Structural Control of the Reservoir Rocks at the Ngawha Field, Northland, New Zealand

Abstract

The low porosity of the Waipapa wacke reservoir rock, and the comparative abundance of youthful faults and lineaments on the Ngawha geothermal anomaly, indicates that fracture zone permeability should be the drilling target. However, the reservoir is blinded by m of low bulk permeability allochthonous cap-rock that has inadequate outcrops for structural studies. Notwithstanding, the physiography has been partly controlled by fault crush zones which appear as lineaments on Landsat and aerial photographs. the northland region, Ngawha lies at a nodal intersection of major northwest, east-northeast, northeast and meridional faults, and is locally covered by a network of predominantly northeast and northwest, often curved, fractures. The late Cenozoic volcanic and hot spring alignments follow the northeast trends, and a sigmoidal graben-like structure follows the general northwest trend of the Ngawha Basin except for a central northeast section on the line of the main Ngawha springs. A dextral rotational torsional stress system is one possible cause of the local fracture pattern, and could have been the result of interacting major east-northeast and meridional faults with respectively, left and right lateral strike-slip components in a northeast-southwest principal horizontal stress system.