Porosity and Permeability in the Basement Rocks at the Kawerau and Ohaaki Geothermal Fields, New Zealand

Abstract

Porosity and permeability of Torlesse basement rocks and volcano-sedimentary sequences at Ohaaki and Kawerau were studied by direct porosity measurement and petrography of blue-epoxy impregnated thin sections. Thin sections show greywacke has near-zero primary porosity; Waikora Formation conglomerates have open pores between greywacke and argillite clasts; pumiceous Ohakuri Group volcaniclastics have greater porosity. Secondary porosity occurs in hydrothermal leach cavities in all lithologies. Clay minerals have intracrystalline microporosity. Basement porosities are generally <lo%, but the Waioeka petrofacies greywackes of Kawerau have a greater porosity range than the Axial- A petrofacies greywackes of Ohaaki. Fracture-vein networks and microcracks could account for the higher Kawerau porosities, and where connected would provide some permeability. Oxygen isotope shifts in Kawerau greywackes are positively correlated with porosity. Porosity measurements indicate that at Ohaaki, pre-3 3 0 ka volcaniclastic formations have similar fluid storage capacity to the younger Rautawiri Breccia aquifer, but less than the shallow Waiora Formation aquifer.