The Hengill Geothermal System, Conceptual Model and Thermal Evolution

Abstract

The Hengill volcanic system in SW-Iceland is a central volcano that produces basaltic and subordinate amount of more evolved rock types and is cut by an active NE-SW fissure swarm. Recent seismic activity indicates that the South Iceland Seismic Zone (SISZ) intersects the eastern part of the fissure zone forming a triple junction with the volcanic zone. The main geological features include a fissure zone and a graben into which most of the volcanic products (lavas and hyaloclastites) accumulate forming highlands in its central part. Three basaltic fissure eruptions of 9, 5 and 2 thousand years before present are found within the fissure swarm. Reykjavik Energy has explored and exploited the huge geothermal resource developed in this volcanic system, first in Nesjavellir in the northern sector and then Hellisheiði in the south. Extensive geophysical surveys including resistivity (TEM) and MT have been done to delineate the geothermal anomaly. A total of about 90 deep exploration, production and re-injection wells have been drilled into the geothermal resources at Hengill, and a few exploration wells have also been drilled in the Bitra and Hverahlíð fields to the east and south of Hengill central volcano respectively. Temperatures within the Hengill geothermal resource varies from about 200°C to about 320°C. In one well at Nesjavellir, probable superheated conditions prevail at about 2100 m depth. The dominant hydrothermal alteration indicates that the geothermal system reached a peak during the last glaciation, but has since then been gradually cooling. The evidence suggests that Holocene volcanic fissure eruptions opened up new flow paths and locally intensified the geothermal system. This postglacial episode has not changed the overall alteration pattern in the reservoir, and may not be seen in the resistivity, but becomes visible by comparing formation and alteration temperature in the wells.