Geochemistry of the Germencik Geothermal Fields, Turkey

Abstract

The Germencik geothermal fields that are among the most important fields of Turkey. The fields contain two geothermal reservoirs. The first reservoir consists of fractured gneiss and quartz schists, and karstic marbles of the Menderes Massif rocks. Overlying Neogene conglomerates comprise the second geothermal reservoir. Neogene aged clastic sediments, especially clayey levels form the cap rock of the system. The heat source may be near surface magma intruded along the active graben fault zones developed by an extensional tectonic regime. The geothermal waters are high enthalpy, meteoric origin and old. Meteoric waters recharge the reservoir rock, are heated at depth, and move up to the surface by convection. Five hot water and seven cold water locations were sampled each month for one year to evaluate the hydrogeochemical evolution of geothermal waters and the hydrodynamics of groundwater systems. All thermal waters in the area are of the sodium-bicarbonate-chloride type and the chemistry does not vary temporarily. The measured temperatures of the thermal springs vary from 50oC to 70oC. Estimated reservoir temperatures using various chemical geothermometers and mixing models vary between 150oC and 250oC, which correspond with a high enthalpy system. The estimated reservoir temperatures obtained by Na/K and Na-K-Ca geothermometers coincide with the downhole temperatures (200oC - 232oC) measured directly in nine deep wells. Alteration minerals, which may be formed by the reactions between host rock and thermal fluid, include Ca and Na montmorillonite, Na and K feldspars, zoisite and albite.