The contribution of shortwave and longwave infrared imaging spectroscopy (IRIS) to mapping alteration indicator minerals in geothermal reservoirs

Infrared spectroscopy has been successfully applied for mapping clay alteration in drill samples of high-enthalpy (i.e. volcanic) geothermal reservoirs. Particularly the ‘clay rank’ ranging (with increasing temperature) from smectite, to mixed-layer illite/smectite minerals, to pure illite is useful for mapping the clay cap of a geothermal system and help decide where the actual, exploitable reservoir starts. These clay minerals can be detected and differentiated with off-the-shelf portable infrared spectrometers. Many other useful indicator minerals, however, such as epidote, zeolites, Ca-sulphates and others, are less common and spectrally less active than those clay minerals. This means that in mixed rock spectra their signal may be ‘swamped out’ by those of the clay minerals, and the other indicator minerals may not be detected successfully. In this talk we will look at recent and ongoing research in using imaging spectrometers on geothermal drill cuttings to detect indicator minerals which may occur in small percentages (or in sub-percent levels) in the drill cuttings but have a vital impact on the correct geothermal reservoir interpretation. We will look at the potential of the SWIR IRIS and at the complementarity of LWIR IRIS data to answer some of the questions the SWIR range by itself cannot produce.