Use of Short-Wave Infrared Reflectance(SWIR) Spectroscopy to Characterize Hydrothermal Alteration Minerals in Olkaria Geothermal System, Kenya

Abstract

Hydrothermal minerals and mineral assemblages formed in a geothermal system provide information on both present and past geothermal reservoir conditions. Conventionally, identification and characterization of hydrothermal minerals in Olkaria geothermal system involves use of binocular microscopes, petrographic, fluid inclusion and XRD techniques. These methods though useful are time consuming and relatively expensive in terms of sample processing and analysis procedures. With high volume of samples being generated during geothermal drilling, a fast and cost-effective technique to map hydrothermal alteration minerals is required. Over time, Infrared spectroscopy has shown to be fast, cost-effective, non-destructive, reproducible and repeatable analytical technique. In this regard, this paper describes infrared spectroscopy technique and its potential to identify and characterize hydrothermal alteration minerals in a geothermal system. Particularly, it presents SWIR hyperspectral imaging results of three geothermal wells in Olkaria geothermal system. Hyperspectral images were acquired from drill cuttings samples using SWIR camera (Specim Ltd, Finland) located in the Geo-science Laboratory in the University of Twente ITC, Enschede, Netherlands. It had a pixel size of 260 µm and a spectral range of between 1000-2500 nm consisting of 288 spectral bands. Spectral identification was done by comparing mineral spectra with published spectra. Smectites, zeolites, illite, chlorite, calcite, epidote and amphiboles, were positively identified using this technique. Results were successfully used to interpret hydrothermal alteration minerals and mapping of alteration zones in each borehole studied. Lastly, information derived were used to decipher the location of borehole as either in-flow, out-flow or cold-influx zone which is critical information in borehole siting.