Insights to Geothermal Subsurface Processes From Core at Steamboat Springs, USA, and Tauhara, New Zealand, Using 3D Scanning Electron Microscopy Imaging

Abstract

Scanning Electron Microscopy (SEM) enables micro-scale 3D imaging of rock constituents. This provides unique information about the complexity of subsurface environmental conditions and fluid-rock interactions within geothermal fields. These dynamic thermal environments drive chemical and physical changes within rock units altering their physical properties, such as porosity and compressibility. Core recovered from drilling within two geothermal fields; Steamboat Springs, Nevada, USA and Tauhara, Taupo Volcanic Zone, New Zealand has been characterized using multiple techniques. At Tauhara, core was recovered as part of a subsidence study to specifically identify weak and strong horizons at depth, in an effort to determine the cause of local subsidence bowls. Analytical techniques included X-Ray Diffraction (XRD) and petrographic microscopy to identify the mineralogy, as well as a range of geotechnical techniques to determine rock compressibility. It was SEM imaging that provided a window into the complex nature of these subsurface environments and identified processes driving the physio-chemical conditions acting on and either strengthening or weakening rocks at specific depths. SEM imaging of core from Steamboat Springs revealed multiple pulses of thermal fluid, changes in fluid chemistry and the relative timing of such events, as well as the intricate nature of sinter (hot spring rock) diagenesis. As geothermal environments are constantly changing and each environment leaves a footprint which is preserved within the rock, SEM can be used to track these changes placing them in a spatial and temporal context. SEM 3D imaging compliments 2D petrographic examination and other standard analytical methods traditionally used to determine rock mineralogy and physical properties.