Carbon Dioxide Diffuse Soil Degassing: A Precious Tool For Identifying Productive Geothermal Reservoirs

Abstract

that deep wells find high temperatures but are not productive because they don’t cross any permeable fractured reservoir. Because of the high cost of deep drillings, this aspect represents one of the main economic risk of geothermal exploration. The aim of this paper is to show that a detailed survey of diffuse CO2 soil flux allows to identify from the surface the permeable portions of a deep-seated actively degassing geothermal reservoir, drastically reducing that risk. The first application of the method has been made in the Quaternary Latera caldera, North of Rome. We will show that productive wells were all located on high CO2 flux zones, whereas the not-productive wells were sited on low flux areas. In addition the survey allows to identify some as yet unexplored portions of the geothermal reservoir where future wells should be conveniently located. The same technique has been applied in the geothermal exploration of Platanares and Azacualpa, Honduras and of Las Pailas, Costa Rica. Obviously, CO2 flux cannot provide any estimate of temperature at depth, which has to be assessed with other geochemical or geophysical exploration techniques.