A Comparison of the Greywacke Basement in Kawerau and Rotokawa Geothermal Fields in TVZ, New Zealand

Abstract

This paper aims to present the results of the petrological and mineralogical analyses conducted on selected greywacke basement samples from the Kawerau and Rotokawa Geothermal Fields. These two fields are both found within the Taupo Volcanic Zone (TVZ), as with most of the active geothermal fields in New Zealand. The Kawerau Geothermal Field lies near the northeast boundary of TVZ while the Rotokawa Geothermal Field is situated close to the southeast boundary. The study is conducted to determine if there are distinct differences in the petrography and mineralogy in the greywackes that may provide insight on the observed difference in injection well performance in the two fields. Results of petrographic assessments indicate that the Kawerau greywacke is characterized by well-sorted, clast-supported metasandstone to siltstone with common volcanic clasts of intermediate composition whereas the Rotokawa greywackes vary from matrix-dominated, well-sorted metasandstone to argillite. Overall, the samples from Rotokawa can be classified as part of the Torlesse Composite Terrane (Kaweka Terrane) while samples from Kawerau can be grouped under the Waipapa Composite Terrane (Morrinsville Facies). A variation in the mineralogy of the two greywackes is also observed with the Rotokawa greywacke having rare calcite / chlorite while Kawerau greywacke has abundant calcite / chlorite. The abundance of these two minerals in the Kawerau greywacke is possibly influencing the rock reactivity to acid-dosed brine resulting to buffering of the injection fluid pH. This may potentially help explain the observed difference in injection well performance between the two fields, however other contributing factors cannot be discounted (e.g. temperature differences).