Subsurface Structures And Relation To Steam Production In The Broadlands Geothermal Field, New Zealand

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Journal Article: Subsurface Structures And Relation To Steam Production In The Broadlands Geothermal Field, New Zealand

Abstract
Broadlands geothermal field is underlain by a stratified acid volcanic sequence of Quaternary age resting unconformably on early Mesozoic or late Paleozoic indurated quartzose feldspathic sandstone and argillite, commonly referred to as the 'greywacke basement'. The surface of the basement slopes westward from 500-1000 ft above sea-level on the Kaingaroa plateau to 7000-8000 ft below sea-level in the Taupo-Reporoa basin. Bouguer negative gravity anomalies increase steadily from east to west across the field marking the descent of the basement. Major buried fault-block structures (horsts) interrupting the steady westward descent of the basement, are associated with major thermal anomalies with temperatures exceeding 300°C. Wedges of clastic greywacke derived sediments (Waikora formation) thin westward from buried basement scarps and interdigitate with pumiceous sediments and pyroclastics of the Ohakuri group. Five important confining layers are marked by two extensive rhyolitic flow units, and three thick ignimbritic ash-flow units. These separate four aquifers - the Waiora, Rautawiri, upper Ohakuri, and lower Ohakuri - which progressively decrease in permeability with depth. Steam production has so far been obtained from the Waiora and Rautawiri aquifers in the Ohaki production area west of the Waikato river. Productive drillholes are sited along faults transecting a rhyolite dome which caps the Waiora aquifer. About 1000 klb/hr of high-pressure (200 psi) steam has been proved by 12 drillholes, equivalent to 80 MW of generated power. Steam production east of the Waikato river is negligible except for a small and unreliable flow of almost dry steam from fractured rhyolite on the crest of a horst in basement rocks.

Author 
G. W. Grindley








Published Journal 
Geothermics, 1970





DOI 
Not Provided
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Citation

G. W. Grindley. 1970. Subsurface Structures And Relation To Steam Production In The Broadlands Geothermal Field, New Zealand. Geothermics. (!) .