| Title | OBSERVING DYNAMIC PERMEABILITY IN THE HEYUAN GEOTHERMAL FAULT FROM MICROSTRUCTURAL ANALYSIS |
|---|---|
| Authors | L. Tannock, M. Herwegh, A. Berger, K. Regenauer-Lieb |
| Year | 2018 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | Heyuan fault, quartz reef, microstructures, geothermal resources, hot springs, quartz reef |
| Abstract | Microstructural textures can provide insights into how the permeability of a fault zone evolves on both long and short timescales. A fault can act either as a permeable pathway (conduit) or an impermeable seal (barrier) evolving over time. A seismic event temporarily increases permeability by fracturing; subsequent fluid flow and oversaturation can then precipitate and seal the fractures again. Thus, the fault zone can become less permeable over time (and therefore less favourable for geothermal fluid flow). Here we investigate in particular the seismic-interseismic relationship between fluid flow and quartz precipitation, which can occur in multiple cyclical and/or episodic stages, as shown by detailed microstructural analysis on the Heyuan geothermal fault zone in South China. The Heyuan geothermal fault zone provides a unique study area in which to investigate this phenomenon. It features a giant quartz reef, formed through continued precipitation of different minerals from hydrothermal fluid flow. The geothermal fluid flow continues to the present to feed hot springs. Our working hypothesis is that the exposed quartz reef can be used as a chemical and structural proxy into fluid circulation patterns at deep sections of the active fault, since exhumation exposed the formerly deeply situated structures. Detailed microstructural analysis of this unit and the fault zone facies unearths a complex history of multiple brittle-ductile deformation cycles, and recystallisation. These processes operate over vastly different timescales. Initial findings validate the relationship between seismic-interseismic periods. Over long-timescales gradual precipitation during inactivity will decrease permeability, whereas seismic cycles can lead to short-timescale increases in permeability in the fault zone, and subsequent fluid flow. Coupled with macro-scale analysis, this information can be used to identify the upflow “sweet spots” and may be utilized for targeting key zones which are most sensitive to these permeability increases, such as presently still acting for the active hot springs along the Heyuan fault. |