| Title | Comparison of Down-Hole Data and Surface Resistivity Data from S-Hengill, a High Temperature Geothermal Field in SW-Iceland |
|---|---|
| Authors | Svanbjörg Helga HARALDSDÓTTIR, Hjalti FRANZSON and Knútur ÁRNASON |
| Year | 2015 |
| Conference | World Geothermal Congress |
| Keywords | Resistivity-logs, neutron-logs, natural gamma-logs, rock temperature, 3D resistivity, MT, TEM, high temperature geothermal field, alteration, Hellisheidi, S-Hengill, Iceland, Petrel |
| Abstract | Resistivity logs from 72 boreholes in southern part of Hengill high temperature field, and resistivity interpreted from surface measurements, TEM and MT soundings, are compared. The resistivity is also compared to the first appearances of alteration minerals. The full comparison was made with an interpolated 3D model based on joint 1D inversion of TEM and MT electromagnetic soundings, where the resistivity was projected on the well paths and formed pseudo logs, which were compared to the resistivity logs which had been up scaled to the same resolution as the resistivity from TEM/MT. In addition pseudo logs were made based on a 3D resistivity model from a joint inversion of the TEM/MT data. These were compared to averaged logs from the resistivity logs in a similar resolution and to the already made pseudo logs from the 1D model. For preparing the data the available resistivity logs from each section of drilling were combined for each well and inserted into Petrel, a 3D visualization software, for geological and geophysical data. Previous studies of resistivity from high temperature geothermal systems in Iceland have shown a low resistivity cap on the outer margins of the reservoir underlain by higher resistivity and further down a deep conductor. The resistivity pattern has been explained with variations of resistivity of temperature alteration minerals and represents a “maximum thermometer” if a geothermal system has cooled. 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. The 3D resistivity model deduced from the TEM/MT electromagnetic soundings has much lower resolution than the interpolated model from the 1D inversion and the former showed less of the features seen in the averaged resistivity logs than the pseudo logs deduced from the 1D inversion. Resistivity well logs in 5 selected wells are shown in the paper. The resistivity model found by interpolating the 1D inversions of TEM/MT data and the more recent 3D model found by inversion of the same data and the averaged logs as well as the alteration minerals as well as an example of the neutron-neutron and gamma-logs will be given. |