Geothermal energy in its broadest sense is the heat stored within earth's crust. Total heat stored, estimated with global heat flow approach, reaches 405× (10) °24 joules above 20 C for 10 km of crustal depth. This estimate drops to 35× (10) °24 joules if we consider 3 km of depth. However only a small portion of this enormous energy is exploitable due to geological, technical and economic constraints. The different types of resources classified according to their temperatures, will require different energy extraction methods and used. No matter which mode is chosen for utilization, electricity production or direct use, exploitation and reservoir management will be that important part the must be handle with skill. OCCURRENCE OF GEOTHERMAL SOURCES There is four main prerequisites for a geothermal resource to become exploitable. A heat source, magma body or hot dry rock at depth Carrier of heat, a fluid Permeable bed which will transmit and enable the production of carrier fluid through well bores Cap rock which will confine the fluid convection in the reservoir These hydrothermal systems are classified according to their temperature range as: Low enthalpy systems High temperature systems Hot dry rock systems. Low enthalpy systems are candidates for direct applications and fluid exists as a liquid in the reservoir. High temperature resources are exploited for electricity production and they may be all liquid, two phase or all vapor systems. Man made reservoirs, hot dry rock systems are still at the stage of demonstration but exhibit a challenging method of mining the heat of hot rocks. Location of geothermal areas on the crust is dictated by global plate tectonics. Mainly six geothermal belts exist according to this theory.
1. Pacific belt
2. Mid Atlantic belt
3. Alpine Himalayan belt
4. East African belt
5. Central Asian belt
6. Pacific belt



