The Permian-Pennsylvanian Paradox basin of the Colorado Plateau contains an elongate wedge of sedimentary rocks which is more than 20,000 feet thick near the Uncompahgre front.

Cambrian shelf-carbonate and detrital sediments were deposited over the general area as the sea transgressed eastward from central Utah onto the southwestern extension of the Transcontinental arch. Ordovician and Silurian strata are absent, but a carbonate unit of questionable age may prove to be Ordovician. Upper Devonian shelf deposits and Mississippian carbonates were deposited over the area, again along the eastern flank of a western sea. Regional uplift and exposure of the Mississippian carbonate resulted in the formation of a karst topography, on which the regolithic Molas Formation of Atokan age was developed. Tectonic activity associated with the growth of the Ancestral Rockies then broke up the old shelf area in Early Pennsylvanian time and formed the Paradox basin with its p esent northwest-southeast structural grain.

The Hermosa Group, Des Moines-Virgil in age, is a heterogeneous rock unit composed of: marine sandstone and carbonate with plate-algal mud-mounds and bioherms; restricted marine sapropelic "black shale" dolomite, anhydrite, and salt; conglomeratic marine and continental arkose, red sandstone, and shale. The Hermosa Group is divided into three formations where the anhydrite facies is present: lower Hermosa or Pinkerton Trail Formation, dolomite and black shale; Paradox Formation, evaporites; and upper Hermosa or Honaker Trail Formation, carbonate, sandstone, and red, green, and gray shale. The rock units are cyclic and the black shales are used as time-markers for a finite time-rock zonation; these cycles have been named the Alkali Gulch, Barker Creek, Akah, Desert Creek, and Ismay. Hy rocarbons are produced from the sediments of each of these cycles, but this paper is concerned only with the Desert Creek and Ismay cycles.

During Pennsylvanian time the Paradox basin was almost encircled by highlands (Uncompahgre and San Luis uplifts) on the northeast and east, and by lowlands (Defiance, Kaibab, and Emery uplifts) on the south and west. The sea entered the basin from the southeast through what is now the San Juan basin. The detritus for approximately 15,000 feet of conglomerate, arkose, and red shale (Permian-Pennsylvanian Cutler Group) was shed southwestward from the highlands and was deposited in a narrow frontal belt. In the southwestern part of the basin, the Permian is only 2,000 feet thick and is represented by four formations: Halgaito, red siltstone, sandstone, and shale; Cedar Mesa, sandstone; Organ Rock, red siltstone and shale; and DeChelly, eolian sandstone. Approximately 3,000 feet of Jurass c-Triassic continental shale and fluvial and eolian sandstone blanketed the Permian-Pennsylvanian basin and highlands, and these sedimentary rocks now crop out in the western two-thirds of the basin. Upper Cretaceous marine detritus, remnants of which are present only on the eastern basin edge, were derived from a distal southern and western source.

Major tectonic phases in the Paradox basin area were: (1) Ordovician-Silurian, regional uplift; (2) latest Mississippian, regional uplift; (3) Early Pennsylvanian, Paradox basin formed; (4) Late Pennsylvanian-Cretaceous, salt flowage producing salt anticlines; (5) Late Cretaceous, Laramide orogeny developed new structural lineations and rejuvenated the Pennsylvanian trend; and (6) Tertiary, regional uplift and igneous intrusion and extrusion.

Major hydrocarbon production is from reservoirs developed in Mississippian and Pennsylvanian carbonates with only minor production from Devonian and Permian sandstones. The important Cretaceous reservoirs are essentially a northwestern continuation of the bar-sandstone trend of production of the San Juan basin. Traps in the Mississippian are structural whereas those in the Pennsylvanian are mostly stratigraphic. Approximately 90 per cent of the current ultimate recovery estimate, 425-450 million barrels of oil, will be from Pennsylvanian reservoirs, with the Aneth field ultimate recovery from Desert Creek-Ismay plate-algal mud-mounds estimated to be 350 million barrels.