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AAPG Rocky Mountain Section Meeting

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Overview of Organic-Rich Facies of the Permian Phosphoria Formation, USA

Abstract

The Permian Phosphoria Formation of the U.S. Northern Rocky Mountain region is a major petroleum system that includes thermally-mature worldclass petroleum source rock facies, in addition to high quality conventional reservoir facies and a regionally-extensive overlying seal. Middle Permian organic-rich mudstone facies of the Phosphoria Formation occur in two separate depocenters, each named an organic-rich member of the Phosphoria. The Retort depocenter of southwestern Montana contains a phosphatic mudstone facies with up to 25% measured TOC. Retort mudstone decreases in %TOC and increases in %P away from the depocenter. At the depositional margins of the Retort depocenter, very well-sorted ore grade peloidal phosphorite is present. The Meade Peak depocenter in southeastern Idaho into northern Utah and parts of northeastern Nevada. In the Oquirrh basin of northwestern Utah Carboniferous and Permian strata are ~7.5 kilometers thick, and the Meade Peak Member is ~150 meters thick. The thick accumulations of Middle Permian organic-rich source rocks in northwestern Utah effectively expands the size of the 'classic Phosphoria basin' as described by the USGS and other publications, and it also greatly expands the size of the Phosphoria Total Petroleum System. For example, DeMaison (1977) suggested that Phosphoria mudrocks provided the source for the ~16 billion Bbl 'Tar Sands Triangle' of southeastern Utah. Most paleogeographic reconstructions interpret the Phosphoria Formation to have been separated from the open panthalassa Ocean by an archipelego of islands and larger landmasses that included the Golconda Allochthon, the Sonoma Arc, and possibly a second more outboard arc. Southwesterly connection of the Phosphoria basin to Panthallasa is consistent with regional stratigraphic information and would have introduced more nutrient-rich waters from the north equatorial gyre than a south-flowing current that entered the Phosphoria basin from the northwest.