Geothermal resources assessment using temperature–depth relationships in the fault-controlled hydrothermal system of Aristino-Traianoupolis area, Northern Greece

Abstract

Aristino-Traianoupolis area hosts one of the most significant water-dominated low-temperature geothermal fields in Greece. It is located on the southwestern uplifted margin of the Tertiary Evros Delta molassic basin, 10 km east of the town of Alexandroupolis (Thrace, NE Greece). The upper hydrothermal system of the Aristino Geothermal Field (AGF), one of the most promising in continental Greece, contains fluids with temperatures ranging from 51 to 99 °C, within a series of overlapping aquifers at very low depths (100–430 m). The main geothermal anomaly for temperatures higher than 90 °C covers an area of 6 km2, to a maximum prospected depth of 500 m below ground surface. The scattered regional anomaly exceeds 50 km2 and is characterized by excessively high and abruptly changing thermal gradient (42 to 450 °C/km) and heat flow (80–800 mW/m2), that are both typical of a fault-controlled hydrothermal system. Since 1993, the AGF has undergone non-systematic geothermal investigation, with emphasis on low-depth (100–500 m) drilling. This paper provides, for the first time, a synthetic and detailed evaluation of all available temperature data gathered in the last 25 years. The steady-state temperature logs reveal the dominant role of conduction for the upper geothermal system, accompanied, in most cases, by rapidly changing and abnormally high thermal gradients (100–450 °C/km), triggered, most probably, by a deeper system of higher temperature. This hypothesis is also supported by the applied chemical geothermometers, which suggest initial fluid temperatures at 140–150 °C, the hydrochemical characteristics of the fluids hosted in the deeper and most promising investigated reservoir (ignimbrite) of the upper system, and the extrapolated temperatures from the conductive temperature–depth profiles. The lower widespread medium enthalpy hydrothermal system should extend at depths 500–1000 m within volcanics and the expected Eocene limestones and basal clastic series of the Tertiary sequence that have filled the basin. Nevertheless, these assumptions need to be verified by appropriate investigations and new drillings at depths greater than 600–700 m, which would confirm the presence of a productive medium enthalpy reservoir.