| Title | Validation of the Influence of Cation-Exchange Capacity on Resistivity Logs |
|---|---|
| Authors | Weisenberger, T B; Ingimarsson, H; Eyjólfsdóttir, E I; Lévy, L; Hersir G P; Flóvenz, O F |
| Year | 2016 |
| Conference | European Geothermal Congress |
| Keywords | Cation-exchange capacity, resistivity, Iceland, Krafla, Hellisheidi |
| Abstract | Resistivity models based on TEM (Transient Electro Magnetic) and MT (Magneto Telluric) soundings are widely used in geothermal exploration to select sites for exploration drilling. Resistivity models within high-temperature geothermal fields (e.g. Nesjavellir, southwest Iceland) mimic the dominant alteration mineralogy conditions in the subsurface. Thereby the typical observed boundary between the low-resistivity cap and the underlying high-resistive core in high-temperature systems can be related to changes in the secondary mineral inventory which have different cation-exchange capacity (CEC), i.e. from smectite dominated cap rock to the chlorite dominated core (Árnason et al 1987a; Árnason and Flóvenz 1992). Alteration minerals with a high CEC favour the conduction of an electrical current between the pore fluid and the pore walls in the rock. This interface or surface conduction is caused by highly mobile ions that form a conductive layer on the surface of the pore walls. The mobility of ions is thereby related to the CEC of the mineral phase (Flóvenz et al 2012). CEC measurements were conducted on drill cuttings in the Krafla and Hellisheidi geothermal fields in Iceland. High CEC values are associated with the clay dominated alteration zone and can be used as proxy for the degree and type of alteration of the volcanic succession. Thereby the CEC reflects the change in clay mineralogy with depth. Rock samples within the smectite alteration zone yield the highest CEC, whereas rocks within the chlorite zone and the epidote-actinolite zone result in a significant lower CEC and corresponding lower conductivity. The CEC, resistivity logs and temperature profile are compared to quantify the degree of physical and chemical parameters that control the electrical conductivity. This allows us to quantify the direct influence of each of those parameters on the electric resistivity. |