An underground volume of rock that contains such an amount of geothermal heat that it can be economically exploited. It may also be an underground hot water deposit. The geothermal reservoir must be at a reachable depth, be at a sufficient temperature and have the right rock composition to, for example, conduct heat well or not leak water used for heat removal through cracks. If the geothermal reservoir already contains groundwater that will be used to power the geothermal power plant, it is a useful idea to know its chemical composition in order to prevent corrosion when dissolved minerals come into contact with the plant equipment.