ALBERT

Description: We determined the structure and permeability variations of a 4 km-long normal fault by integrating surface mapping with data from five boreholes drilled through the fault (borehole to tens of meters scale). The Big Hole fault outcrops in the Jurassic Navajo Sandstone, central Utah. A total of 363.2 m of oriented drill core was recovered at two sites where fault displacement is 8 and 3-5 m. The main fault core is a narrow zone of intensely comminuted grains that is a maximum of 30 cm thick and is composed of low-porosity amalgamated deformation bands that have slip surfaces on one or both sides. Probe permeameter measurements showed a permeability decline from greater than 2000 to less than 0.1 md as the fault is approached. Whole-core analyses showed that fault core permeability is less than 1 md and individual deformation band permeability is about 1 md. Using these data, we calculated the bulk permeability of the fault zone. Calculated transverse permeability over length scales of 5-10 m is 30-40 md, approximately 1-4% the value of the host rock. An inverse power mean calculation (representing a fault array with complex geometry) yielded total fault-zone permeabilities of 7-57 md. The bulk fault-zone permeability is most sensitive to variations in fault core thickness, which exhibits the greatest variability of the fault components. Zoe Shipton received her B.S. degree from the University of Leeds and her Ph.D. from the University of Edinburgh, United Kingdom. She recently completed a postdoctoral fellowship at Utah State University. Her research emphasizes three-dimensional fault growth and fluid flow through time, using integrated surface mapping and drill core data. She is currently a lecturer in structural geology at Trinity College, Dublin.James Evans received B.S. degrees in geology and engineering from the University of Michigan and an M.S. degree and Ph.D. from Texas A&M University. He is a professor at Utah State University and chief editor of the Journal of Structural Geology. His research examines rock deformation in the brittle and semibrittle fields, relationships between fluid flow and deformation, and deformation mechanisms of seismogenic faults. After working as a technical draftsman in the aerospace industry, Kim Robeson received his B.S. degree in geology from California State University at San Bernadino and his master's degree on fault development from Utah State University. Kim died in November 1998. Craig Forster received his B.S. degree from the University of British Columbia, an M.Sc. degree from the University of Waterloo, and his Ph.D. at the University of British Colombia. He is a research professor at the University of Utah and Utah State University. His research examines permeability heterogeneity in flow simulators, the hydraulic properties of faults, and water resources and environmental issues. Stephen Snelgrove received a B.S. degree in geophysics, an M.S. degree in geological engineering from the University of Utah, and is currently a Ph.D. student in civil engineering at the University of Utah. He worked for Phillips Petroleum from 1978 to 1986. His research interests are in the construction of geologically sound subsurface models and flow simulations.