Characterisation of fracture properties in greywacke reservoir rock utilising the Kuaotunu epithermal deposit as a fossil analogue

Abstract

Geothermal fields that are operational within the Taupo Volcanic Zone (TVZ) intersect the underlying greywacke, the informal name applied to the metasedimentary basement rocks that underlie Zealandia. Within this reservoir rock, the nature of fracturing is often constrained by information derived from well-bore studies and is limited by one-dimensional wells or two and three-dimensional subsurface geophysical images often with low resolution. Fractures are conduits for geothermal fluid flow; therefore, a surface analogue—where fracture patterns can be explored in more than one direction—can be a vital source of information for geothermal energy production. This thesis focuses on describing the fracture permeability of greywacke outcrops utilizing the Kuaotunu epithermal deposit as a fossil analogue, in order to expand the understanding on how fractures influence permeability within an important reservoir rock in the TVZ. These rocks are exposed along a 2 km coastal cliff section at Kuaotunu, Coromandel Peninsula, comprise Mesozoic age, well-indurated, inter-bedded sandstones and argillites with a complex structural history. Data for this study were obtained from field mapping, focussing on host rock lithological character, stratigraphy, structural elements that contributed to paleo-permeability, and fracture styles. For the purposes of this study, a simplified lithological classification scheme of two divisions, Type A (sandstone-rich thick beds, 60 cm to 50+ m) and Type B (interbedded sandstone-argillite thin beds, <5 cm to 60 cm), were defined based on bed thickness ranges and lithological compositions. Euhedral, drusy and microcrystalline quartz vein types along with two dominant fracture styles, namely extension fractures and mosaic breccias were identified in the field. The paleo-permeability at Kuaotunu is sub-divided broadly into the Western and Eastern study area. The fracture and vein character varied between these study areas. TheWestern area is dominated by type A beds, where elongate, anastomosing veins with isolated occurrences of increased fracture intensity are mostly observed. The Eastern area is dominated by type B beds with widespread mosaic brecciation, quartz veneers along sheared argillitic bedding horizons and extension fractures within fault damage zones. The paleo-permeability at Kuaotunu varies along-strike and acrossstrike, and was controlled by the extensional east-west paleostress field active during geothermal activity, heterogeneity of the greywacke, and geometry of the faults. The observed and quantified difference in fracture styles between the two lithostratgraphic units support the conclusion that brittle failure and permeability was influenced by the host rock.