Geothermal systems in global perspective

Abstract

Geothermal systems are classified in three main categories: (a) volcanic geothermal systems with the heat source being hot intrusions or magma chambers in the crust, (b) convective systems with deep water circulation in tectonically active areas preferably of high geothermal gradient, (c) sedimentary systems with permeable layers at great depth (2-5 km), including geo-pressured systems often found in conjunction with oil resources. Some 50 years ago geothermal fields were divided into high- and low- temperature fields or areas. This division was based on (arbitrarily) inferred temperature at 1 km depth. It was soon realized that most of the former are related to volcanism (>160 - >300 °C) (category a) and that the latter draw heat from the general heat flow of the crust (just above ambient to >160°C) (categories b and c). The temperature range given here is based on values measured in boreholes in Iceland or silica and gas geothermometry. Other subdivisions have been proposed by adding an intermediate group between the two main ones. There are several types in each of the two main groups. They are described below, emphasizing the geological control. A fourth category constitutes the so called hot dry rock. The thermal resource there is abnormally hot, usually intrusive rock of very low permeability and not harnessable so far.