| Abstract |
Discerning the contributions to fluid flow in a geothermal reservoir from intrinsic and structural permeability components is an important, yet difficult task. High-quality resistivity borehole image log data (Formation MicroImager; FMI) was collected between -669 and -1565 mRL (meters relative to sea level), from a series of lavas, ignimbrites and volcaniclastics in well WK271, Wairakei Geothermal Field, New Zealand. This image log has been used to investigate the textural characteristics of volcanic rocks in the geothermal reservoir, with a view to improving inputs into numerical reservoir models. Textural analysis of the resistivity image log was able to provide a much-improved internal stratigraphy compared to that derived solely from drill-cuttings, revealing the volcanic processes that created the rocks. Several ignimbrite facies, textures inferred to represent volcaniclastic and sedimentary deposition, intervals of breccia, and various rhyolite lava textures are observed and linked to core and outcrop analogues. Based on volcanic textures identified on the WK271 resistivity image log, for the first time we have been able to recognise seven individual flow units in the regionally extensive Whakamaru Group ignimbrite, which are separated by texturally characteristic intervals of airfall tephra, and a high degree of variability and interbedding in the thick Tahorakuri Formation silicic deposits. Permeability of volcaniclastic formations in geothermal reservoirs is typically controlled by a mixture of fractures and matrix, with some intervals of high density fractures being permeable. There is no clear correlation between rock type and fracture density with most rock types showing variable densities, though non-welded ignimbrites and tuff beds generally have a low fracture density. The textural analysis of this resistivity borehole image log, combined with permeability information from completion test data, thus gives insight into the potential contribution to geothermal fluid flow from intrinsic permeability, along with the volcanic and geothermal processes that formed the geothermal reservoir rocks. |