| Title | Stratigraphy and Hydrothermal Alteration Encountered by Monitor Wells Completed at Ngatamariki and Orakei Korako in 2011 |
|---|---|
| Authors | Catherine Boseley, Greg Bignall, Andrew Rae, Isabelle Chambefort, Brandon Lewis |
| Year | 2012 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | Ngatamariki, Orakei Korako, Monitor Wells, Geology, Hydrothermal Alteration |
| Abstract | As part of the Ngatamariki Geothermal Development, in September – November 2011 Mighty River Power Limited drilled four pressure monitoring wells. These are, two “sentinel wells” ~3 km north of NM4, between the Ngatamariki and Orakei Korako Geothermal Fields (NMM17 and NMM18), and two “monitoring wells” a further 3.5 km to the north, within the Orakei Korako Geothermal Field (OKM1 and OKM2). Each of the monitor well pairs has one deep (~1500 mD) and one shallow (~400 mD) well. The deep sentinel well (NMM18) was designed to target volcaniclastic sediments, tuff and variably welded ignimbrite of the Tahorakuri Formation, which is a known reservoir-hosting unit at Orakei Korako and Ngatamariki. NMM18 is sited to monitor subsurface pressures in the area corresponding to the inferred overlap of the resistivity boundary zones of the two fields. The deep monitor well (OKM1) was located near the abandoned OK1 exploration drillhole (drilled to 1403.6 mD in 1965), 1 km east of the main thermal area at Orakei Korako, in order to monitor reservoir depth pressures. Both NMM18 and OKM1 show a typically argillic alteration assemblage, characterised by smectite, mordenite, quartz, calcite, leucoxene, pyrite and iron oxides. Indications from drilling and testing results are that these wells are in optimal locations for ongoing subsurface pressure monitoring as part of resource consent requirements. This paper describes the geology from the wells, and compares the geology and alteration mineralogy logged, with the geology recorded in the Orakei Korako geothermal exploration wells drilled in the 1960’s, and at Ngatamariki since the mid 1980’s. |