The H·gˆngur High-Temperature Geothermal Field, Central-Iceland. Surface Exploration and Drilling of the First Borehole: Lithology, Alteration and Geological Setting

Abstract

The H·gˆngur high-temperature area in Central-Iceland is considered to represent one of the larger geothermal fields in Iceland. The field is being explored by Landsvirkjun. Surface exploration started with reconnaissance geological mapping and mapping of geothermal manifestations in 1995 in association with plans to construct a hydrodam in the area and partially submerging the thermal area. Acid volcanics suggested the presence of a central volcano, and a semi-circular alignment of the acid eruptives imply the possibility of an underlying caldera. TEM resistivity survey in the area showed a large (20-40 km2) geothermal anomaly indicating a subsurface high-temperature anomaly. Sampling of gas and fluid from geothermal manifestations indicated subsurface temperatures up to 290? C. The first exploration borehole was sited in the central part of the resistivity anomaly. The well has a 9 5/8" production casing down to 800 m and a production part drilled down to 2360 m. The stratigraphy shows basaltic extrusives and thick acidic units in the upper 700 m and basaltic hyaloclastite and lava formations down to 1900 m. From 1900 m basaltic intrusives prevail with 100 m thick acid intrusion at the very bottom. The hydrothermal alteration shows a progressive increase in alteration from smectite-zeolite zone near surface succeeded by a chlorite-epidote zone below 580 meters depth. A preliminary comparison with measured temperatures indicates an equilibrium.