Geology of Minnelusa Oil in Northeast Powder River Basin, Wyoming: ABSTRACT

Abstract

Since 1957, with the opening of the Donkey Creek Minnelusa pool, 35 successful exploratory tests have proved the potential of upper Minnelusa (Permo-Pennsylvanian) sandstones. Oil is produced from structural, permeability, and unconformity traps, or from combinations of these. Trapping conditions are illustrated by the Robinson Ranch and Raven Creek fields. Oil fields usually are associated with low-relief structures, and Minnelusa accumulations are accompanied by stratigraphic changes in younger beds of Permian, Triassic, and Jurassic age. The most obvious change occurs in the superjacent Opeche Shale. Updip thickening of the Opeche coincides with wedging-out of Minnelusa pay sandstones. Therefore, current geologic exploration relies largely on subsurface isopach mapping of the Opeche. Future exploration should consider other techniques of geologic interpretation, such as thickness changes in all younger units and use of the seismograph for stratigraphic as well as structural control. More than 10 million barrels already have been produced from the Minnelusa. It is evident that many new fields are yet to be discovered. Average size of fields is small, but excellent reservoir sandstones may yield as much as one million barrels per well. Exploratory drilling, aided by moderate costs, will continue at a high level of activity for many years. Improved geological interpretation will help maintain an adequate success ratio. End_of_Article - Last_Page 118------------