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The AAPG/Datapages Combined Publications Database

Wyoming Geological Association


Topics in Wyoming Geology: Wyoming Geological Association Guidebook, 2008
Pages 35-56

Detailed Core Interpretation Allows a New Perspective on Tensleep Sandstone Correlations at Teapot Dome Field, NATRONA CO., Wyoming

Mark Milliken


Recent detailed interpretation of Pennsylvanian-Permian Tensleep Sandstone Formation (Tensleep) core has resulted in upgrades to the stratigraphic hierarchy traditionally used at Teapot Dome Field (NPR-3). The purpose of the coring project is to characterize the sealing nature of the overlying Permian Goose Egg Formation (Goose Egg) cap rock, and to evaluate vertical and horizontal permeability heterogeneities in the Tensleep in support of future CO2 EOR projects. Well 48-X-28 was drilled and cored to a TD of 5758 ft in the lower Tensleep in 2004. A total of 353 ft of core was cut over the interval 5300 to 5653 MD, between the Forelle Member of the Goose Egg and the lower Tensleep “D Sandstone.” Routine core analyses and thin section examinations were conducted over the cored interval. The Opeche sandstone, at the base of the Opeche Shale Member of the Goose Egg, contains abundant dead oil and appears to be fluvial in origin. Its basal contact is a major unconformity on top of the Tensleep “A Sandstone.” At NPR-3 and elsewhere, the Opeche sandstone has been recognized as Tensleep. Core work combined with formation imaging logs have allowed more effective log top and base definitions, and lithology correlations to neutron-density porosity log character. A major unconformity at the base of the Opeche sandstone led to a redefined log interpretation of the upper Tensleep contact. Traditional Powder River Basin units, such as the “B Dolomite,” have been repositioned in the Tensleep stratigraphic hierarchy to correlate with sequence boundaries. In addition to core, field work on Tensleep outcrops confirms the repetitive nature of major flow units. A typical sequence consists of cross bedded eolian dunes eroded during a marine advance, with the uppermost zones being diagenetically altered by carbonate cementation. The dune sandstones are commonly overlain by high stand dolostones and dolomitic marine sandstones. The marine rocks are often bioturbated and contain marine fossils. Where evidence suggests subaerial exposure, the dolostones form sequence boundaries. Evidence for lowstand subaerial exposure includes oxidization and apparent paleosols. Overlying the subaerially exposed marine units are dune and interdune sandstones, completing the sequence. Sequence boundaries observed in core and defined by log character can be correlated among other Tensleep wells within NPR-3, allowing improved understanding of major field-wide flow units.

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