Volcanic texture identification and influence on permeability using a borehole resistivity image log in the Whakamaru Group Ignimbrite, Wairakei Geothermal Field, New Zealand

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) collected from the Whakamaru Group ignimbrite in well WK271, Wairakei Geothermal Field, New Zealand, has been used to investigate the textural characteristics of volcanic rocks in the geothermal reservoir, with a view to improving reservoir model inputs. Textural analysis of the FMI log in the Whakamaru Group ignimbrite was able to provide an improved internal stratigraphy compared to that derived solely from drill-cuttings, and offers insights into the volcanic processes that generated it. Based on volcanic textures identified on the WK271 FMI image log, seven individual flow units in the Whakamaru Group ignimbrite are recognized, and are separated by texturally characteristic intervals of airfall tephra. Comparison of fracture density and rock type (including welding intensity) shows a low fracture density in non-welded ignimbrites and tuff beds. The logged interval occurs over a fault zone where permeability has previously been inferred to be strongly influenced by fractures, precluding an accurate assessment of the ignimbrites intrinsic permeability.