Abstract

Three sequences comprise the Permian Phosphoria of the Bighorn Basin in Wyoming: the Grandeur (basal), Franson (middle), and Ervay (upper). Internally, each sequence contains cycles (rhythmic bedding) of mainly ramp carbonates. The cycles record both couplets of regional transgression/regression and coincident marked changes in climate. Paleostructure also influenced facies distributions and thicknesses, both locally and regionally.

Because Permian rocks reflect progressive marine flooding of the Wyoming Shelf, only the youngest (Ervay) cycle displays all the depositional variations of a complete transgressive/regressive couplet. The Grandeur and Franson cycles consist of rocks deposited mainly in the most landward portions of the deposystem. The Grandeur cycle, where it is not completely lost to post-Grandeur/pre-Franson erosion, is composed of shallow marine carbonates. The Grandeur partially fills erosional relief developed on the Tensleep Sandstone during pre-Phosphoria, post-Tensleep time. The Franson cycle consists of redbeds and evaporites in all but the westernmost part of the basin.

In most of the Bighorn Basin the Ervay cycle consists of basal phosphate-rich cherty siltstones overlain by carbonates. The Ervay can be subdivided into at least four regionally correlative subcycles. In the western part of the basin, these carbonate subcycles have at their bases open-marine siliciclastic, phosphatic, crinoid- and bryozoan-rich packstones and wackestones. These grade upward into restricted-marine molluskan and pellet-rich packstones and wackestones. Siliciclastics loosely referred to as the Shedhorn Sandstone occur throughout these open-marine dominated cycles. By contrast, on the eastern side of the basin, Ervay carbonate subcycles are primarily restricted-marine rocks that grade laterally and upward into fenestral stromatolitic boundstone and pisolitic “diagenetic terrain” island complexes, and subtidal redbed-evaporite deposits. Each Ervay subcycle includes transgressive, highstand, and lowstand components. Transgressive deposition took place when cold marine water flooded the ramp and allowed only sponges and calcific faunas to grow under slow rates of mixed clastic/carbonate sedimentation. The climate was cool and wet.

Subsequently the climate evolved to hot and arid during sea-level rise and highstand conditions. Average bottom-water temperature increased. Aragonitic faunas flourished. Sedimentation rates increased. Islands and local tidal pools composed of peloidal grainstones and fenestral, coated-grain stromatolitic units formed. The islands restricted water circulation to the east and created broad redbed-dominated lagoons and salinas. Each subcycle ended with a lowstand represented by 1) pisolite-rich, tepee-structured units that formed on the highest parts of islands and 2) anhydrites laid down in the salinas.

Reservoir types in the basin reflect these depositional patterns. Restricted-marine, sucrosic dolomites form most reservoirs on the western side of the basin (e.g., Hamilton Dome, Little Grass Creek, Pitchfork fields). Pisolitic Fenestral stromatolitic boundstones form the best reservoirs on the eastern flank of the basin, especially at Cottonwood Creek Field.

Ervay subcycles can be mapped in detail at Cottonwood Creek Field and further subdivided into local progradational deposits. Per-well volumes of production correlate mainly to the thickness of porous peritidal facies. Reservoir quality within each peritidal unit commonly changes at exposure surfaces due to subtle changes in depositional fabric and early diagenesis. Abrupt lateral changes in both deposition and diagenesis create a complicated reservoir. Cyclic deposition also is apparent in fields on the western side of the basin, but because these cycles consist of laterally-extensive restricted- and open-marine facies, variations in production are related mainly to structural position and fracture intensity.