Borehole Ce-2/95 at Cerkno at the boundary between the Southern Alps and the External Dinarides (Slovenia)

Abstract

Thermal water has been tapped by a hydrogeologic-exploration and exploitation borehole Ce-2/95, drilled at Cerkno in 1995 to a depth of 2004 m. It is located in the northwestern margin of the External Dinarides that are characterized by the nappe structure. The borehole has drilled through the Trnovo nappe composed of low permeable Carboniferous- Permian clastics (down to a 95 m depth), then through the upper part of the Hrušica nappe with clastic and pyroclastic rocks of the Pseudozilian Formation (down to a 545 m depth), the Main dolomite Formation (down to a 835 m depth), and finally the Malmian limestone of the middle part of the Hrušica nappe. The borehole is important in structural sense since it confirms the model of the nappe structure of the NW margin of the External Dinarides. On the basis of analysis of the section structure between Gorizia and Cerkno it was possible to determine the movement between the Trnovo and Hrušica nappes that is roughly 30.5 km, therefore, almost the same as in the section over Idrija (32 km). The main aquifer drilled by the borehole is Malmian limestone, while the predominant part of the recharge area for the borehole is in the Blegoš-Vrhnika range that is a part of the Hrušica nappe. Geothermal measurements in the borehole have revealed several main inflow zones of thermal water in a depth range of 847 and 1528 m. At the deepest measuring point (1940 m) 46°C has been determined. Due to deep water circulation this is probably not the real formation temperature for this depth. The calculated heat flow density of 50 mW/m2 is in agreement with regional HFD map.