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A study of 980 wells, 24 outcrop sections, and 29 cores was conducted in the Powder River basin of northeastern Wyoming in order to develop predictive depositional models for the Leo sandstones of the middle member of the Minnelusa Formation. Given the limited amount of data available, an approach was devised that relied on synthesis of information from modern analogs with data derived from the ancient to predict regional patterns of deposition. This information was then used to determine genetic relationships between the patterns of regional sedimentation and proven stratigraphic traps. Six Leo oil and gas fields were examined in detail; the Qatar Peninsula and Um Said sabkhas serve as modern analogs.
The results of the study show that the Leo member of the Minnelusa and equivalent units of the Powder River basin were deposited as dune sequences within and adjacent to the Lusk embayment, a northward-extending arm of a large epeiric sea that existed southeast of the study area. Situated approximately 15° north of the paleoequator, the study area was the site of accumulation of sands transported from a northerly source by northeasterly trade winds. Accumulation and distribution of these windblown sands in the area surrounding the Lusk embayment were controlled by the local depositional setting, tectonic framework, and a series of minor fluctuations of eustatic sea level. The greatest potential for preservation of these dune deposits occurred during periods of rising base level.
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