The geological framework of the Wairakei-Tauhara Geothermal System, New Zealand

The geology of the Wairakei-Tauhara geothermal system has been revealed in increments over more than 50 years of field development. Only two major reviews of geo-scientific information have been completed: the first was made more than 40 years ago, the second (unpublished) was completed more than 25 years ago. This paper is an overview and update of the stratigraphic and structural framework of the system and its controls on fluid flow and hydrothermal alteration. We provide information on new areas of drilling exploration in the west of the Wairakei Geothermal Field and on the first production-focused drilling in 40 years at the Tauhara Geothermal Field. The lithology, thickness and extent of several units have been refined, while new units have been discovered by recent deep wells; five new members of the Waiora Formation are proposed. Nomenclature of formations and members is also updated. We review controls on fluid flow in the system and find that fault zones are likely up-flow channels, but their correlation with well feed points is equivocal, whereas intra- and inter-formational permeable zones are directly located by drilling and well completion data. New mineralogy data confirms an earlier known prograde trend of increasing hydrothermal alteration rank and intensity with depth. In the west of the Wairakei-Tauhara system thermal and chemical evolution has created a lower temperature and/or pH overprint on the older propylitic assemblage. Conditions at the eastern boundary of the system appear to have long-term stability. (c) 2009 Elsevier Ltd. All rights reserved.