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Diagenetic characteristics of the Jurassic Navajo Sandstone in the Covenant oil field, central Utah thrust belt

¹ Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112-0111; wparry{at}comcast.net
² Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112-0111; marjorie.chan{at}utah.edu
³ Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112-0111; barb.nash{at}utah.edu

W. T. Parry is professor emeritus of geology and geophysics at the University of Utah. His former positions include associate professor of geoscience 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, where he taught geology and engineering. His research interests are geochemistry and mineralogy related to ore deposits and faults.

M. A. Chan is a professor of geology and the department chair at the University of Utah. She received her B.S. degree from the University of California, Davis, and her Ph.D. from the University of Wisconsin, Madison. Her recent and current research focuses on Mesozoic sedimentology and stratigraphy on the Colorado Plateau, with applications to eolian reservoirs and terrestrial iron oxide concretion analogs to Mars.

B. P. Nash is a professor of geology and geophysics at the University of Utah where she directs the electron microprobe laboratory. Her research interests are in electron beam analysis, mineral chemistry, tephrochronology, and magma genesis and evolution. She is currently investigating isotopic and geochemical characteristics of supereruptions from the Yellowstone hotspot to understand what makes them so super.

ABSTRACT

The Jurassic Navajo Sandstone core in the Covenant field includes eolian dune interbedded with carbonate playa lake and fluvial interdune facies. Dune facies samples are bleached but not depleted in iron; bleached dune facies outcrop samples are depleted in iron. Bleached dune facies in the core samples contains ferroan dolomite, quartz overgrowths that do not completely fill pore spaces, grain-coating and pore-filling illite, coarse-grained gray hematite, kaolinite, and trace pyrite. Reddish brown interdune facies are typically very fine-grained sandstone and siltstone and contain dolomite and ferroan dolomite cement, illite pore-filling, and very fine-grained, red hematite. Diagenetic mineralogy and chemical compositions overlap the mineralogy and compositions of outcrop samples.

The carbon and oxygen isotopic composition of dolomite in interdune facies and adjacent dune facies is derived from groundwater discharge modified by evaporation in a playa lake interdune environment, not from interaction with hydrocarbons.

The iron in bleached dune facies is incorporated in coarse-grained hematite, ferroan dolomite, and trace pyrite. The bleached diagenetic mineral association of ferroan dolomite-hematite-pyrite with is metastable relative to more reducing conditions produced by petroleum. The reservoir temperature of 188°F (87°C) is too high for bacterial sulfate reduction and too low for geologically significant thermochemical sulfate reduction accounting for the association of abundant in produced water and trace pyrite in the core.