Putauaki Mt Resistivity Survey

Proceedings 24* New Zealand Geothermal Workshop 2002 PUTAUAKI MT RESISTIVITY SURVEY C.J.BROMLEY Institute of Geological Nuclear Sciences, Taupo, N.Z. SUMMARY - New magnetotelluric resistivity soundings, supplemented by Schlumberger traversing measurements, have been collected in the vicinity of Putauaki (Mt Edgecumbe) to investigate the southern and eastern extent of the Kawerau Geothermal Field. They show that this field is associated with a large area of low resistivity totalling about Approximately 50% of this area is administered by the Putauaki and Te Tahuna Putauaki Trusts, and only a small portion has been tested with exploration drill holes. The volcanic peak of Putauaki itself yrs old) is on the southern boundary of the field. An association is inferred between the magma body feeding this vent and the heat source for the geothermal system yrs old). An area of particularly low resistivities ohm-m at lkm depth) and cold geothermal gas vents geothermometry is located near the centre of the revised low resistivity anomaly, about 1.4 east of the hottest existing Kawerau geothermal wells From this data, a geothermal hydrological model is inferred: a deep upflow, with associated intense hydrothermal alteration, occurs on faults centered beneath the gas vents, and supplies lateral flows to the northwest into the area of present production at Kawerau, as well as to the northeast. 1. INTRODUCTION normal step faults cause this drop in basement depth towards the centre of the Whakatane Fluids from the Kawerau Geothermal Field have Graben (Nairn, 1986). However, Wood and been utilised for process steam (up to 300 Brathwaite (1999) show that the most likely and electricity production (up to by the orientation of local basement faults, based on Tasman Pulp and Paper Mill since 1957 contours of basement elevations in Kawerau (Bloomer, 1997). Two binary plants also generate drillholes, is about 80". Indeed, about 60" about 6 from brine. The existing was the dominant trend of the most accurately steam field is managed by Century Resources on measured faults in the area - an east-west zone of behalf of the New Zealand Government. The bulk en-echelon fractures that ruptured just north of of the drilling and subsequent fluid extraction Kawerau geothermal field during the 1987 took place in a 2 area at the north-western Edgecumbe earthquake. Furthermore, the gravity corner of the field near the largest area of thermal anomaly contours in the south-eastern sector of features (Onepu Springs) and close to the Kawerau (MacDonald et al, 1970) imply a Tarawera River (Figure 1). Temperature predominantly east-west trend to the down- contours, pressure contours and chloride faulted,high-density basement. concentrations of the discharged fluid from the existing boreholes all imply that the fluid source The location of basement faults has important (hot, high pressure, saline fluid upflow) is to the implications for further exploration drilling, and southeast of the drilled area, in the direction of for management of production and injection Putauaki (Mt Edgecumbe) (Allis et al., 1997). strategies at Kawerau, because of the lack of Two exploration wells drilled to the southeast of extensive permeable stratigraphic aquifers. Where the Tasman Mill and were found these faults pass through basement they can, in to be very hot (up to but impermeable. places, be highly permeable (eg. but at Subsequent efforts concentrated on the more other places the structures are impermeable (eg. permeable outflow area of cooler temperatures to Because of large uncertainties in the northwest. determining strike and dip of fault zones, and in predicting their productivity, targeting of inferred The known subsurface geology consists of a faults at Kawerau has had variable success (Wood sequence of volcanics (rhyolites, ignimbrites, and Brathwaite, 1999). tuffs and sediments) overlying a basement of greywacke at about 1 depth near the mill, but Some permeable aquifers do occur in the deepening to the northwest, and shallowing to overlying volcanics to the northwest, particularly about 700 m depth to the southeast. It was at depths above about 400 m and at about 800 initially presumed that north-east trending (White et al., 1997). The shallower aquifer has 135 suffered from pressure drawdown of several bars and Te Tahuna Putauaki Trusts. The objective of causing inflows of cooler water from above. this work was to determine the resistivity- Calcite deposition in the well bore was also a structure of this sector, to resolve uncertainties problem, particularly in wells tapping the regarding the eastern boundary of the Kawerau shallower aquifer. The deeper zone has shown Geothermal Field, and to provide information to little, if any, evidence of pressure drawdown after assist with planning future exploration drillholes. more than 25 years of exploitation. This implies The scope of the work included DC and MT that the deeper zone is recharged from a highly resistivity measurements and gas analyses. permeable source of hot water, and that the entire field could probably sustain a much higher rate of DC