Geothermometry of Sofia Field Hydrothermal Systems, Bulgaria

Abstract

The major reservoir rock types in Sofia geothermal field are Triassic limestone and sandstone, Jurassic limestone, Cretaceous sandstone and andesite. As secondary reservoirs, Pliocene sands accumulate the hot water and directly cover the bedrock. The chemical composition of 30 natural springs and geothermal wells in this field were taken into consideration. Several geothermometers (chalcedony, Na-K-Ca, K-Mg, Na-K and Na-Li) have been used to predict subsnrface temperatures. It can be concluded that for Sofia field the chalcedony geothermometer is found to be the most reliable one of the studied geothermometers. According to it the highest predicted temperatures in the geothermal reservoir are above 90?C. Based on all geothermometer data it is possible that water with a relatively high temperature can be found in the northern part of the city of Sofia and in the towns of Mramor, Dobroslavtzi, Gniliane, Trebich and within an area east of Sofia, between the towns of Kazichene and Ravno pole where 81?C geothermal water has already been found. All these prospects areas are in the sedimentary rocks of the field. The maximum temperature for the accumulated water in the volcanic rocks of the southern parts of Sofia is not expected to be higher than 45?C. These results are in agreement with those of the authors obtained by the helium method and other methods of geothermal exploration that have been used in this field. Possibilities of application of the established correlation ìtrace elements abundance - Td-6íí like new indicators, concerning the dynamics and the kinetics of the genetic processes and the migration in the thermal fluids - kom the deep reservoir up to the emergence - are elucidated. Apparently, no geothermal water with significantly higher temperature than the measured temperature of existing waters is likely to be found.