Volcanology and Geothermal Energy "d0e9175"

Geologic Setting

The Taupo volcanic zone (TVZ) of the North Island of New Zealand is a 250- by 60-km zone of young silicic and andesitic volcanoes that occupies a north-northeast-trending structural depression. The TVZ consists of offshore andesitic composite cones to the northeast, a central section containing at least six caldera complexes, and a southern center consisting of andesitic composite cones. The central part of the TVZ, which is ~125 by 60 km, is mostly made up of silicic calderas and has a geothermal resource of ~2000 MW_e Donaldson and Grant, 1978). An assessment of the geothermal resource potential for individual fields in the TVZ is presented in Table 4.2.

The central section of the TVZ reaches elevations of 300 to 600 m and the depth to basement within the graben is 500 to 1000 m below sea level (Healy, 1982). The basement is even lower below the calderas themselves. Rogan (1982) found steep gravity gradients at the TVZ margins and conservatively estimated that the volume of tuffs, lavas, and associated sedimentary rocks within the central TVZ is ~12,000 km³ . Much of the activity responsible for this erupted material occurred during the past 1 million years.

The calderas of the TVZ have been favorable targets for geothermal drilling and development for many decades, but comprehensive volcanological studies of the calderas are more recent (for an excellent summary paper, see Wilson et al ., 1984). We chose to discuss here the well-known Wairakei geothermal field, which is the site of one of New Zealand's largest geothermal generating plants.

― 171 ―

Table 4.2. Assessment of New Zealand High-Grade Hydrothermal Geothermal Resources^a
	*Estimated Electrical Power Potential (MW*_e )^b
*Field*^c	*Area (km*² )	Maximum Temperature (°C)	*Proven*^d	*Inferred*^d	*Speculative*^e
Wairakei	15	270	150	—	—
Tauhara	14–16	280	100	80	—
Broadlands	11	300	120	30	—
Kaweru	6–10	290	100	—	30
Waiotapu-Reporoa	8–12	295	—	150	100
Orakei Korako	6–10	260	—	50	50
Rotokawa	8–12	300	—	50	100
Ngawha	30–50	300 (?)	—	400	500
Tikitere-Taheke	12	270 (?)	—	75	75
Waimangu	12	270 (?)	—	50	100
Te Kopia	5	240	—	20	20
Tokaanu-Waihi	4	steam field	—	—	100
Ketetahi	?	—	—	25	—
Total	140^e		470	930	1075
^a Adapted from Rowley (1982).
^b Installed capacity, accounting for reservoir recovery factor and conversion efficiency.
^c Lack of sufficient information has precluded the inclusion of areas like Atiamuri, Mokai, and Ngatamariki, which are under consideration.
^dProven = That part of the accessible resource base, resource, or reserve whose size can be computed from geochemical, geological, and geophysical data, drillhole data and reservoir engineering measurements. Inferred = That part of a geothermal resource base, resource or reserve whose size can be inferred on the basis of geochemical, geophysical, or geological evidence, but for which there is little if any corroborating drillhole data. Speculative = Regions where useful geothermal heat is likely to exist, but has not yet been positively identified.
^e Projected 110,000-MW_electrical for 140 km² of high-grade, liquid-dominated reservoirs.