The Deeper Structure of the Wairakei Geothermal Field

Abstract

Most drillholes at Wairakei are between 600 and 800 m deep, a few extend down to 1000-1200 m where thick impermeable ignimbrites or rhyolites have discouraged deeper drilling over much of the field. penetrated through these ignimbrites. One in the centre of the Wairakei Field drilled almost 1 km of ignimbrites and bottomed in impermeable andesites and tuffs. Wairakei Field drilled only 200 m of ignimbrites and bottomed in pumice- lapilli tuff and rhyolite. Re-evaluation of drillhole data indicates that the Wairakei Ignimbrites are composed of three separate sheets, that can be discriminated on phenocryst content. Only the upper sheet extends north from W121 to A major tectonic boundary, probably a caldera margin o r NW-trending cross- fault separates these two holes and has been suggested as the target for a deep (2.5-3 km) exploratory hole. Other deep exploration well sites in the southern and western parts of the Wairakei Field are also discussed.