--> Sedimentologic and Diagenetic Analysis of Source and Reservoir Facies Within the Phosphoria and Park City Formations, Northern and Central Rocky Mountains

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Sedimentologic and Diagenetic Analysis of Source and Reservoir Facies Within the Phosphoria and Park City Formations, Northern and Central Rocky Mountains

Abstract

The Permian Phosphoria Formation and time-equivalent strata in parts of Montana, Wyoming, Idaho, and Utah are the likely source of high API, sulfur-rich petroleum produced widely across western and central Wyoming and much of the estimated 16 billion Bbl of in-place hydrocarbons in the Tar Sands Triangle of SE Utah. In order to better document regional variations and controls on facies and the diagenetic/paragenetic history of potential source rock and reservoir rock facies within the Phosphoria, this ongoing study involves detailed sedimentologic and diagenetic analysis of Phosphoria-equivalent rocks across the northern and central Rocky Mountain region. Over 25 field locations were examined and sampled across southern Montana, eastern Idaho, Wyoming, and northern Utah, with an emphasis on the Meade Peak and Retort Members of the Phosphoria and interstratified Franson and Ervay Members of the Park City Formation. Three organic-rich facies from the Meade Peak and Retort are differentiated: 1) massive to thinly-bedded organic-rich silty dolomudstone; 2) bioturbated organic-rich silty dolomudstone; and 3) weakly-laminated moderately-organic phosphatic dolomudstone. Spiculitic chert and interbeds of peloidal phosphorite packstone and grainstone as well as phosphatic hardgrounds and isolated nodules are commonly associated with the mudstone facies. Collectively, these facies represent deposition within a partially restricted basin that was at times both anoxic and euxinic and characterized by episodic current activity, as indicated by hardgrounds, phosphatic ooids, and small scale scours. Commonly, the base of the Meade Peak is marked by a quartzose phosphorite facies, reflecting partial reworking of underlying Pennsylvanian quartz arenites. In SW Montana and NW Wyoming, the Shedhorn Sandstone Member occurs as a shallow marine siliciclastic sandstone facies reflecting offshore dispersal of sandy detritus from exposed portions of central Montana. Much of Wyoming and northeastern Utah comprised a shallow marine shelf characterized by several penecontemporaneous sub-basins in which more argillaceous facies accumulated and several bathymetric highs dominated by shallow marine carbonate deposition. Normal marine carbonate packstone and wackestone facies in the Franson and Ervay Members pass eastward into steinkern-bearing, bioturbated phosphatic dolowackestone and dolopackstone facies, dolomite with shelly fossil molds, and dolomite with evaporite molds.