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Sub-surface Rock Alteration Mapping of OW 205 at Olkaria Geothermal System Using Conventional Laboratory Techniques and Hyperspectral Imaging of Drill Cuttings
Mathew KAMAU
[KENGEN, Kenya]
Hydrothermal alteration is a common process in active geothermal systems. It can significantly change the physio-chemical properties of the parental rock material. The dissolution and transformation of primary minerals and the formation of hydrothermally altered minerals depends on the prevailing sub-surfaces conditions of a geothermal reservoir. Identification and characterization of the hydrothermal minerals is key to unraveling the predominant sub-surface conditions. In Olkaria, conventional techniques such as binocular and petrographic analysis have been used to identify hydrothermal alteration minerals. However, with introduction of spectroscopy technique in Olkaria, it is imperative to compare hydrothermal alteration minerals identified petrographically with those identified by infrared spectroscopy techniques. Hence, this study compares infrared spectroscopy and petrographic analysis of hydrothermal alteration minerals in OW 205 drilling cutting samples. The study will also include results from binocular observation of the drilling cuttings at the rig site. Binocular analysis was done by visually observing drilling cutting samples using a binocular microscope while petrographic analysis involved first preparing thin section with thickness of about 30 micrometers by mounting drilling cuttings on glass slides. Petrographic studies involved viewing samples in a thin section using a petrographic microscope. On the other hand, spectroscopy analysis was done by first imaging drill cutting sample using short wave infrared (SWIR) camera. Images of the samples were made by passing samples on a translation stage underneath a camera. Minerals such as chlorite, epidote, actinolite, zeolites were positively identified by the two techniques. However, minerals such as chalcedony, prehnite, quartz, and albite were only identified using petrographic analysis. Generally, the integrated use of binocular observations, thin section petrographic analysis, and infrared spectroscopy provide reliable data for hydrothermal alteration characterization in geothermal wells and development of a conceptual model.
Topic: Geology