Lithological and Structural Controls on Fluid Flow and Hydrothermal Alteration in the Western Ohaaki Geothermal Field (New Zealand)--Insights from Recent Deep Drilling

Abstract

Ten production wells have been drilled in the western Ohaaki steamfield since 2005, to a maximum depth of -2136 m.a.s.l.. Drilling results show a propylitic mineral assemblage occurs below about -1000 m.a.s.l., with permeability proximal to faults in, and extending from, the greywacke basement. The faults provide a conduit for vertical flow of >290° C fluid into the overlying pyroclastic units, although formational permeability predominates in the postbasement stratigraphy. Fluid inclusion microthermometry points to past temperatures in the reservoir as high as 300-320° C, although late-stage calcite overprinting of epidote in some wells reflect cooling (and change in reservoir chemistry), and predominance of somewhat more CO2-rich fluids than in the past. Evidence of deformed veins points to long-lived faulting, which has maintained open pathways during the life of the hydrothermal system.