Petrology as a Practical Tool in Geothermal Studies

Abstract

Petrological studies of subsurface samples are useful and cost effective in helping to determine the geology and hydrology of geothermal fields. Specific hydrothermal minerals form in response to water/rock reactions at elevated temperatures. The equilibrium assemblages of minerals are know for a wide range of natural conditions of temperature, salinity and fluid pressure. The degree to which equilibrium is attained in a given geothermal reservoir is a function of age, effect on permeability and the rates of change of temperature and water chemistry. Permeability varies during the life of a geothermal system due to leaching and precipitation of minerals and because of faulting and hydraulic fracturing. Examples drawn from recent studies in New Zealand and California illustrate how sequences of minerals preserve a record in the reservoir rocks of successive episode of water/rock reaction and change in permeability. These studies have obvious applications to resource assessment and management.