The Nestos basin lies E of Kavala where the Delta of Nestos has been formed. The onshore part of the Basin covers an area of 600 km (the offshore extension of this basin is
known as the Prinos Basin).
The basin is rather deep with an axis of ENE - WSW direction, The basin is seperated from Xanthi - Komotini Basin by the Abdira - Fanari Ridge, which is mainly composed of metamorphosed rocks. Towards the north, south and west the basin boundaries are granites and metamorphosed rocks sequences of the Serbomacedonian massif.
The Nestos sedimentary basin started its formation at the end of the Lower Miocene (Serravalian).
The Nestos Basin attains depths of 3,500 - 4,000 m at several points. The typical stratigraphic column for the basin consists of 800 - 900 m of Plio-Quaternary rocks at the upper part and 700 - 900 m of Miocene rocks in the lower parts.
The rocks types which are represented in these sequences include alternations of sands, gravels, sandstones, conglomerates, limestone layers, siltstones amd the Miocene evaporite sequence and in the deeper parts varve sequences with alternating sandstones and siltstones.
The basement of the Nestos basin consits of metamoprhic rocks of the Rhodope massif (mainly gneisses, amphibolites and marbles).
The paleogeographical and stratigraphical evolution of the basin has been reconstructed on the basis from 3 deep oil exploratory boreholes (NESTOS-1, NESTOS-2, NESTOS-3), one deep thermal exploratory borehole (N-1G), 14 slim geothermal exploratory boreholes of indermediate depth and some water supply wells.
A point of interest for the upper part of the Tertiary rock sequence in the basin is its good porosity and permeability.
For borehole NESTOS-3 the porosity, for the upper 800 m, for all interesting strata has been estimated to vary from 25 - 32% followed by a good to very good permeability. In the boreholes NESTOS-1 and NESTOS-2 several horizons with geothermal interest have been detected within the upper 1500 m.
The geothermal gradient for the Nestos basin has been estimated to fluctuate from 31 to 37 oC/km. This value is considerably higher than the normal geothermal gradient.
In the basin of Nestos several geothermal manifestastions and geothermal fields exist (for example, the geothermal fields of Eratino - Chrysoupolis and Neo Erasmio - Magana).
Considering the fact that the geological basement is relatively "hot" over the whole area of the basin (178 oC in borehole NESTOS-1 and 122 oC in N-1G), we can assume that the thermal character of the area is influenced mainly by the active extension tectonic and correlated with the metamorphic basement.
The thermal anomaly is located above a buried horst of the basement. The anomaly is connected with an important N140o fault system that affects the basement.
The hot (70 oC) water-bearing formation drilled between 550 - 650 m, by the borehole N-1G consists of a secondary resevoir within the Neogenic sediments and is possibly heated by conduction. That is based on the fact that the only water samples considered as hot in the geochemical study seems not to have been contaminated by waters of deeper origin. The reservoir contains Na-CI waters with TDS of 7-8 g/l (as determined from measurements during the drilling of N-1G) and its extend should be considered as equal to that of the area of high geothermal gradient.
On the western side of the sedimentary basin of the Nestos River delta, two main hydrothermal targets exist:
(a) the permeable zone in the lowermost part of the sedimantary sequence (transitional sedimantary zone calcanerites oolithic limestones) and
(b) the fractured rigid and/or permeable (carbonate?) parts of the metamorphic formation below the lower marble amphibolitic series of the Rhodope Massif.
No well-defined fracture zone was encountered by borehole N-1G, drilled in the center of the area of high thermal gradient, except for some fractures in the altered lower marble amphibolitic series which favor for heat transfer (both conductive and convective) towards the sedimentary host (secondary) reservoir.
Low enthalpy (70 oC) fluids exist at 600 m depth and hotter, medium enthalpy fluids probably exist in the basement. These hot fluids have an equilibrium temperature of 150-180 oC and probably constitute an important and extensive reservoir within the basement.
The existence of such a reservoir is necessary to justify the extended thermal anomaly, covering an area of about 40 km2 in a region without active magmatism. The geology and lithology of the basement together with the high values of the geothermal gradient indicate that the crystalline series acts as a cover for the hot hydrothermal system, and that this aquifer is located at a depth of 1500 - 1800 m.
A hot primary reservoir with a temperature in the order of 150 - 180 oC should be the source which heats, by conduction, the overlying formations and the secondary reservoir.
Summarizing, all the data point to the existence of a medium enthalpy hydrothermal system within the metamorphic basement; the so-called lower marbles of the Rhodopian Massif, or, alternatively, the fractured zones of the "rigid" formations (e.g. migmatic gneisses, intensively fractured plutonites), are likely the main reservoir.
In this area, 20 exploratoy nad 3 productive boreholes have been drilled. A reservoir with a temperature of 40o - 65o C has been discovered at depths 200 - 400 m. The fluids are characterised as Na-Cl, Na-HCO3 and their TDS is 1-12 g/l.