A Petrographic Analysis of the Relationship between Porosity and Organic Matter in the Permian Phosphoria Formation of Northeastern Utah

The Permian Phosphoria Formation is a reservoir for oil and gas in the western United States, as well as a major source of phosphate. This study examined the relationship between phosphate richness and porosity exhibited in the formation. Petrographic analysis was carried out on rock samples collected from the Phosphoria (Park City) Formation located north of Vernal, Utah, on the southern flank of the Uinta Mountains. The analysis demonstrated an inverse relationship between organic richness and porosity in the Phosphoria Formation. Porosity is controlled by lithology, amount of cementation, weathering, and amount of fecal pellets, which are the source of phosphate within the unit. Fecal pellets in the Phosphoria Formation contain and concentrate phosphate. Porosity is highest in strata with low levels of organic matter, near the middle of the geological unit, with increased organic matter near the upper and lower contacts, yet the relationship between porosity and organic matter is not necessarily a linear one, as other factors likely control the amount of porosity observed in the unit. Contributing factors include acidic ground waters, diagentic history, and the amount of calcite cement. A clear understanding of what controls porosity within the Phosphoria Formation has important implications for evaluating the unit as it is an important reservoir unit, as well as assessing phosphate richness for mineral extraction in the Western United States. Introduction As an important source for phosphate, a century of research has gone into the study of the Phosphoria Formation (Cheney, 1957 & Maughan, 1979). The formation is Permian in age and varies in thickness from 0 meters up to 67 meters (Maughan, 1979).