Abstract

Slattery field was discovered in 1963 on the eastern flank of the Powder River basin, Wyoming. The field has produced over 6.5 million bbl of oil in the past 22 years. It is located 20 mi south-southeast of Gillette, Wyoming (T49N, R68-69W) and is 2 mi long and 1.4 mi wide. Although there is minor Cretaceous Muddy Sandstone production, sandstones in the upper part of the Permo-Pennsylvanian Minnelusa Formation are the principal reservoirs. Previous interpretations have suggested a continuous, widespread nature for the Minnelusa sandstone reservoirs. However, this study indicates the limited nature of each reservoir and its relative stratigraphic position. Dune sandstones are present in the Minnelusa Formation at five separate zones. These zones partially overlap southwest to northeast. Four of them form separate reservoirs as determined by geological correlations, log analysis, reservoir information and production data. The southern end of Slattery field abuts a south-southwest trending structural nose. An Opeche Shale-filled valley parallels the field on its eastern updip edge and truncates upper reservoirs to form part of the trapping mechanism. Previous testing and completion practices, which often include several zones at a time, are not able to determine individual reservoir fluid content and pressure. Oil production in one well may have been missed.