Nature and Classification of Geothermal Resources

Abstract

Geothermal resources are classified on basis of heat source, heat transfer, reservoir temperature, physical state, utilization and geological settings. They are e.g., separated into low-temperature systems, where temperature at 1km depth is below 150°C, and high temperature systems, where it exceeds 200°C. These are divided into subgroups based on the regional geological setting. The low-temperature systems are divided into: (1) shallow resources, mainly used through ground-source heat pumps, (2) sedimentary systems found in deep permeable layers in sedimentary basins of the world, (3) geo-pressured sedimentary systems in stratigraphic traps where pressures are close to lithostatic and (4) fracture-controlled convective systems where the heat source is technically active hot crust at depth, where water circulates to depth (>1km), through mostly vertical fractures, to extract heat. High-temperature geothermal systems are found in volcanically active areas of the Earth. The sub-classification applied here is based on tectonics and volcanism: (a) rift zone geothermal systems in volcanic systems on mid ocean ridges, on supra-marine rifts or in continental rifts, most on diverging plate boundaries, (b) hotspot volcanism is accompanied by geothermal activity based on magmatic intrusions, and (c) geothermal systems associated with stratovolcanous and calderas in compression regions, where oceanic plates collide and volcanic arcs are created on the continental side. Low-temperature resources are spread over most of the Earth while volcanic geothermal systems have a more limited distribution. Geothermal utilization has so-far been restricted to the most accessible and economically feasible resources while a few frontiers in geothermal research are being pursued, including enhanced (engineered) geothermal systems, deep roots of volcanic geothermal systems and geothermal resources on mid-ocean spreading ridges, all of which constitute an enormous energy potential for the future.