HR: 15:10h
AN: V53F-07    [Abstracts]
TI: The geo-scientific basis for the geothermal evolution in Iceland
AU: * Flovenz, O G
EM: ogf@isor.is
AF: ISOR - Iceland GeoSurvey, Grensasvegur 9, Reykjavik, IS-110, Iceland
AB: More than half of the primary energy use in Iceland is economically produced from geothermal resources. The main reasons for this unique success in Iceland are favourable geological conditions and highly developed technology in geosciences and engineering. Iceland is a sub-aerial part of the ocean floor, located where the central axis of the Mid-Atlantic Ridge intersect the Iceland hot-spot, resulting in abnormal crustal thickness and complicated tectonic patterns. The ridge axis crosses the island from South-West to North East forming a volcanic rift zones that is characterized by many active central volcanoes and associated high temperature geothermal fields (T more than 200°C at 1 km depth). The rift zone is highly faulted and the uppermost 1 km is composed of permeable young basaltic material. Outside the volcanic zone the crust is normally made of altered basaltic lavas of low primary permeability due to secondary mineralization. However, recent tectonic activity, probably due to glacial rebound and relative movement of the ridge axis and the hot spot, has formed permeable fractures that are pathways for geothermal fluid and result in numerous low temperature geothermal fields ( T less than 150°C at 1km depth). The background heat flow in Iceland varies with age from 70 to 250 mW/m2 and the crustal thickness varies from 20 to nearly 40 km. Geothermal exploration is done with a multidisciplinary approach where geological mapping, geochemistry and geophysics interact. The geological mapping with emphasis on tectonic structure, stratigraphy, hydrothermal alteration and eruption history is usually the first step. If hot springs or fumaroles exist, chemical methods are used to predict the reservoir temperature and the fluid properties prior to drilling. Geophysical surveys are the most widely used methods to detect subsurface high temperature fields and to estimate their size and properties. Resistivity soundings, mainly based on TEM and MT measurements, play the key role, but analysis of natural seismic events, aeromagnetic and gravity surveys are also helpful. In case of exploring for low temperature fields heat flow measurements are also important as well as geophysical methods to detect water-bearing fractures. The exploratory work leads to a conceptual model of the geothermal field. During the exploratory drilling phase borehole geology supported by geophysical well logging are the main tasks, which together with results from well testing make the basis for a revised geological and hydrological model of the reservoir. During exploitation geo-scientific research is still important to understand the response of the geothermal field to exploitation. This includes monitoring of temperature, pressure, induced seismicity as well as changes in fluid chemistry, geodesy and gravity.
DE: 0925 Magnetic and electrical methods (5109)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting