Upper Part of Minnelusa Formation in Outcrop Near Beulah, Wyoming, and at West Mellott Field, Powder River Basin

James W. Schmoker, Christopher J. Schenk, Richard M. Pollastro

The upper part of the Minnelusa Formation near Beulah, Wyoming, consists of dolomite, gypsum (anhydrite in the subsurface), and sandstone units that provide a three-dimensional sedimentologic and structural analog for upper Minnelusa oil fields such as West Mellott. Each dolomite records the initiation of marine transgression across relict topography of the previous cycle. Gypsum (anhydrite) above the dolomite records restriction of the water body as sea level began to fall. Low-energy marine, sabkha, and eolian-dune sandstones record continued regression.

The extent and distribution of eolian dunes differ from cycle to cycle. Where eolian sandstones were not preserved, marine and sabkha sandstones are directly overlain by marine dolomites of the next cycle. At Beulah, gypsum onlaps and covers an eolian-dune complex tens of feet thick, forming a visual model of the reservoir-trap system at West Mellott field.

Original intergranular pore space of the eolian-dune complex at West Mellott was about 40-45% and was probably pervasively filled with early gypsum (anhydrite) cement. Relatively late dissolution of anhydrite cement has preceded and overshadowed burial compaction, resulting in an increase of fluid-filled pore space (to as high as 30%) and a concurrent and somewhat incongruous decrease of intergranular pore space. At West Mellott, intergranular pore space is reduced from original levels but is still relatively high, and reservoir quality of sandstones is limited by incomplete anhydrite dissolution rather than by burial compaction.

AAPG Search and Discovery Article #91033©1988 AAPG Rocky Mountain Section, Bismarck, North Dakota, 21-24 August 1988