The Permian Phosphoria Formation: Stratigraphy, Paleo-Environments, and Petroleum Potential

Hendrix, Marc S.; Hofmann, Michael

The Permian Phosphoria Formation has long been regarded as a major source of hydrocarbons in the northern Rocky Mountains, yet little recent information regarding the detailed sedimentology, stratigraphy, and geochemistry has been reported from the formation. Given the Phosphoria's great potential as a resource play, it is therefore critical that the formation's sedimentology, ichnofacies, mineralogy and geochemistry be investigated in order to evaluate the hydrocarbon storage potential and geomechanical properties of the formation. The ultimate goal of our work is to better understand the entire Phosphoria petroleum system through the establishment of a basin-wide sedimentary database for the Phosphoria formation and time-equivalent strata in the northern Rocky Mountains.

Here, we report results from regional stratigraphic relations, detailed outcrop-scale sedimentology, and fabric analysis of polished rock slabs and thin-sections along an east-west transect located between northwestern Colorado and north central Utah and trending roughly perpendicular to depositional strike. Our results suggest that the Phosphoria Formation was deposited across a westward deepening ramp connecting a hypersaline shelf on the east with an anoxic baisn to the west. Shelf environments range from sub- through supratidal and include preservation of abundant evaporate molds and a rich assemblage of trace and body fossils. Where present, the Meade Peak organic rich facies range from grain supported phosphorites in the east with TOC% generally <1% to phosphatic dolomite further west with TOC% ranging up to several percent. Abundant small-scale scours and low angle laminae in the western facies are consistent with bottom current activity there. Phosphatic, fossiliferous dolomite of the Franson Member of the Park City Formation gradationally overlies the Meade Peak. Widespread reflux by shelf brines completely dolomitized the Franson Member there, while exposure to meteoric water resulted in nearly complete dissolution of all shelly debris and development of extensive secondary moldic porosity. Further west, equivalent carbonate facies within the Franson are tight and not dolomitized, suggesting the deeper water there prevented reflux dolomitization and formation of widespread moldic porosity through exposure to meteoric water.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013