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Well ThG-17 is in the Theistareykir geothermal field under the slopes of Mt. Baejarfjall. The geothermal system is caused by an active central volcano. The well was directionally drilled to a depth of 2500 m. This report will focus on data from geological logging down to 2300 m depth. Binocular microscopic, thin-section petrography and X-ray diffraction analyses of samples were the major analytical methods used to analyse rock cuttings collected from a depth of 18 m down to 2300 m to determine the lithology, mineral alteration and the incidence of intrusive rocks in well ThG-17. Well ThG-17 is characterised by different hyaloclastite formations. Glassy basalt is dominant, but also tuff, reworked tuff and basaltic breccia, fine- to medium-grained and medium- to coarse-grained basalts were identified. The hydrothermal alteration minerals in well ThG-17, which were found with increasing depth, provide information on the paleotemperature of the geothermal reservoir as indicated by the presence of alteration minerals that represent low temperature ranging from 50°C to 200°C (e.g. smectite, scolecite, mesolite), intermediate temperature ranging from 200°C to 250°C (e.g. quartz, chlorite) and high temperature ranging from 250°C and higher (e.g. prehnite, epidote, wollastonite, actinolite and garnet). Six alteration mineral zones defined by the first appearance of alteration minerals were established: unaltered (0-24 m), zeolite-smectite (24-172 m), mixed layer clay (172-300 m), chlorite (300-600 m), chlorite-epidote (600-1200 m) and epidote-actinolite (1200-2300 m).
All feed zones in well ThG-17 are of small to moderate size and the most permeable feed zones are located at 2030 m and 2300 m. An analysis of the temperature logs illustrates that the geothermal system follows the boiling point curve in the upper 300 m. However, a temperature reversal occurs around 400 m. This decline in measured temperature reflects a cooling of the geothermal system possibly due to an influx of cold water through highly permeable zones. Notwithstanding, the temperature recovers and continues to follow the boiling point curve until another temperature reversal occurs below 2000 m. This decline in measured temperature exhibits a cooling of the geothermal system in the lower section which again could possibly be due to an influx of cold water through highly permeable zones and/or fractures intersecting the lower section of the well. |
