THREE DIMENSIONAL CONCEPTUAL MODEL OF ORAKEIKORAKO AND TE KOPIA GEOTHERMAL SYSTEMS, TAUPO VOLCANIC ZONE, NEW ZEALAND

Abstract

Orakeikorako and Te Kopia are two geothermal fields of the Taupo Volcanic Zone located 5 km apart along active NE trending rift faults. While it is acknowledged that the geothermal activity at Orakeikorako and Te Kopia are closely related with the active faults, the connection between the two fields is not well understood and it is possible that these two fields were once a single larger geothermal system. The purpose of this paper is to better define and visualise the subsurface geological settings and structural framework of the Orakeikorako and Te Kopia area.
A three dimensional geological model of these two geothermal fields has been developed using Leapfrog Geothermal. The model is built from continuous log data from six exploratory wells from Te Kopia and Orakeikorako, constrained at surface by the 1:250,000 regional geological map, and complemented by recent mapping in the Paeroa Block area. The 3-D geological model is subdivided in six stratigraphic layers (metasedimentary basement, Reporoa Group, Whakamaru Group, Huka Group, Maroa Group and surficial deposits), and basaltic and rhyolitic intrusions. The displacement in the Te Kopia area by the Paeroa fault and its subordinate faults produces a relay ramp that dips northeast. In the Orakeikorako area, the displacement by the Matangiwaikato and East Wainui faults also produce a relay ramp that dips southwest.
Complementary to the stratigraphic model, a clay alteration model and a temperature model are presented as part of this paper. Clay alteration ranges from smectite through smectite-illite, to illite. The comparison of alteration and formation temperatures seems to indicate cooling in the Orakeikorako and Te Kopia geothermal fields that may be due to groundwater incursions along the active faults.