| Keywords |
hyperspectral, infrared spectroscopy, hydrothermal alteration, Nevada, great basin, drill core, Fallon FORGE, Tungsten Mountain, EGS, SWIR |
| Abstract |
Infrared spectroscopy is effective for analysing geologic materials because most rock-forming minerals have absorption features in the short-wave infrared (1000-2500 nm, SWIR) and long-wave infrared (2.5-25 μm, LWIR). Applied to drill core or cuttings samples, infrared spectroscopy is useful for characterising the lithologies and minerals encountered in geothermal wells. The type and distribution of hydrothermal alteration mineralogy within a well provides information about past temperatures, field hydrology, and rock physical properties. In this study, we apply advances in hyperspectral imaging technology to create high-resolution mineral maps of geothermal drill core and cuttings samples to characterise reservoir lithologies and alteration assemblages. The primary instrument used in this study is equipped with VNIR-SWIR (500-2500 nm) and LWIR (7.6-11.9 μm) spectral cameras that have a pixel size of 1.2 mm, as well as an RGB camera, to create a high resolution (0.12 mm pixel size) image of the same drill core or cuttings sample. Hyperspectral imaging allows for rapid analyses with minimal sample preparation and allows for the evaluation of the spatial relationships of alteration minerals within samples, with depth, and between wells at higher spatial resolutions than typically applied to geothermal systems. We investigate two field sites from Nevada, western USA: the Tungsten Mountain geothermal field, operated by Ormat Nevada, Inc., and the Fallon FORGE Engineered Geothermal System (EGS) site. These two locations represent contrasting geothermal endmembers: an active hydrothermal system (Tungsten Mountain, currently with 37 MWe installed capacity) and low permeability, conductive geothermal system (the Fallon FORGE EGS site). At both locations, we analyze samples from several wells in the 300 to 2800 m depth range and use infrared spectroscopy to characterize the distribution of minerals at various scales. Comparison of these new hyperspectral data with existing datasets (e.g. structural, stratigraphic, geophysical, mineralogical, hydrologic, temperature) can reveal new insights on the relationship of hydrothermal alteration to reservoir properties, with implications for geothermal reservoir characterization, and hydrothermal alteration history. |