Geothermal Exploration of Ascension Island, South Atlantic Ocean

Abstract

Exploration on Ascension Island in the South Atlantic Ocean has resulted in the discovery of high temperature geothermal fluids. Ascension is a volcanic island that has seen eruptions within the last 1000 years. However, there are no geothermal manifestations at the surface and only limited amounts of hydrothermal alteration exposed. Exploration began with geologic mapping and was followed by geophysical investigation that included electrical resistivity and aeromagnetic surveys. These data were used to site seven core holes for temperature gradient measurements. A deep test well, Ascension #1, was drilled to a depth of 10,255 feet. Several fluid entries were encountered and tested, but they lacked the permeability required to support production. Extensive quartz and epidote alteration was responsible for sealing the lower fracture zones. In spite of this, a bottom hole temperature of 480°F was recorded. A second leg was drilled to explore for greater permeability. This leg wads terminated by a mechanical failure before the target depth was reached.