Geological Results of Production Well Drilling in the Western Steamfield, Ohaaki Geothermal System: 2005 - 2007

Abstract

Drilling of 10 production wells since 2005 in the western Ohaaki steamfield has provided important insights into the deep stratigraphy, including new lithological types, the depth and contour of the greywacke basement and likely influences of basement faults, and the deep hydrological structure. New andesite and dacite subunits of the Tahorakuri Formation have been identified and may be of extrusive or intrusive origin. The depth to the greywacke basement increases towards the northwest, where it lies beyond -2150 mRL, and north (-1724 mRL), with a possible NW-trending horst-like structure. The basement is likely to be faulted by both NW- and NE-trending basement faults, which provide pathways for upwelling fluids into the post-basement volcano-sedimentary sequence where both formational and fault-related permeability are likely to provide pathways for fluid movement. The northwestern part of the steamfield, where greywacke basement is deepest, contains favourable mineralogical indicators for good permeability and high temperatures (>280° C) and is a highly prospective part of the field for steam production.