Porosity, permeability and fluid flow in the YellowstoneGeothermal System, Wyoming

Journal Article · Fri Mar 29 04:00:00 EST 2002 · Journal of Volcanology and Geothermal Research

OSTI ID:861545

Dobson, Patrick F; Kneafsey, Timothy J; Hulen, Jeffrey; Simmons, Ardyth

Cores from two of 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. Porosity and matrix permeability measurements and petrographic examination of the cores were used to evaluate the effects of lithology and hydrothermal alteration on porosity and permeability. The intervals studied in these two core holes span the conductive zone 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 rhyolitic 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 localized permeability barriers within 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 porosity and permeability. In rocks with inherently low matrix permeability (such as densely welded ash-flow tuff), fluid flow is controlled by the fracture network. The Y-5 core hole penetrates a thick intracaldera section of the0.6 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 generally to reduce matrix permeability and focus flow along fractures, where multiple pulses of fluid flow and self-sealing have occurred.

• Research Organization: Ernest Orlando Lawrence Berkeley NationalLaboratory, Berkeley, CA (US)
• Sponsoring Organization: USDOE Office of Civilian Radioactive WasteManagement
• DOE Contract Number: AC02-05CH11231
• OSTI ID: 861545
• Report Number(s): LBNL--50044; BnR: 820101000
• Journal Information: Journal of Volcanology and Geothermal Research, Journal Name: Journal of Volcanology and Geothermal Research Journal Issue: 3-4 Vol. 123; ISSN 0377-0273; ISSN JVGRDQ
• Country of Publication: United States
• Language: English

Related Subjects

15 GEOTHERMAL ENERGY
58 GEOSCIENCES
FLUID FLOW
FRACTURES
GEOLOGIC SURVEYS
GEOTHERMAL FLUIDS
GEOTHERMAL SYSTEMS
HYDROTHERMAL ALTERATION
LAVA
LITHOLOGY
MINERALIZATION
PERMEABILITY
POROSITY
SANDSTONES
STREAMS
TEXTURE
TUFF
YELLOWSTONE NATIONAL PARK
Yellowstone hydrothermal alteration self-sealing permeabilityporosity