Abstract

Charles Tenney, consultant, and I measured the interval from Madison (Mississippian) to the top of the Tensleep (Permian) in one of the finest exposures in Wyoming (Sec 9, T 23 N, R 83 W). The total thickness was 321 feet of erg sand sea clastics of excellent reservoir quality, expressed in 5–25° foreset dips. There exist six tight, thin carbonate sabkhas (<3 feet thick) and two “Skolithos” or gastropod vertically- burrowed interdunes. There were five laminated, wavy calcareous interdune sandstones up to twelve feet thick, and three convoluted white sandstones that were lee-side slumps at possibly water-saturated interfaces.

These facies are illustrated in Figures 1–4. The tight facies can easily prevent lateral migration for oil traps or explain water level variations in big fields, such as enigmas of water over oil, in oil and gas accumulations. Most mechanical logs won’t read them, though some one- to three- foot zones may be discerned by Sonic Logs.

Wind direction during Permian was northeast or tradewind direction as Wyoming was 3500 miles south of its present latitude. Hot winds, evaporation and facies changes formed barriers to lateral or vertical oil or gas migration. An idealized profile of a dune is presented in the Formation (a little closer to the shallow sea, but still Tensleep) of the nearby Powder River Basin.