The Upper Minnelusa, a viable conventional oil target in the Powder River Basin

Abstract

The Upper Minnelusa sands of the Powder River Basin have produced more than 650 MMBO of oil since their discovery in the early 20th century. Early explorers looked for surface structures downdip of surface oil seeps on the western flank of the Black Hills Uplift. More recent exploration efforts, from the mid-20th century to present, have used subsurface geological, geophysical, and petrophysical data to better define the eolian and evaporitic marine system which existed in the Powder River Basin some 300 M.Y.A. The scope of this discussion pertains chiefly to the Upper Member of the Minnelusa Formation (Upper Minnelusa) of Campbell, Crook, and Weston Counties, Wyoming. In this area, the Early-Permian Upper Minnelusa sits disconformably atop the Late-Pennsylvanian Middle Minnelusa; the eastern distal equivalent to the Tensleep Formation which was deposited to the west. Within the Upper Minnelusa, several regionally correlative sequences are recognized, the upper three of which are known to be hydrocarbon bearing. Each depositional sequence is composed of a prograding eolian dune complex moving seaward during relative sea level regression. The marine transgression which followed inundated, reworked, and buried the older eolian sands beneath shallow marine and evaporitic facies. The best Upper Minnelusa reservoirs are the preserved relics of eolian sand dunes and dune complexes, which formed along the northwestern shoreline of a shallow evaporitic seaway. The resulting reservoirs are laterally complex due to interfingering of subaerial, shallow marine, and evaporitic facies. Dune and dune complex poro-perm preservation was influenced by rate of burial, differential compaction, and selective cementation. Trapping configurations for Minnelusa fields are predominately tied to incision and infill of impermeable Opeche Shale and, to a lesser degree, structural traps and geomorphically high relief dunes. Regional dip reversal from dipping east to dipping west followed deposition of the Upper Minnelusa provides implications for the source of the Minnelusa oil as well as for subsequent deposition of highly productive Mesozoic hydrocarbon reservoirs.

AAPG Datapages/Search and Discovery Article #90357 ©2019 AAPG Rocky Mountain Section Meeting, Cheyenne, Wyoming, September 15-18, 2019