| Title | Preliminary investigation of seismic velocity variation at the Rotokawa and Ngatamariki Geothermal Fields |
|---|---|
| Authors | S. Sewell, M. Savage, J. Townend, S. Bannister, L. Hutchings |
| Year | 2017 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | Ngatamariki, Rotokawa, seismic velocity |
| Abstract | Imaging of seismic properties has the potential to provide important information on the physical and thermodynamic properties of rocks and their pore fluids within and adjacent to geothermal reservoirs. Obtaining accurate seismic velocity models is also important for reducing uncertainty in microseismic event locations, particularly for event depths. As part of a preliminary investigation into seismic property variations at the Rotokawa and Ngatamariki geothermal fields, we have completed a first-pass interpretation of well logging data and preliminary 1D and 3D seismic tomography inversions. The well logs show a large contrast in P-wave velocity (~1 km/s) for the Tahorakuri Formation between NM9 in the north and NM10 in the south of the Ngatamariki field. This contrast appears to be related mostly to a reduction in matrix porosity in NM9 for the Tahorakuri Formation (~10% lower in NM9) as shown by the neutron porosity logs. To investigate the cause of the difference in matrix porosity, bulk rock chemistry measurements were made on drill cuttings in NM9 and NM10 at 5m depth spacing using portable XRF. The portable XRF measurements show an increase in silica concentration of ~10-20% for the Tahorakuri Formation in NM9. It therefore appears that silicification of the Tahorakuri Formation in the north of the field, formed by intense relict magmatic-hydrothermal alteration above an intrusive, has lowered the matrix porosity and increased seismic velocity in the vicinity of NM9. As the northern wells at Ngatamariki generally have low permeability through the Tahorakuri Formation, imaging the high seismic velocity zone using seismic tomography offers a potential way to map the low permeability in 3D. Preliminary seismic tomography has been conducted utilizing a Monte Carlo 1D inversion using the VELEST code and 3D inversion using the Simul code. This has shown that a high to low P-wave velocity contrast (~0.5-1 km/s) occurs in the north-northwest of the Ngatamariki field that may be due to the silicification above the intrusive. This contrast appears to strike approximately NNE-SSW through NM9, suggesting that the silicification of the Tahorakuri Formation, and hence low permeability, does not continue east of this well. However, synthetic Simul tests show that the spatial resolution of the current seismometer array is generally poor. In order to improve the spatial resolution, an expanded array of 30 additional seismometers was installed in February 2017 which will record for a year. |