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Department of Geosciences, The University of Tulsa, Tulsa, OK

Abstract: Facies Architecture of the Tensleep Sandstone, Bighorn Basin Wyoming: Outcrop Analogs to Subsurface Reservoirs

Outcrops of the Middle Pennsylvanian Tensleep Sandstone are useful analogs for understanding reservoir characteristics in the subsurface. Outcrop study sites include Alkali Creek and Ziesman dome, and subsurface study sites include Bonanza field and Byron field.

Facies architecture hierarchy consists of 4 elements, described in the order of decreasing rock volume: parasequence; facies within a parasequence; eolian first-order surface bounded elements; and eolian second-order elements. Marine-eolian facies couplets typically comprise parasequences. A wide variety of marine, marginal marine and eolian facies are found in the Tensleep. First-order surface bounded elements rise gently from contact with marine facies to the south-southwest. First-order surfaces bound very large-scale tabular-planar sets, and are regarded as the erosional contact between climbing eolian dunes. Eolian intrasets are contained within first-order elements, and include wind- ripple- and avalanche-dominated trough cross-strata with axes plunging to the west-northwest. Second-order surfaces bound eolian cross strata within the tabular-planar sets; the upper part contains southward dipping grainfall-dominated strata, and the lower part contains west-southwest plunging wind- ripple- and grainfall-dominated strata with rare avalanche strata.

Facies architecture is identified in producing oil fields using conventional logs, borehole images, and core. Petrophysical properties are controlled by the architectural elements. Thus, facies architecture, based on outcrop and subsurface analysis, has profound implications for reservoir management strategies.

AAPG Search and Discovery Article #90919©1999 AAPG Rocky Mountain Section Meeting, Bozeman, Montana