of the relative importance of different factors and processes in a hydrogeological system. In the case of thermal effects of such a system, the Peclet number provides a good measure of the importance of convective versus conductive heat flow and of thermal disturbance produced by groundwater flow (Van der Kamp

12 454 D. Rajver, D. Ravnik / Physics and Chemistry of the Earth 28 (2003) and Bachu, 1989). However, improper use of various characteristic parameters of the hydrogeological system can lead to wrong conclusions and erroneous conceptual models of subsurface fluid flow and heat transfer processes. We hypothesize that recharge for the Catez geothermal field most likely takes place in the Gorjanci Mountains, where Triassic dolomites and limestones outcrop at least 5 km away. The flow descends to a depth between 2 and 3 km, before rapid ascent to the spring discharge point. More than 30 years ago thermal spring Perisce at its southern rim flowed at a rate of 2 l/s and a temperature of 32 C. Nowadays, thermal water flows further to the northeast towards the exploitation boreholes, while thermal spring has run dry. Heat flow in the basin is assumed to be on average 54 mw/m 2, somehow lower than estimated in a regional context earlier (Ravnik et al., 1995). If thermal spring Perisce is discussed, its thermal yield above a non-thermal groundwater temperature of 11 C was 180 kw. This yield would take all the basal heat flow from 3.3 km 2 of the basin. With a length of flow path of 5 km, the width of the flow path must be at least 0.66 km. There is certainly also some conductive heat loss. Therefore, this width can be estimated at 0.90 km. The flow per unit width then becomes 0.002/900 ¼ m 2 /s. With a thermal conductivity of 2.8 W/m K, a length of 5 km and a maximum depth for the flow system of 3 km, the resulting value for geothermal Peclet number Pe is 2.0. This value is a rough estimate only due to obviously three-dimensional nature of the flow. If more probable much longer length of flow path of, say, 30 km is taken into account, the width of the flow path must be at least 0.10 km, or considering some conductive heat loss, 0.14 km. The flow per unit width is then 0.002/ 140 ¼ m 2 /s. With a thermal conductivity of 2.8 W/m K, a length of 30 km and a maximum depth for the flow system of 3 km, the resulting value for geothermal Peclet number becomes A geothermal Peclet number of about 2 or 3 implies strong disturbance of the subsurface temperature regime by groundwater flow (Van der Kamp and Bachu, 1989). In the recharge end of the basin temperatures are likely to be well below normal, while in the discharge zone temperatures should be well above background. If, for example, the Peclet number had turned out to be much less than one, subsurface temperatures would be near normal, a general characteristic of conduction dominated hydrogeological systems, and thermal springs would be unlikely to occur. The conceptual model would then have to be revised. If, on the contrary, the Peclet number had turned out to be much greater than one (ca 10 or more), the whole system would be strongly cooled by convection and the groundwater discharge would be relatively cool. Thermal springs only occur for geothermal systems with a Peclet number of about unity, and any conceptual model of such springs must be consistent with this criterion. In our case, according to such analysis of the Catez geothermal system, this is a quantitative verification of the real conditions. 