Shterev et al. GEOTHERMAL RESOURCES AND SYSTEMS IN THE STRUMA (STRYMON) RIFT VALLEY (BULGARIA AND GREECE) Kostadin Ivan Dimitar Laboratory of Mineral-Thermal Waters, NCFTR, 1618 Sofia Geological Institute, Bulgarian Academy of Sciences, 1 113 Sofia University of Mining and Geology, 1156 Sofia Key words: geothermal exploration, geothermal resources, Struma rift valley, Bulgaria, Greece Introduction Precambrian age, intruded by Palaeozoic, Late Cretaceous and Palaeogene granitoids, and partially covered by Palaeogene volcanic The Struma rift valley (graben system) is one of the most interesting and sedimentary formations. and rich in thermal waters regions on the Balkan Peninsula. The geothermal activity is manifested by about 100 natural and borehole The principal fault sets strike NNW-SSE (Struma Lineament), thermal sources, and many temperature and hydrogeochemical SSW-NNE to SW-NE, and WNW-ESE to W-E. The grabens are anomalies provoked by hidden deposits of thermal waters in the usually elongated along the strike of the lineament although some of basement, boards and sedimentary filling of the grabens. them follow the strike of oblique or transverse fault sets Thus, the horst-and-graben pattern of the region consists of slightly elongated The Struma geothermal zone is a perfect subject for fundamental but almost isometric horsts surrounded by grabens studies of the structural, hydrodynamic and geochemical characteristics of non-stratified (fault- and fracture-bounded) The neotectonic Serbo-Macedonian Swell formed in the beginning hydrogeothermal deposits and circulation systems that have of the Miocene, and has been subject to continuous subsidence since developed in tectonically active granite-metamorphic terrains Late Badenian or Sarmatian times. During a first stage (Late outside active contemporary volcanic structures and belts. Such Badenian - Sarmatian), a small area along the Struma Lineament has studies are needed also for exploration and evaluation of the been the site of lacustrine sedimentation. The second stage geothermal resources in view of their utilisation in the tourist (Maeotian?) followed to the south of the threshold of the Middle- industry, urban needs, agriculture (geothermal greenhouses), Mesta fault zone (lineament) with a marine ingression, and to the bottling industry, and other economic activities important for north, with alluvial to lacustrine sedimentation. New intense vertical Bulgaria and Greece. Industrial natural C02 will be possibly block movements began in Pontian time when coarse proluvial and available in the Greek part of the valley. alluvial sediments filled the grabens during the third stage (Pontian - Romanian) A fourth stage (Pleistocene and Holocene) was marked The present paper is a summary of our estimations and prognoses by the most prominent horst uplift The proluvial deposits of this for the character, spatial distribution, amounts and quality of the stage form huge fans along the active Quaternary faults. The vertical geothermal resources emplaced and reproducing themselves in the amplitude (uplift) along the main neotectonic faults reached 3.5 - 4 Struma rift valley. Detailed studies on some particularly important km. The total thickness of the Neogene and Pleistocene deposits in and impressive hydrogeothermal deposits in different parts of the the grabens varied from 1 to 2 km in the northern (Blagoevgrad, valley will be subject of special publications. Simitli, Sandanski) grabens, and up to 3 - km, in the Serres Graben. 2. Methods and information base The grabens are usually asymmetric, with steep (60 - bounding The present study is based on deductive interpretations and normal faults However, the dip of some faults gradually decreases summaries of existing evidence about the geological structure, the towards south, and the extension perpendicular to the rift increases hydrogeology and geothermal conditions of the Struma rift valley considerably In consequence, block rotation and mutual and the surrounding horsts. Methods and ideas already presented in adjustments led to an en-echelon arrangement, and the grabens were our previous publications and prognoses on the thermal waters and tilted against the fault zones with great vertical offset Some of the hydrogeothermal systems and provinces (Shterev, 197 1, 1984, fault structures and fault intersections control earthquake foci at 1989; Shterev and Penev, 1991) are also used as well as all available depths of 2 - 10 km, and in rare cases, down to 45 - 50 km data for hydrogeological studies and shallow geothermal drilling (Sidirokastro, Lithotopos, Nigrita, Serres). Precious information has The fault structures and the movements in the basement and at the been supplied also by the two deep oil-prospecting boreholes