| Abstract |
In the last 4 years to mid 2009, more than 45 geothermal wells (i.e., production, shallow (<600 m deep) steam, injection and exploration) have been drilled by Contact Energy Ltd. in the Wairakei and Tauhara Geothermal Fields. New information on the stratigraphy below ~1500 m drilled depth at Wairakei (including the deepest well WK247, drilled to 2759 mCHF VD; -2246 mRSL), and previously unexplored parts of the Tauhara Geothermal Field, has prompted a revision of their geology and stratigraphic nomenclature. Shallow formations comprise Oruanui Formation, Huka Falls Formation (including Rautehuia Breccia), Waiora Formation (5 members), Karapiti (2A, 2B, 3) Rhyolites and Wairakei Ignimbrite (Whakamaru Group). The previously unknown deeper stratigraphy (below -1000 mRSL) comprises plag-px phyric rhyolite lava (Poihipi Rhyolite), and several welded and non-welded pyroclastic units of the Tahorakuri Formation (including previously unrecognised Stockyard ignimbrite member). The inferred depth to the greywacke basement is >400 m deeper at Wairakei (below -2250 mRSL) than previously believed, whereas it has now been encountered by deep drilling (in TH17) from -1484 mRSL in the NE sector of Tauhara. Interpretation of recent drilling results by GNS Science, complemented by detailed petrology, provide a geological framework for identifying future drilling targets. Whilst fault zones provide up-flow channels, we now appreciate intra- and inter-formational permeable zones provide major inflows to the production wells, and an important control on lateral fluid flow at Wairakei and Tauhara. Hydrothermal mineral studies confirm a prograde trend in alteration rank and intensity with depth, and point to thermal and chemical change in the western Wairakei-Tauhara system that has created a lower temperature mineral overprint of the early propylitic assemblage, whilst conditions at the eastern boundary of the system have long term stability. Defining the nature and extent of the geological formations, and lithological / structural controls on permeability, has proven invaluable to understanding the hydrology of the geothermal system and essential to the continued long-term, sustainable utilisation and management of the resource. |