Geophysical Structure and Densification of Producing Aquifers in Broadlands-Ohaaki Field.

Abstract

Isopach maps of the main lithostratigraphic units of the Broadlands- Ohaaki geothermal field have been constructed from integrated geophysical models and well logs, Hot fluids are produced mainly from two pyroclastic flows which exhibit moderate permeability within a small (3 elongated area in the centre of the field. Significant densification of the rocks in both layers within this small area is described. In the upper layer (Waiora Formation), densification is associatedwith mineral deposition (mainly quartz); in the deeper layer (Rautawiri Breccia), densification is probably due to compaction by explosive, natural hydraulic fracturing. Ancient hydrothermal affected the top layer. A model is proposed which links the various phenomena, assuming that later fracturing was confined to the small area with hydrothermal eruptions which coincide with the area affected by densification and enclosing most of the productive wells.