Abstract: Abstract: Organic Geochemistry, Incipient Metamorphism, and Oil Generation in Black Shale Members of Permian Phosphoria Formation, Western Interior United States
George E. Claypool, Alonza H. Love, Edwin K. Maughan
Nearly the whole range of organic metamorphism is reflected in the composition of sedimentary organic matter in shale members of the Permian Phosphoria Formation of western Wyoming and adjacent states. The different degrees of thermal maturity are recognizable on the basis of the amount, composition, and molecular nature of the extractable organic matter, and the color and elemental composition of the kerogen.
Organic matter in black shale members of the Phosphoria exhibits maximum conversion to petroleum hydrocarbons at temperatures corresponding to a 2.5 to 4.5-km range of burial depth. At shallower depths, Phosphoria deposits are rich in extractable organic matter of asphaltic composition, and contain an immature hydrocarbon assemblage which is unlike mature petroleum. At greater depths there is extreme depletion of extractable organic matter and loss of hydrocarbons by thermal destruction and expulsion.
Oil which is believed to have been derived from the Phosphoria black shales also appears to be limited regionally to areas where the reservoir rocks were buried to depths of from 2.5 to 4.5 km at the end of the Cretaceous. This suggests that there is an effective "window" for Paleozoic oil in the Cordilleran region.
If all of the oil in Paleozoic reservoirs in central Wyoming were derived from the Phosphoria black shales, and if hydrocarbon generation and migration efficiencies were on the order of 10 to 20% each, then much of the oil must have been generated as far away as eastern Idaho and migrated prior to metamorphism, because of insufficient quantities of organic matter in the immediate area of the producing reservoirs.
AAPG Search and Discovery Article #90969©1977 AAPG-SEPM Rocky Mountain Sections Meeting, Denver, Colorado