Facies and Diagenesis of the Park City Formation, Wind River Basin, Sheep Mountain Anticline, Fremont County, WY

Daniel Hallau and J. Frederick Sarg
Colorado School of Mines

This study examines the Permian carbonates on Sheep Mountain Anticline in Fremont County, Wyoming, which sits on the southern margin of the Wind River basin. From a hydrocarbon exploration perspective, the Park City Fm. on Sheep Mountain represents a critical position between the landward Goose Egg shales and the basinward cherts and organic mudrocks. Most Park City Fm. hydrocarbon production to date is from anticlinal structures and is almost exclusively associated with Tensleep (Pennsylvanian) production. There is a possibility that the basin is largely underexplored with respect to Permian stratigraphic traps. While the world-class phosphate deposits of the Phosphoria Fm. have been extensively studied, an examination of less phosphate-rich facies helps elucidate the conditions under which this enormous phosphate resource was deposited. Four measured sections, petrographic analysis of ~60 thin sections, descriptions of over 200 hand samples, well log analysis of 10 nearby well logs, and handheld x-ray fluorescence have been used to interpret the depositional and diagenetic history of these rocks. Three 3rd order cycles of the Park City Fm. outcrop on Sheep Mountain, each of which represents a very different position on the homoclinal carbonate ramp. From youngest to oldest, the Ervay cycle is dominated by peloidal pack- & grainstones, the Franson cycle by echinoderm bryozoan brachiopod wacke- & packstones, and the Grandeur cycle by sandy and silty mud- to packstones. Silicification of bioclasts and bioturbation traces is common, and at least two silicification events are recorded. Phosphate precipitation appears to have been an early diagenetic event, and apparently occurs throughout the sea level cycle, notable during transgression and throughout the highstand. The preferential phosphate replacement of echinoderm fragments instead of bryozoans and brachiopods suggests a relationship between the Mg-content of calcite and phosphatization.

AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62^nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013