Fluid Rock Interaction Fluid flow, vein formation and permeability evolution in the Earth's Crust

Eric Tenthorey, Stephen Cox and Christoph Hilgers

---

Chemical reactions in the Earth's crust can profoundly influence various mechanical and hydraulic properties of the host rock. One such property, permeability, can be altered dramatically as reactants dissolve and metamorphic products form within fractures or intergranular pore space. In addition to the obvious importance of permeability in the formation of certain ore deposits and for hydrocarbon migration, rock permeability can also have an important role in controlling pore pressures and lithologic strength. In the case where permeability reduction is accompanied by deformation, pore pressures may rise and induce mechanical failure of the rock, thus creating new fracture conduits for fluid flow and reaction. If such a process is repeated over time, extensive vein networks can form, potentially containing significant precious metal concentrations. Although such ideas are generally accepted among the scientific community, the incorporation of chemical fluid-rock reaction processes into understanding linkages between fluid transport properties and deformation processes is relatively new. As a result, these processes are very poorly understood in terms of how pore and fracture topology evolve, the magnitudes of permeability change involved and the role of temperature and pore pressure on chemical reactivity and mechanical behaviour. We are investigating these questions with the aim of understanding large scale issues such as hydration and fluid flow in the lower crust, mechanics of faulting and the formation of economically valuable mineral deposits. The first phase of this project is exploring hydrothermal sealing in fractured quartzite. These experiments are being performed in a Paterson high pressure rig. The fractured specimens are subjected to temperature gradients of up to 25°C/cm with the high temperature end of the sample at temperatures ranging from 500-900° C. As fluids diffuse from the high T/high solubility end of the specimen to the lower T end, precipitation of quartz occurs. We are exploring the temperature dependence of fracture closure processes and rates. Previous studies on reactive flow have shown that such topologic changes in sealing fractures have drastic effects on flow properties, namely permeability. Experiments, currently in preparation, will combine diffusion driven vein formation with permeability and porosity measurements, so that the link between various vein morphologies and flow properties may be quantified. The next step in this program will investigate fracture sealing and permeability evolution in a dynamic environment, where episodic shear failure and associated permeability enhancement competes with fracture healing and sealing processes which destroy permeability between slip events. These experiments are providing a quantitative understanding of processes controlling fluid-driven nucleation of earthquakes.