PRELIMINARY ANALYSIS OF SEISMIC VELOCITY VARIATION AT THE ROTOKAWA AND NGATAMARIKI GEOTHERMAL FIELDS IN NEW ZEALAND

Seismic properties (compressional wave velocity - Vp, shear wave velocity - Vs and attenuation) are known to vary with temperature, porosity, permeability and saturation (steam content) and thus imaging seismic properties via seismic tomography has the potential to yield useful information on geothermal reservoirs. We present preliminary seismic tomography results from two operating geothermal fields in New Zealand, Rotokawa (174 MWe) and Ngatamariki (84 MWe). The preliminary tomography shows that a large contrast in seismic velocity likely exists in the north of the Ngatamariki field between 1 to 2 km below seal level (bsl). Analysis of sonic velocity logs from two wells in the field accompanied by portable XRF measurements made on drill cuttings show that the velocity contrast is likely due to high intensity, acidic alteration above a diorite/tonalite pluton which intruded approximately 0.6-0.7 million years ago. The alteration associated with the intrusion appears to have dramatically altered the properties of the Tahorakuri formation in the north of the field, resulting in lower matrix porosity, higher seismic velocity and higher density. Sonic logs show a measured difference of ~1 km/s (approx. +20%) in seismic velocity between the north and south of Ngatamariki for the Tahorakuri Formation between ~1 to 2 km below sea level. Neutron porosity logs show this is due mostly to an ~10 % lowering of matrix porosity in the northern TH formation (porosity is ~15-20% for the Tahorakuri Formation in the south of the field vs. 0-10% in the north). pXRF measurements made every 5m over the logged sections of the two wells show that the lower porosity and higher velocity appears to be mostly due to silicification. As the northern wells at Ngatamariki are generally low permeability, if the highly altered, low porosity, high velocity Tahorakuri Formation can be imaged with seismic tomography future wells in this area of Ngatamariki can be targeted away from the low permeability area and we plan to acquire a high resolution seismic tomography dataset in 2017 with this goal.