Under foreseeable economics and technology, extraction of geothermal resources is limited to the upper few kilometres of the earth's crust. At these depths, the global distribution of geothermal resources is primarily controlled by plate-tectonic features. Geothermal resources related to igneous intrusions in the upper crust occur along spreading ridges, subduction zones, inter-arc basins, and melting anomalies. Geothermal resources unrelated to igneous intrusions in the upper crust occur most commonly in porous sedimentary rocks near convergent or divergent plate boundaries where regional heat flow is high. Geothermal reservoirs at pressures well in excess of hydrostatic occur commonly in young tectonic basins characterized by high rates of sedimentation and subsidence; these reservoirs are commonly termed "geopressured." Except for the scattered melting anomalies, the central parts of crustal plates have low heat flow, little volcanism or tectonism, and accordingly no geothermal potential except at depths beyond present drilling limits. The hydrologic properties of crustal rocks are very important in determining location, size, and type of geothermal resource. Hot Dry Rock can result from solidification of a young intrusive body or from conductive heating of impermeable rock around such a body. Convective hydrothermal systems result either from convection of meteoric water around young intrusive bodies or from deep circulation of meteoric water along fracture zones. Geopressured reservoirs are formed in deep sedimentary basins when escape of connate water and water produced by the thermal dehydration of clays is impeded by sediments of low permeability