Borehole Geology of Well THG-15 at Theistareykir Geothermal Field, NE-Iceland
[Tanzania Geothermal Development Company, Tanzania, United Rep of]
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 on 16th of March 2017. 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 temperatures after three months of well heating-up indicate 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. Generally, drilling of geothermal wells (boreholes) is the best option for confirming surface exploration data and resource estimation. Detailed subsurface geology (lithology, alteration minerals, intrusives, cracks/faults) and well data (well orientation, measured depth, feed zones, and temperature profiles) provide useful insights regarding the geologic past and prevailing conditions of the geothermal system. Therefore, information decoded from drill cuttings and temperature logs in a particular geothermal field are vital for refining existing conceptual models and sitting of future drilling targets.
|        Topic: Geology||Paper Number: 12063|