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Abstract

AAPG Bulletin, V. 88, No. 2 (2004), P. 175-191.

Copyright copy2004. The American Association of Petroleum Geologists. All rights reserved.

Chemical bleaching indicates episodes of fluid flow in deformation bands in sandstone

W. T. Parry,1 Marjorie A. Chan,2 Brenda Beitler3

1Department of Geology and Geophysics, 135 S. 1460 E. University of Utah, Salt Lake City, Utah 84112-0111; email: [email protected]
2Department of Geology and Geophysics, 135 S. 1460 E. University of Utah, Salt Lake City, Utah 84112-0111
3Department of Geology and Geophysics, 135 S. 1460 E. University of Utah, Salt Lake City, Utah 84112-0111

AUTHORS

William T. Parry is professor emeritus of geology and geophysics at the University of Utah. Former positions include associate professor of geosciences at Texas Tech University, Lubbock, Texas, and exploitation engineer for Shell Oil Company, Midland, Texas. He received his B.S. and M.S. degrees and his Ph.D. in geological engineering from the University of Utah. His research interests are geochemistry and mineralogy related to faults and ore deposits.

Marjorie A. Chan is professor of geology at the University of Utah, where she joined the faculty in 1982 and currently serves as department chair. She received her B.S. degree from the University of California, Davis, and her Ph.D. from the University of Wisconsin, Madison. Her current research focuses on sedimentology and stratigraphy in Mesozoic deposits of the Colorado Plateau.

Brenda Beitler is currently a Ph.D. candidate in the Department of Geology and Geophysics at the University of Utah. She earned a B.S. (1998) degree and an M.S. (2000) degree in earth sciences from the University of California, Santa Cruz. Her thesis research focuses on color variations and iron mineralization resulting from micro- to regional-scale fluid flow in the Navajo Sandstone.

ACKNOWLEDGMENTS

The Petroleum Research Fund of the American Chemical Society provided financial support for this study. We gratefully acknowledge the United States National Park Service for permission to conduct scientific research under Permit Number CARE-2001-SCI-0002. Dennis Netoff assisted with deformation band localities.

ABSTRACT

Jurassic sandstones on the Colorado Plateau have been variably bleached through interaction with hydrocarbon-bearing solutions or other reducing agents. Deformation bands in the Navajo Sandstone have a variety of colors in comparison with the host rock color that indicate the timing of bleaching relative to deformation-band formation. White deformation bands in red sandstone indicate that deformation bands were likely permeable at an early dilatant stage in their development history. Field characteristics, petrography, bulk rock chemistry, clay mineralogy, and geochemical modeling show that bleached deformation bands experienced an episode of chemical reduction where fluids removed some iron and left the remaining iron as pyrite and magnetite. Mass-balance calculations show that as much as 10 kg of chemically reducing fluid per 100 g of rock (1500 pore volumes of fluid) are necessary to remove 0.1 wt.% iron from a deformation band. These large pore volumes suggest that moving, reducing solutions regionally bleached the sandstone white, and bleached deformation bands resulted where deformation bands provided localized fluid access to unbleached, red sandstone during an initial dilatant stage. Alternatively, access of reducing soil solutions may be provided by gravity-driven, unsaturated flow in arid to semiarid vadose zones. Color and chemical composition is a valuable index to the pathway and timing of hydrocarbon movement through both host rocks and deformation bands.

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