| Abstract |
To understand what controls the geophysical properties at the Ngatamariki geothermal field, New Zealand, particularly seismic velocity, an analysis of geophysical logs was undertaken. The geophysically-logged interval in the south of the field (well NM10) spans the propylitic altered, tuff-dominated, Tahorakuri Formation volcaniclastics and andesite. In contrast, the logged interval in the north of the field (NM9) spans the Tahorakuri Formation that experienced an earlier phase of potassic, advanced-argillic and phyllic alteration related to intrusion of a tonalite-diorite intrusive complex. Geochemical analyses of drill cuttings using portable X-ray fluorescence (pXRF) were obtained at a 5m depth interval spanning the logged intervals in each of the wells. These were combined with automated mineralogy using a Tescan Integrated Mineral Analyzer (TIMA) and quantitative X-ray diffraction (XRD) data on selected samples that provided quantitative mineralogy data. The geochemical and mineralogy data were then used to interpret the factors that affect the geophysical properties of the rocks. The Tahorakuri Formation was found to have markedly different geochemistry, mineralogy and petrophysical properties in the north of the field (NM9) compared to the south of the field (NM10) which we interpreted as due to the wide-spread quartz deposition and ductile deformation that took place during the intrusion of the tonalite-diorite complex. These processes reduced porosity within the Tahorakuri Formation in the north of the field, resulting in higher seismic velocity, higher density and higher resistivity relative to the south of the field (north (NM9) –average porosity of 7.5%, average P-wave velocity (Vp) 4.59 km/s, average density 2.64 g/cm3, average resistivity 118 ohm.m versus south (NM10) – average porosity 18%, average Vp 3.78 km/s, average density 2.47 g/cm3, average resistivity 20 ohm.m). The approximately 400m thickinterval of Tahorakuri Formation in NM9 that overlies the intrusive has particularly low porosity (<5%) and consequent high seismic velocity (4.5–5.5 km/s), density (2.6–2.8 g/cm3) and resistivity (500–1000 ohm.m). Abundant andalusite at the top of this interval provides evidence that the rock in this zone was >350 °C during the intrusive event. The inferred temperature, as well as deformation textures observed in an FMI log over this interval, suggest that the very low porosity is the result of ductile deformation. The interval between the andalusite altered tuff and the intrusive body appears to be relatively unaltered with abundant plagioclase and only subtle potassic alteration. Abundant small aperture (<0.1 mm), low-angle (<20° dip) fractures occur within this interval which are interpreted as being due to hydraulic fracturing due to pressure-transients that occurred within the lithostatic-pressured zone above the magma. The lack of alteration within this interval is interpreted as being due to the fluid within this zone being supercritical when the intrusive was emplaced with the low porosity/permeability that resulted from ductile deformation preventing further alteration as the magma cooled and during the present-day geothermal activity. The dataset provides insights into the changes in geophysical properties that occur close to intrusive bodies and has potential implications for geophysical imaging of magmatic intrusions and supercritical zones. |