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Application of Rhenium-Osmium Geochronology to Phosphoria Oils, Wyoming

Paul G. Lillis1, David Selby2, and Michael D. Lewan1
1U.S. Geological Survey, Denver, CO
2University of Durham, Durham, United Kingdom

The Permian Phosphoria Formation (eastern Idaho / western Wyoming) is estimated to have generated petroleum between the Late Triassic and Early Cretaceous. In this study, we applied the rhenium-osmium (Re-Os) geochronometer to oils considered to be derived from the Phosphoria Formation to constrain the timing of petroleum generation. Results show that the majority of the Phosphoria oils yield a Permo-Triassic age (255±73 Ma), but with considerable scatter about the regression (mean square weighted deviate, MSWD = 696). For isotopic data to yield a linear array, all samples must have formed at the same time and possess the same initial 187Os/188Os composition. The scatter may reflect different periods of oil generation or variations in the initial 187Os/188Os composition of the source rock. A subset of oils with similar initial 187Os/188Os values (0.37 to 0.54) yield a Middle Triassic Re-Os age (244±20 Ma). This older age may reflect early oil generation from Type II-S kerogen, or may record the timing of bitumen generation, which precedes oil generation. In contrast, Re-Os data for samples of Phosphoria oils from Torchlight field, Big Horn Basin, yield a Miocene age (9.30 ± 0.67 Ma, MSWD = 0.65). This young age supports studies that suggest that Phosphoria oil may be locally derived at younger times in parts of the basin. Alternatively, the young Re-Os age may reflect the timing of alteration by thermochemical sulfate reduction, which has been proposed to affect the Torchlight and nearby fields. Although these preliminary results are promising, research is continuing to fully understand the Re-Os isotope systematics of petroleum generation.

AAPG Search and Discovery Article #90092©2009 AAPG Rocky Mountain Section, July 9-11, 2008, Denver, Colorado