Structural control on the mountainous geothermal reservoirs distributed across the Oligocene andesite-type volcanic complex (Lece massif, southern Serbia)

Abstract

Upper crustal faults and fractures control a natural thermal and non-thermal fluid circulation in the Lece andesite complex (LAC) (southern Serbia, Vardar Zone). By exhibiting differential conduit-barrier behavior, regional faults, and associated tensional fractures are key elements to this effective natural geothermal system. The combination of field-based hydrogeological prospecting and structural geology (water-bearing structural lineaments) yields several types of geothermal and cold mineral groundwaters that appear to be genetically linked to a cluster of fluid conduit-type extensional faults (hydraulic boundaries). The results further show that geothermal fluid production occurs in the naturally fractured and non-stimulated subsurface reservoirs distributed around the volcanic body (LAC). The geothermal reservoir doublets lie along the external perimeter of the LAC, whereas the abutting geological assemblage is composed of peri-volcanic, much older Neoproterozoic-Paleozoic metamorphic rock. The embedded thermal fluids are formed in the near-surface zones controlled by abundant fractures, particularly those occurring within the propylitized and hydrothermally altered subsegment of the Lece volcanic body. The resulting productivity of these composite aquifers is reaching very high values. The study further yielded several of the most important factors controlling the natural geothermal reservoirs: (i) the important effects of the extensional (neo)tectonic activity and their influence on the reservoirs (fractured lithological assemblage); (ii) the subsurface hydrodynamic conditions within the interval characterized by the highest quantitative mixing of the thermomineral groundwaters, including (iii) the physical and chemical properties of the LAC mineral groundwaters. The study underlines the presence of structural hydraulic boundaries (including the tectonically maintained permeability) having implications for a better assessment of the ongoing fault zone-related geothermal extraction. It further contributes to the overall hydrogeological potential (quantitative and qualitative).