7. Conclusions Characteristics of the Krsko basin are relatively numerous thermal and subthermal (<20 C) springs along its southern and southwestern rim from Smarjeta Spa to the Catez field in the 2 3 km wide zone along the Krka river. Only about 10% of the basin area (300 km 2 ) has been investigated for geothermal purpose in more detail. The basin is filled with Tertiary low permeable sediments what is reflected in geotherms with expressed conductive temperature field, while mostly convective temperature field dominates according to geotherms in Mesozoic rocks of chiefly Triassic and Jurassic, less of Cretaceous age. With results of gravity, seismics, geoelectrical measurements and deep drilling the shape of syncline has been determined, only here and there also lithologic composition and separation of Tertiary and Mesozoic beds of the syncline base. The most important geothermal anomaly is the broader Catez field area where geological, hydrogeological and geothermal research has been predominantly concentrated. The boreholes there have reached temperatures of C. Less investigated have been anomalies of Smarjeta Spa, around Kostanjevica and Buseca vas, where temperature higher than 36 C has not been determined. Geothermal gradient in Tertiary sediments is, but a few exceptions, generally positive, while in Mesozoic sediments it is frequently around zero. This is noticeable mostly in the geotherms of the Catez anomaly, and less on anomalies at Kostanjevica and at Smarjeta Spa. Such geotherms are characteristic of all artesian boreholes of the region. The zero gradient of longer depth span is a sign for isothermal conditions in a borehole, showing at least the topmost part of geothermal reservoir. Due to deficiency of geological and geophysical data from greater depths we can not foresee the state and shape of geothermal reservoir in deeper parts of Mesozoic complex. Deeper boreholes with acquired thermal data are concentrated in the Catez field, one at Kostanjevica and few at Smarjeta Spa. Elsewhere temperature data are obtained mostly from shallower boreholes, north of Brezice and between Brezice and Krsko. Therefore, in other, much larger parts of the basin, a data transformation has been used for calculation of geotherms: from the results of VES the lithological structure and thickness of individual sediment beds have been approximately derived, and to the latter the suitable thermal

13 D. Rajver, D. Ravnik / Physics and Chemistry of the Earth 28 (2003) conductivity has been attributed for calculation of depth temperatures. The highest temperatures, say over 50 C, at certain depths (i.e. 300 m) after data acquired to date are found in the Catez field and further to the ENE towards Dobova, much lower near Kostanjevica (up to 36 C), while elsewhere the anomalies have not yet been discovered. Below the Catez field a free water circulation exists, deeper than in case of the other anomalies where the borehole depths are generally low. The results of geothermal and geoelectrical research confirm so far mostly a presumption about the vertical circulation of groundwater in depths of at least 2 3 km. The characteristics of geothermal anomalies will be more thoroughly established only with more comprehensive future surface investigations and especially with boreholes drilled deeper into Triassic and Jurassic rocks with all necessary measurements and tests. To answer the question how far away the Catez hydrogeothermal system spreads towards east, north and west, it would be interesting to drill some gradient boreholes in these directions. Deep geophysical investigations and later drilling is in the same sense needed between Kostanjevica and Smarjeta Spa to the west and perhaps around Krska vas. Acknowledgements The authors greatly appreciate the useful and constructive suggestions by S. Veliciu and one anonymous reviewer. They are grateful to the Geological Survey of Slovenia for support and permission to publish this paper. D. Zivanovic helped to improve the figures. References Andrews, J.N., Burgess, W.G., Edmunds, W.M., Kay, R.L.F., Lee, D.J., The thermal spring of Bath. Nature 298, Brezigar, A., Tomsic, B., Stopar, R., Zivanovic, M., Overview and reinterpretation of geophysical research in the surroundings of the NPP Krsko. Unpublished Report, Geological Survey Ljubljana, 36 pp (in Slovene). Duprat, A., Geophysics in geothermal prospecting. In: Economides, M., Ungemach, P. (Eds.), Applied Geothermics. John Wiley & Sons, Ltd., Chichester, pp Fournier, R.O., Truesdell, A.H., Chemical indicators of subsurface temperature applied to hot spring waters of Yellowstone National Park, Wyoming, USA. U.N. Symp. Development Utilization Geothermal Resources, Pisa. Gosar, A., Seismic reflection method in structural investigations for assessment of earthquake hazard in the Krsko basin. Ph.D. Thesis, University of Ljubljana, Ljubljana, Slovenia (in Slovene, with English abstr.). Gosar, A., Seismic reflection surveys of the Krsko basin structure: implications for earthquake hazard at the Krsko nuclear power plant, southeast Slovenia. J. Appl. Geoph. 