| Title | The Resistivity Structure of High-Temperature Geothermal Systems in Iceland |
|---|---|
| Authors | Knutur Arnason, Ragna Karlsdottir, Hjalmar Eysteinsson, Olafur G. Flovenz and Steinar Thor Gudlaugsson |
| Year | 2000 |
| Conference | World Geothermal Congress |
| Keywords | Exploration, Resistivity structure, Thermal alteration |
| Abstract | Electrical and electro-magnetic methods have been used extensively to identify and delineate high-temperature geothermal reservoirs in Iceland. All high-temperature systems, within the basaltic crust in Iceland, have a similar resistivity structure, characterized by a low resistivity cap at the outer margins of the reservoir, underlain by a more resistive core towards the inner part. This is found in freshwater systems as well as brine systems, with the same character but lower resistivities in the brine systems. Comparison of this resistivity structure with data from wells shows a good correlation with alteration mineralogy. The low resistivity in the low-resistivity cap is dominated by conductive minerals in the smectite-zeolite zone in the temperature range of 100-220?C. At temperatures 220-240?C zeolites disappear and the smectite is gradually replaced by the resistive chlorite. At temperatures exceeding 250?C chlorite and epidote are the dominant minerals and the resistivity is probably dominated by the pore fluid conduction in the high-resistivity core. The important consequence of this is that the observed resistivity structure can be interpreted in terms of temperature distribution. A similar resistivity structure is to be expected in acidic rocks. Due to different alteration mineralogy, however, the transition from the conductive cap to the more resistive core, presumably occurs at temperatures lower than 200?C. |