| Title | DISTINGUISHING LIGHTER SHADES OF GREY: THE USE OF PORTABLE XRF TO DEFINE CHEMOSTRATIGRAPHY IN THE GREYWACKE BASEMENT OF NEW ZEALAND GEOTHERMAL SYSTEMS |
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| Authors | J. H. White, S.L.L. Barker, and A. Calibugan |
| Year | 2017 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | Chemostratigraphy, portable x-ray fluorescence, Kawerau geothermal field, greywacke basement, Taupo volcanic zone |
| Abstract | Several geothermal resources in New Zealand are hosted in the greywacke basement rocks. The basement lithologies are characterized by interlayered meta-sandstone and metamudstone (argillite) and can be grouped into two distinct terranes (e.g. Torlesse and Waipapa). The basement may appear grossly homogeneous (particularly for Torlesse Terrane) making the recognition of changes in lithology challenging, particularly when inspecting rock cuttings from geothermal drilling. A more detailed understanding of the basement lithologies is deemed useful in providing insight on the influence of lithology on fluid flow within the basement, in delineating possible locations and/or displacement of faults as well as in evaluating possible chemostratigraphic controls on geothermal reservoir processes. While recognising that visual changes in basement lithologies can be challenging, there is the potential to define lithology changes on a chemical basis (i.e. chemostratigraphy). In this study, we collected systematic portable x-ray fluorescence (pXRF) chemical data from the greywacke basement of the Kawerau Geothermal Field in an attempt to define the greywacke and argillite lithologies. Approximately 1700 samples were analysed across 12 geothermal wells, with samples collected on every interval of drill cuttings (typically 5 metre intervals). The quality of pXRF data was assessed by comparing a subset to full laboratory wavelength dispersive x-ray fluorescence spectrometry on fused beads and pressed pellets. The pXRF data collected for Kawerau could be the most comprehensive chemical data available for the greywacke basement rocks to date. The results were then compared to existing visual classifications of the basement rocks which had been previously published for the well KA50 by Wallis et al. (2012). It was demonstrated that argillite-rich intervals can be recognised on the basis of elevated K, Rb and Al concentrations, which are likely related to elevated clay content expected for argillite. Thresholds for selected chemical parameters were defined for argillite (metamudstone), meta-sandstone and mixed argillite/metasandstone. This classification was applied across the Kawerau field to reveal the distribution of argillite and meta-sandstone layers within the greywacke basement. |