in the sides of the Struma rift valley control a generalised and deep Serres graben Stry-2). circulation of meteoric waters The result is the formation of stratified (fault-and-fracture bounded) hydrogeothermal deposits 3. Geological setting of the Struma (Strymon) rift valley in view and systems that contain and reproduce low-mineralised thermal of its geothermal perspectives waters with nitrogen gas composition Some of the fault structures in the Serres graben emit intense flows of endogenous The Struma rift valley is a segment of a complex Palaeogene - Carbonated or thermal waters with increased Neogene fault system in the central parts of the Balkan Peninsula. mineralisation are formed around them After the last compressional phase in the Early Miocene, it developed in Neogene and Quaternary time under extensional Permeable sedimentary rocks at the deep stratigraphic levels of the conditions as a NNW-SSE continental rift (Zagorchev, 1992) Serres and Sandanski grabens represent potential autonomous extending over more than 200 km in Bulgaria and Greece (Fig. 1). layered reservoirs of thermal waters as well as possible recipients of hot waters that penetrate from the basement through permeable The basement of the grabens is widely exposed in the surrounding faults or lithofacies windows. Layered reservoirs of thermal waters horsts. It consists of amphibolite-facies metamorphic rocks possibly exist also in karstified marble bodies in the basement of the (gneisses, migmatites, schists, amphibolites, marbles) of supposed Serres graben, and to a lesser extent, of the Sandanski graben 1185 Shterev et Fig. 1. Simplified geological map of the Strymon valley in Bulgaria and Greece. Inset - position of the area studied in the Struma Lineament and other important lineaments in the Balkan Peninsula 1186 Area of formation reproduction of low-minerolised meteoric thermal wafers with gos composition In granites silicate stratified in the + 4 basement of the Valley) + t 4 + I In Neogene terri detrital depo- sits (stratified in - + + donsk! Serres grabens) In marble bodies Area of formation and distribution of car bonated thermal wafers the basement ond deposits Supposed outlines of brackish saline thermal waters the basement the deep Neogene deposits graben) occurrences Manifested by springs discovered by boreholes Identified by geotemperoture, geochemical indications of occurrences (Table 1 and of the volley Stry-1 Deep boreholes J -5 -10 AEGEAN SEA Fig. 2. Hydrogeothermal zoning and activity in the Struma (Strymon) rift valley 1187 Shterev et 0 10 2p 100 110 125 E - 100 200 300 LOO banya-2 600 700 800 9 00 1000 1100 1300 1 1500 -Convectively cooled profile C onvect i vel y warmed prof i Ie 1600 1700 \ \ \ \ \ \ \ \ Deep bore holes in the Serres graben 1900 \ \ Stry-2 2 Fig. 3. Geothermal profiles (geothermograms) and regimes in different points of the Strymon valley 1188 Shterev et al. enet ic classification Chemical composition 1189 Shterev et al. Manifested Thermal 2. IN THE GEOTHERMAL RESERVOIRS OF THE DEEP NEOGENE TERRIGENOUS DETRITAL GRABEN DEPTH PROGNOSTIC Surface Average thermal area temperature power ... Sandanski 300 1200 250 45 0,250 75 -9 Seres 500 1500 1500 50 0,250 375 -55 Total 1750 450 -64 - Resources: R', = 0,264; R or H' 0,370.1 J or 1190 et The Neogene marine transgression and sedimentation controlled the J (2 45 in case of doublet borehole exploitation, and formation of brackish and saline thermal waters at deep stratigraphic 0 370 x J (0 74 in case of singlet borehole levels and in the basement in the southern part of the Serres graben. exploitation 4. Origin, character and geologic localization of thermal 7. waters The Struma (Strymon) rift valley is one of the richest and most Our evaluations of the origin, physicochemical characteristics and diverse geothermal zones on the Balkan Peninsula. According to geologic localization of all exposed and potential thermal waters in preliminary evaluation, about thermal waters with a total the hydrogeological space of the valley are summarized in Table thermal power above 400 are continuously reproducing Most of the thermal waters have a meteoric origin and continuous themselves in its depth. A considerable geothermal potential (about recharge. Only the basement and the deeper Neogene reservoirs of 4.6 x J) exists also in closed (conductive) bedded reservoirs the Serres graben preserved fossil saline thermal waters and brines with salty thermal waters and brines in the Serres graben. of sedimentary marine origin. Considerable resources of natural C02 are outlined in the Greek part of the valley. Most of the meteoric waters possess a nitrogen gas composition, alkali character and very low mineralisation (from 0 3 to I - 5 The predominant part of the geothermal