Exploration Drilling in the Theistareykir High-Temperature Field, NE-Iceland: Stratigraphy, Alteration and Its Relationship to Temperature Structure

Abstract

The Theistareykir high temperature geothermal area is located some 25 km north of Lake Mývatn in North-East Iceland. This remote geothermal area has been the subject of surface exploration off and on for almost 40 years with the utilization of geothermal energy for electricity production as a long term goal.
Exploration drilling started in 2002 with the completion of well ThG-01, a vertical well reaching 1953 m below the surface. Since then 5 more wells have been completed including three wells using directional drilling. In addition one production section was abandoned and a new production section drilled from wellhead ThG-5, providing data from a total of 7 transects in the potential production zone.
In the top ~1 km the stratigraphy is characterized by sub glacial eruption products. These hyaloclastites are typically formed of pillow basalts, breccias and tuffs. The hyaloclastites are emplaced upon a succession of basalt-flows separated by relatively thick breccias and tuffs. The pile is sporadically cut by thin basaltic dikes. Alteration of the bedrock is extensive and minerals indicating high temperatures are found high up in the stratigraphic column. A narrow zone of overpressure is encountered in some wells between ~100 m and ~300 m depth.
Bedrock temperatures are among the highest recorded thus far in Iceland and the chemistry of the fluid is amenable to electrical production.