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Well ThG-15 is located within the Theistareykir geothermal field in NE-Iceland. It is a directional production well drilled to a total depth of 2260 m. 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 16 m down to 1992 m to determine the lithology, mineral alteration and the occurrence of intrusive rocks in well ThG-15. The stratigraphy of well ThG-15 is characterised by lava flows, which are intercalated by reworked sedimentary tuffs in the upper 114 m, hyaloclastites (basaltic breccia, tuff, glassy basalt) from 114 to 920 m and fine- to medium-grained crystalline basalt below 920 m. The hydrothermal alteration minerals in well ThG-15 are wairakite, zeolite, smectite, mixed-layer clay, quartz, pyrite, calcite, chlorite, epidote, prehnite, actinolite and wollastonite. Alteration zones have been established based on appearance and temperature dependency of alteration minerals in well ThG-15. These zones are zeolite-smectite zone (58-210 m), mixed-layer clay zone (210-445 m), chlorite zone (445-610 m), chlorite-epidote zone (610-978 m) and epidote-actinolite zone (978-1992 m). The most permeable zones in the well based on circulation losses and temperature logs were encountered at 1455-1475, 1800-1830 m and at 2090 m depth. Measured temperature after three months of well heating-up indicates that the geothermal system follows the boiling point curve in the upper 1000 m. Alteration temperature correlates with the measured temperatures at this depth, but well ThG-15 experiences cooling below 1000 m, possibly due to cold-water inflow from the feed zone/fractures in the deepest section of the well. |
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