39, Haenel, R., Rybach, L., Stegena, L., Fundamentals of geothermics. In: Haenel, R., Rybach, L., Stegena, L. (Eds.), Handbook of Terrestrial Heat-flow Density Determination. Kluwer Academic Publishers, Dordrecht, Boston, London, pp Ivankovic, J., Nosan, A., Hydrogeology of the Catez thermal springs. Geologija 16, (in Slovene with English abstract). Lapajne, J., Premru, U., Kump, P., Drobne, F., Marinko, M., Exploration of the thermal springsõ area in south-eastern Slovenia. II. Phase, Unpublished Report, Geological Survey Ljubljana, 38 pp (in Slovene). Lapajne, J., Premru, U., Sribar, L., Ivankovic, J., Kump, P., Exploration of the thermal springsõ area in south-eastern Slovenia. III. Phase, Unpublished Report, Geological Survey Ljubljana, 40 pp (in Slovene). Lapajne, J., Mervic, I., Milosavljevic, M., Rihtar, B., Exploration of the thermal springsõ area in south-eastern Slovenia. V. Phase, Unpublished Report, Geological Survey Ljubljana, 52 pp (in Slovene). Nosan, A., Hydrogeology of the Catez thermal springs. Geologija 5, (in Slovene with English summary). Petauer, D., Verbovsek, R., Drobne, F., Optimization of the possibility of thermal water use from the carbonatic aquifer near Catez. V. Phase. Unpublished Report, Geological Survey Ljubljana, 49 pp (in Slovene). Placer, L., Structural meaning of the Sava folds. Geologija (Ljubljana) 41, Poljak, M., Zivcic, M., Tectonics and seismicity of the Krsko basin. Proc. First Croatian Geological Congress, Zagreb, vol. 2. pp Prelogovic, E., Aljinovic, B., Maticec, D., Probability analysis of the earthquake hazard at the NPP site. Seismotectonic study. Unpublished Report, Zagreb, 55 pp (in Croatian). Premru, U., Ivankovic, J., Urh, I., Exploration of the thermal springsõ area in south-eastern Slovenia. I. Phase, Unpublished Report, Geological Survey Ljubljana, 45 pp (in Slovene). Rajver, D., Geothermal characteristics of the Krsko basin with emphasis on geophysical investigations. M.Sc. Thesis, University of Ljubljana, Ljubljana, Slovenia, 203 pp (in Slovene, with English abstr.). Rajver, D., Gosar, A., Zivanovic, M., Geothermal energy resources in Triassic aquifers in the Krsko basin. Unpublished Report, Geological Survey Ljubljana, 18 pp (in Slovene). Ravnik, D., Rajver, D., Poljak, M., Zivcic, M., Overview of the geothermal field between the Alps, the Dinarides and the Pannonian basin. TectonoPhysics 250, Roach, M., Model 2D, Ver. 3.0, User Manual, University of Tasmania, Hobart, p. 32. Van der Kamp, G., Bachu, S., Use of dimensional analysis in the study of thermal effects of various hydrogeological regimes. In: Beck, A.E., Garven, G., Stegena, L. (Eds.), Hydrogeological Regimes and Their Subsurface Thermal Effects. Geophysical Monograph 47, IUGG vol. 2, AGU, Washington, pp Verbovsek, R., Geothermal model for the Krsko Brezice field. Geologija 31, (in Slovene with English summary). Verbovsek, R., Locniskar, A., Nosan, A., Thermal water research in the Catez field. Unpublished Report, Geological Survey Ljubljana, 20 pp (in Slovene). Verbovsek, R., Locniskar, A., Otorepec, S., Report on the exploration exploitation borehole V-15/88 for thermal water at Catez Spa. Unpublished Report, Geological Survey Ljubljana, 25 pp (in Slovene). Verdoya, M., Pasquale, V., Chiozzi, P., Hydrothermal circulation in the Acqui Terme district, Tertiary Piedmont Basin (NW Italy). Bull. dõhydrogeologie no. 17, In: Vuataz, F.-D. (Ed.), Proc. of the European Geothermal Conference Basel Õ99, vol. 1, Neuch^atel, pp