Table 3 Classification of geothermal systems with regard to temperature enthalpy and physical state after Saemundsson et al. (2009); Bodvarsson (1961)

Low-temperature (LT) systems with reservoir temperature at 1 km depth below 150\({}^{\circ }\hbox {C}\). Often characterized by hot and boiling springs.

Low-enthalpy geothermal systems with reservoir fluid enthalpies less than 800 \(\hbox {kJ}\,\hbox {kg}^{-1}\), corresponding to temperatures less than 190\({}^{\circ }\hbox {C}\).

Liquid-dominated geothermal reservoirs with the water temperature at, or below, the boiling point at the prevailing pressure and the water phase controls the pressure in the reservoir. Some steam may be present.

Medium-temperature (MT) systems with reservoir temperature at 1 km depth between 150–200\({}^{\circ }\hbox {C}\).

High-temperature (HT) systems with reservoir temperature at 1 km depth above 200\({}^{\circ }\hbox {C}\). Characterized by fumaroles, steam vents, mud pools and highly altered ground.

High-enthalpy geothermal systems with reservoir fluid enthalpies greater than 800 \(\hbox {kJ}\,\hbox {kg}^{-1}\).

Two-phase geothermal reservoirs where steam and water co-exist and the temperature and pressure follow the boiling point curve.

Vapor-dominated reservoirs where temperature is at, or above, boiling at the prevailing pressure and the steam phase controls the pressure in the reservoir. Some liquid water may be present.