Ngawha geothermal area, Northland

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Skinner, D.B.; Steiner, A.; Hornibrook, N. de B.; Scott, G.H.; Edwards, A.R.; Bowen, F.E.; Healy, J.; Kear, D. 1966 Ngawha geothermal area, Northland. Wellington: New Zealand Geological Survey. Report NZGS 16 34 p.

Abstract: The results of surface geological mapping and shallow drilling of the Ngawha region are given. The stratigraphic, petrological and paleontological results from a subsequent deep drillhole are followed by discussion of these results and recommendations for further investigations. Whilst surface mapping had indicated the presence of only superficial deposits of Onerahi Chaos-breccia, the deep drillhole showed at least 1500 ft of this formation to be present in the Ngawha basin. This precludes the use of surface mapping in interpreting the sub-surface geology of the area. Beneath the Onerahi Chaos-breccia some 170 ft of unsheared sediments are tentatively interpreted as representing an unnamed autochthonous, Eocene formation that might, away from the deep drillhole, provide porous reservoir beds. Waipapa Group basement penetrated by the hole was a fine grained argillite of low porosity but this might overlie shattered sandstones that could also form a porous reservoir. A drillhole from 3000 to 4000 ft deep is recommended to test the permeability of such autochthonous sediments as may overlie the Waipapa Group, and to investigate the possible existence of permeable beds within the Waipapa basement. The drillhole should be sited as close to the present centre of hydrothermal activity as possible. (auth)