Pre-Investigation Geological Appraisal Of Geothermal Fields

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Journal Article: Pre-Investigation Geological Appraisal Of Geothermal Fields

In recent years there has been interest in the possibility of generating electricity from geothermal steam in many countries. The initial stage is the preliminary evaluation of geothermal resources and, apart from economic considerations, the problem is essentially geological. This paper deals with the factors involved in the selection of areas that warrant expenditure on investigation and development. Preferred requirements in geothermal fields for power generation are temperatures above 200°C and permeable aquifers or zones within 2000 m from the surface. The existence of these conditions is usually indicated by the presence of thermal springs and a rapid survey of these is first required. The world distribution of thermal springs is such that the greatest concentrations coincide with areas of late Tertiary and Quaternary volcanism, especially where volcanic rocks of that age are thickly developed. The presence of boiling springs, geysers and fumaroles in such a volcanic district indicates the existence of high temperatures within 1000 m from the surface. If the volcanism is on a regional scale there may be a number of geothermal areas, the presence of which emphasizes the existence of major geothermal resources. Some of the largest geothermal areas are located in regions where the eruption of huge volumes of rhyolitic and dacitic ignimbrites suggests the occurrence of crustal melting or intrusion. If individual thermal springs are scattered over a large area, it may be inferred that there is a relatively shallow aquifer capped by an impermeable unit. Linear distributions of springs should be sought, indicating the presence of faults or fissures that enable the hot water and steam to escape to the surface. Springs with large discharge and temperatures below boiling point indicate low geothermal gradient. Springs with small discharge indicate low permeability at depth or the presence of a cap-rock cut by few or faults. The probable conditions are therefore inferred from study of geological environment, structure and stratigraphy, and the type and distribution of thermal springs in relation to these. If chemical analyses of the waters are available these may indicate if the waters are steam heated of form a single large reservoir, and may give some idea of the temperature of this.

J. Healy

Published Journal 
Geothermics, 1970

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J. Healy. 1970. Pre-Investigation Geological Appraisal Of Geothermal Fields. Geothermics. (!) .

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