resistivity measurements were made using the extraction if this source were to be tapped. Schlumberger array with spacings of 500 m and 1000m (giving nominal probing depths of up The natural surface heat flow from thermal to 500 m). This helped resolve uncertainty, activity is estimated (Allis et al., 1993) to be caused by insufficient data, regarding the eastern about 100 Matahina Ignimbrite, boundary of the Kawerau geothermal system. (which occurs between about 100 m and 200 m of Magneto-telluric soundings were then sited using the surface) appears to act as a “blanket” or the traversing data as a guide. These soundings aquiclude, preventing upflows of hot fluid to the provided resistivity information as a function of surface over most of the geothermal system (east depth (fiom near surface to about two of Tarawera River), but is absent to the west kilometres), and as a function of orientation. where the thermal features are located. An additional subsurface outflow of about 50 Gases discharging through a patch of bare ground possibly extends into the (sometimes a shallow lake), at NZMG co- groundwaters of the Rangitaiki plains to the north ordinates 2838433 6340600 about 1.4 and northeast, contributing to anomalously high east of borehole were sampled and chloride and boron concentrations in these waters. analysed. The purpose was to determine whether these gases are of geothermal origin, and to Putauaki (Mt Edgecumbe) is a relatively young obtain the deep temperature of origin from gas cone (mostly years old), compared to the geothermometry. age of the geothermal system years, Browne, 1979). It may be a recent extrusion from Several magneto-telluric soundings were a shallow magma body that has provided a heat conducted around Putauaki (Mt Edgecumbe) in source for the geothermal system (Christenson, order to investigate the deep resistivity structure 1997). Ejecta fiom three hydrothermal of the volcano and its relationship to the vents, located near Tarawera Road, north of geothermal system. Soundings were also located Putauaki, have been dated at 14,500 and 9,000 in the vicinity of several hydrothermal eruption years old. craters near one of the hottest Kawerau boreholes, KA26 (Figure 1). Early resistivity surveys (before 1970) of Kawerau field (MacDonald et al., 1970; Studt, 3. RESISTIVITY AND GAS ANALYSIS 1958) were shallow penetrating (to about 250 m RESULTS depth) and limited in the area surveyed. They suggested a field area of 10 centred on the The DC Schlumberger array measurements Tasman mill. This was the basis for the conducted in the vicinity of Lake near subsequent exploration drilling. Later the eastern boundary of Kawerau Geothermal measurements, using a deeper penetration Field, reveal apparent resistivities that Schlumberger traversing array = 1000 m), consistently decrease with depth spacings showed a much greater field area of at least 19 500 m to 1000 m). They also increase west with a major eastward extension of the to east (about 15 ohm-mto45ohm-m, at = low resistivity ground (Allis et al., 1000 m). In the vicinity of Te Teko, further 1993). However, the location and cause of the northeast, apparent resistivities are greater than north-eastem and southern resistivity boundaries, 50 ohm-m. Figure 1shows the revised resistivity in particular, remained uncertain. The new boundary of the Kawerau Geothermal Field, resistivity work, reported here, allows for an incorporating these new data. The north-eastern improved resistivity model and an inferred “boundary” is actually a zone of gradual increase hydrological model of the Kawerau field to be in apparent resistivity from 20 ohm-m to 70 ohm- constructed. m consistent with a model of outflowing, and gradually cooling, geothermal fluids. The total 2. GEOSCIENTIFIC PROGRAMME area of the Kawerau resistivity anomaly (defined as ohm-mat = 1000 m) is Additional geoscientific exploration on the south- approximately eastern side of Kawerau Geothermal Field was undertaken with the collaboration of the Putauaki The locations of all the magnetotelluric soundings 136 around Putauaki are shown in Figure 1. intense smectite alteration at temperatures of Wherever possible, the orthogonal sounding about occurring within porous volcanic arrays were oriented NE (Ex) and using units. The underlying moderate resistivity layer 50 m electric field dipoles and perpendicular (about 30 ohm-m) coincides with greywacke at magnetic coil antennas. A measurement high temperature but relatively low frequency range of 8 to 0.02 Hz provided permeability, and moderate type penetration depth range from near surface to alteration. In the central area of the anomaly, low about 3 Notch filtering at 50 Hz was resistivities ohm-m) are found at depths of required because of nearby high voltage 1.2 to 2.5 km. This is beyond the depth of the powerlines. Natural signal strengths in the nearest well and the cause is unknown, but frequency bands near 1 Hz and 2 were, as may be related to a local increase in alteration usual, very low, resulting in some noisy portions intensity associated with basement fracturing.

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