Application of the Vitrinite Reflectivity Method to Samples from the Wairakei-Tauhara and Ohaaki Geothermal Fields

Abstract

Organic material occurs as disseminate grains between 0.2 and 10 microns in diameter in samples of lacustrine sediments of the Huka Falls Formation. Thirty six cores from wells drilled at the Wairakei- Tauhara (TH4, THM4, 219 and 35) and Ohaaki fields (Bfs 10, 1 and 5) were examined; thirtytwo contain organic matter (vitrinite) whose reflectivity was measured using a reflected light microscope with photometer attachment. Mean random vitrinite reflectivities, Rornd, ranged from 0.24 to 1.45% with highest values generally present in samples coming from the greatest depths where the hottest bore temperatures occur. Vitrinite reflectivity has thus been demonstrated to be a useful geothermometric method for sediments of geothermal systems in the Taupo Volcanic Zone where it records maximum temperatures. These reconnaissance results also suggest that the duration of heating of the Ohaaki samples was shorter than those from Wairakei-Tauhara but different parts of the latter field had timespans of different duration.