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Mixed-Layer Clay Geothermometry in the Wairakei Geothermal Field, New Zealand

Published online by Cambridge University Press:  02 April 2024

Colin Charles Harvey  and
Patrick R. L. Browne
Colin Charles Harvey*
Affiliation:
Geothermal Institute and Department of Geology, University of Auckland, Private Bag Auckland, New Zealand
Patrick R. L. Browne
Affiliation:
Geothermal Institute and Department of Geology, University of Auckland, Private Bag Auckland, New Zealand
*
1Present address: Dept. of Geology, Indiana University, Bloomington, Indiana.
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Abstract

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Mixed-layer clays of variable composition and structure occur in core samples from two drillholes (WK207 and WK210) drilled into the Te Mihi sector of the Wairakei geothermal field. These were identified by X-ray diffraction analysis of glycolated and oriented sample fractions at less than 2 μm and less than 0.2 μm.

Low permeability lacustrine sediments encountered by drillhole WK207 contain a well-developed sequence of mixed-layer clays. The shallowest downhole appearance of mixed-layered illite/smectite (I0.6/Sm) occurs at 146 m depth where temperature is only 100°C. Discrete illite is present only below 297 m (200°C) in the finer size fraction (less than 0.2 ¼m). Chlorite first appears downhole, in association with illite-smectite, at 177 m depth (110°C).

Drillhole WK210 encountered predominantly ignimbrites and rhyolites, and fluid flow here is mainly in channels. Within these rocks, a sequence of interlayered clays is poorly developed. Discrete illite and chlorite are present in core from only 244 m (180°), but the measured temperatures where interlayer clays occur ranges from 140 to 209°C.

Differences in the identity of clay minerals present in the Wairakei reservoir, where conditions are otherwise the same, demonstrate the strong control that the type of fluid flow has on their formation. In poorly-permeable sediments, where diffuse fluid flow prevails, a clearly-defined sequence of mixed-layer clays occurs. These are absent where channel flow dominates, the discrete chlorite and illite deposit directly from solution.

Keywords

Geothermal systemHydrothermal alterationMixed-layer claysReservoir permeability
Type
Research Article
Information
Clays and Clay Minerals , Volume 39 , Issue 6 , December 1991 , pp. 614 - 621
Copyright
Copyright © 1991, The Clay Minerals Society

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