THE ROLE OF LITHOLOGY AND ALTERATION ON PERMEABILITY AND FLUID FLOW IN THE YELLOWSTONE GEOTHERMAL SYSTEM, WYOMING

Abstract

Cores from two of the 13 U.S. Geological Survey (USGS) research holes at Yellowstone National Park (Y-5 and Y-8) were evaluated to characterize lithology, texture, alteration, and the degree and nature of fracturing and veining. Matrix permeability measurements and petrographic examination of the core were used to evaluate the effects of lithology and hydrothermal alteration on permeability. The intervals studied in these two core holes span the conductive zone (including the reservoir cap) and the upper portion of the convective geothermal reservoir.

Variations in porosity and matrix permeability observed in the Y-5 and Y-8 cores are primarily controlled by lithology. Y-8 intersects three distinct lithologies: volcaniclastic sandstone, perlitic rhyolite lava, and nonwelded pumiceous ash flow tuff. The sandstone typically has high permeability and porosity, and the tuff has very high porosity and moderate permeability, while the perlitic lava has very low porosity and is essentially impermeable. Hydrothermal self-sealing appears to have generated the existing cap to the reservoir. Changes in pressure and temperature in Y-8 correspond to a zone of silicification in the volcaniclastic sandstone just above the contact with the perlitic rhyolite; this silicification has significantly reduced permeability.

For rocks with low matrix permeability (such as densely welded ash flow tuff), fluid flow appears to be controlled by the fracture network. The Y-5 core hole penetrates a thick intracaldera section of the 0.6 Ma Lava Creek ash flow tuff. In this core, the degree of welding appears to be responsible for most of the variations in porosity, matrix permeability, and the frequency of fractures and veins. Fractures are most abundant within the more densely welded sections of the tuff. However, the most prominent zones of fracturing and mineralization are associated with hydrothermal breccias within densely welded portions of the tuff. These breccia zones represent transient conduits of high fluid flow that formed by the explosive release of overpressure in the underlying geothermal reservoir and that were subsequently sealed by supersaturated geothermal fluids. In addition to this fracture sealing, hydrothermal alteration at Yellowstone appears to generally reduce matrix permeability and focus flow along fractures, where multiple pulses of fluid flow and self-sealing have occurred. The results of this study can be used to constrain and validate coupled thermal-hydrological-chemical models that predict potential changes in fluid flow resulting from the thermal impact of storing high-level nuclear wastes in fractured ash flow tuffs.