GROUNDWATER CIRCULATION IN THE NEA KESSANI LOW-TEMPERATURE GEOTHERMAL FIELD (NE GREECE)

The low-temperature geothermal field of Nea Kessani, located in NE Greece, is characterized by a thermal reservoir made up of arkosic sandstones. The temperature distribution at depth, inferred from exploratory and productive wells, indicates that hot fluids rising from depth enter the arkosic reservoir in a restricted area of the field and flow towards local thermal springs. Well production tests have revealed the presence of hydrogeological boundaries within the arkosic reservoir. The geochemical characteristics of the thermal waters, which have an Na-C1/HCO3 composition and salinity varying between 5 and 6 g/L, indicate that these waters undergo conductive cooling within the reservoir. No admixture of waters from the aquifers in the cover has been observed. The slight chemical differences existing between the thermal waters are probably caused by CO2, which represents about two thirds by volume of the discharged fluid. This CO2, as indicated by its isotopic composition, could originate from decomposition of marbles of the Paleozoic basement underlying the arkosic reservoir and may also affect the isotopic composition of the thermal waters, which exhibit an interesting positive oxygen shift. However, such a shift could also be the result of water-rock exchange processes at low temperatures, since the water feeding the field comes from a regional circulation which, as indicated by its deuterium content, has recharge areas on the Rhodope Chain. Alternatively, the shift could be attributed to the contribution of a deep-seated high-temperature geothermal reservoir, but at present there is no evidence of high-temperature resources in the region. A maximum temperature of 110°C has been estimated by quartz geothermometry. The physical, chemical and hydrogeological data available so far have permitted us to formulate a fluid circulation model for the Nea Kessani geothermal field.