Cation-Exchange Capacity Distribution within Hydrothermal Systems and Its Relation to the Alteration Mineralogy and Electrical Resistivity

Abstract

Cation-exchange capacity (CEC) measurements are widely used to quantify the smectite content in altered rocks. Within this study, we measure the CEC of drill cuttings in four wells from three different high-temperature geothermal areas in Iceland. The CEC measurements in all four wells show similar depth/temperature related pattern, and when comparing the CEC with electrical resistivity logs, we could show that the low resistivity zone coincides with CEC values >5 meq/100 g. The measurements show, in general, an exponential decrease of the CEC with increasing depth. At the facies boundary between the mixed-layer clay and epidote-chlorite zone, the CEC reaches a steady state at about 5 meq/100 g and below that it only decreases slightly within a linear trend with increasing depth. The facies boundary overlaps with the transition where the electrical resistivity logs show an increase in resistivity. It is shown that the measured CEC can be related to the clay mineral alteration within the geothermal system and the CEC reflects the smectite component within the interstratified chlorite/smectite minerals for similar alteration degree. Furthermore, CEC was measured in seven core samples from different alteration zones that had previously been studied in detail with respect to petrophysical and conductivity properties. The results show a clear correlation between CEC and the iso-electrical point, which describes the value of the pore fluid conductivity where transition from surface conductivity to pore fluid conductivity occurs. The presented study shows that the CEC within hydrothermal altered basaltic systems mimics the expandable clay mineral alteration zones and coincides with electrical logs. The presented method can, therefore, be an easy tool to quantify alteration facies within geothermal exploration and drilling projects.