PRELIMINARY STUDY OF DOWN-HOLE RESISTIVITY FROM 72 BOREHOLES IN THE S-HENGILL GEOTHERMAL FIELD, SW-ICELAND, WITH RESPECT TO SURFACE RESISTIVITY DATA AND ALTERATION MINERALS

摘要

Resistivity logs from 72 boreholes in southern part of Hengill high temperature field, and resistivity interpreted from surface measurements (TEM and MT) are compared. The resistivity is also compared with the uppermost appearance of alteration minerals with special emphasis on clay minerals. The available resistivity logs from each section of drilling are combined for each well and inserted into Petrel, a 3D visualization software, for geological and geophysical data. A 3D model of resistivity was also made in Petrel based on 1D resistivity found previously from TEM and MT data by doing a joint inversion. The resistivity well logs are averaged to the same resolution as the TEM and MT data. Previous TEM and MT resistivity models show high-resistivity formations underlain by low-resistivity ones below which resistivity increases again. This variation was interpreted as being due to the different conductivity of alteration mineral assemblages. Low resistivity was linked with zeolites and smectite and higher resistivity with chlorite deeper down in the wells. The comparison between the resistivity from the surface TEM and MT soundings and the resistivity logs shows a good match in many of the wells. A further comparison with alteration zones in the wells also shows that the onset of the smectite zeolite zone correlates with the lowering of the resistivity, and that resistivity starts to increase again at the upper boundary of epidote-chlorite alteration and in some cases amphibole.