Distribution of Clay Minerals through a Conductive (MT) Zone on the Margins of a High-Temperature Geothermal Reservoir, Wairakei Geothermal Field, Taupo Volcanic Zone, New Zealand

Abstract

Recent magnetotelluric (MT) surveys of the Wairakei-Tauhara Geothermal Field, Taupo Volcanic Zone, New Zealand, have defined a 200 ? 300 m thick conductive layer (i.e., <5 ?m) across the shallow parts (~0 ? 500 m depth) of the reservoir. This layer is shallowest above the upflow regions of the reservoir, draping to greater depths on its margins. It shows good stratigraphic correspondence to the top of a rhyolite lava unit (i.e., Karapiti 2A) and the base of the Huka Falls Formation. Our study investigates the clay mineral distribution through the conductor in ten geothermal wells in the Otupu area, a region of infield injection in the eastern parts of the Wairakei field where the conductor deepens. We use a combination of methylene blue (MeB) testing to estimate the abundance of smectite; shortwave-length infrared (SWIR) analysis and X-ray diffraction (XRD) clay separate analysis. The results show that smectite is at greatest abundance at the top of the conductor (i.e., 3 ? 7 ?m), in the Huka Falls Formation. The most conductive intervals (i.e., 1 ? 5 ?m) are dominated by interlayered illite-smectite, occurring at the top of the Waiora Formation directly beneath the Huka Falls sediments. Illite is present in most wells below illite-smectite dominated intervals. Beneath the conductor, with increasing depth and resistivity in wells located close to the reservoir boundary, there is a reversal of clay rank to smectite at low abundances. For wells located away from the boundary, illite remains the dominant clay mineral with increasing depth. The conductor reflects the geohydrological structure of the shallow reservoir, with geothermal fluids trapped beneath a relatively impermeable Huka Falls sediments. The deepening of the conductor to the north is where the Karapiti rhyolite is absent and the base of the Huka Falls Formation is deepest.