Joint Meeting: University of Wyoming Department of Geology and Geophysics Wyoming Geological Association Geological Survey of Wyoming: April 2-4, 1982 Laramie, Wyoming: Subsurface Practices in Geology and Geophysics Abstracts of Papers - Compiled by James R. Steidtmann

Middle to Upper Pennsylvanian rocks of the Leo (middle member of the Minnelusa Formation) consist of inter bedded sandstone, dolomite, anhydrite, limestone, bedded chert and radioactive carbonaceous shale. They were deposited in the perched late Paleozoic Alliance Basin largely as a restricted evaporite-carbonate tidal flat. Eustatic sea-level fluctuations produced intercalation of eolian sands, organic “black shales” and supratidal to subtidal carbonates in close vertical and lateral proximity. During the Late Pennsylvanian, the development of the Lusk embayment provided a supply of less restricted marine water into the southwestern Hartville uplift area. This resulted in the deposition of relatively thick crinoidal limestones in lieu of evaporites and carbonaceous shales.

Criteria suggesting wind-blown deposition of the bulk of Leo sandstone include deflationary lag surfaces, low-amplitude eolian ripples, irregular sand distribution within correlative stratigraphic zones and cross-strata dip directions corresponding to the paleo-tradewind belt location. The black shales are mostly shallow-water deposits of lagoons, sabkhas, and interdunal ponds.

Sandstone units deposited as isolated eolian dunes and dune fields provide superb stratigraphic traps for hydrocarbons generated in the carbonaceous shales. The current flurry of Leo drilling began in 1978 and has affirmed the inherent potential of this play. Seismic, dipmeter and core data along with the establishment of depositional trends provide the key to stratigraphic Leo exploration.