Abstract

Integrated studies of over 1000 subsurface control sections and over 300 measured surface sections indicate that paleostructural growth of late Paleozoic structural elements, penecontemporaneous with deposition, markedly influenced the distribution of Permian cyclic lithofacies and carbonate reservoir belts in Wyoming and southeastern Idaho. Major paleostructural features include the Antler orogenic belt, Sublett Basin, and Bannock high of southeastern Idaho, the Cortez-Uinta axis, the Wyoming shelf margin, and several paleostructures on the Wyoming shelf, some of which may represent early growth of features that later were involved in Laramide orogenic movements. Identification of paleostructural growth is based on interpretations of depositional environments of Permian facies, along with thickness patterns of Permian as well as pre-and post-Permian stratigraphic units.

The Phosphoria Formation and the equivalent beds of the Park City Formation are divisible into two main depositional cycles, comprising the Franson and Ervay Members, each of which are separated into subcycles, based on a marker-bed correlation framework, applicable throughout most of the Wyoming shelf province. Marker beds are thin sandy and shaly units that represent widespread clastic deposition across the shelf during low sea level stages at the termination of each subcycle. Petroleum reservoir beds are associated with early diagenetic dolomitization of coarse-grained skeletal carbonate mounds or banks, concentrated along paleostructural high areas of the shelf. Initial dolomitization of these beds is believed to have been caused by seepage-reflux of high-magnesium waters during low sea-level stages at the termination of depositional cycles, with a general westward shelf-to-basin gradient. Deposition of phosphorite and organic-rich dark shale occurred at the time of the maximum regressive and early transgressive stages of the two main cycles. Carbonate mound or bank buildup of bryozoan, crinoid, brachiopod, oolite-pellet, phylloid algae, and other skeletal debris occurred during maximum transgressive stages when optimum normal seawater circulation systems were present across the shelf.

Thick, organic-rich, dark shales sufficiently rich in organic matter to have been petroleum source rocks are present in the Meade Peak and Retort Members of the Phosphoria Formation, with maximum thickness in southeastern Idaho and westernmost Wyoming. A substantial thickness of dark shale is also present in the Green River, and parts of the Wind River and Bighorn Basins. Isopach maps of post-Permian strata suggest that burial depths in the western area were sufficient for generation of petroleum from the Phosphoria beds to have begun as early as Jurassic time. At that time an eastward migration gradient existed across west-central Wyoming, perhaps as far east as the Bighorn Mountains and Casper Arch. Late in Mesozoic time, generation and migration should have begun from the source-rock facies farther east in the Green River, Wind River, and Bighorn Basins, with a migration gradient into local reservoir belts.

The nature and distribution of stratigraphic facies in the overlying Dinwoody Formation suggest that cyclical depositional patterns characteristic of the Permian continued into the Triassic. The striking difference between the Phosphoria and Dinwoody sediments is interpreted to have resulted from the worldwide decline and extinction by Triassic time of major elements of the Paleozoic marine biotic assemblages which were largely responsible for the buildup of skeletal carbonate mounds or banks and associated organic-rich sedimentary facies in the Permian and earlier Paleozoic beds.