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The Case for Another Look at the Paleocene Fort Union Formation in the Eastern Greater Green River Basin, Wyoming


The Paleocene Fort Union Formation in the eastern Greater Green River Basin is a thick succession of shale, sandstone, coal, and siltstone, deposited as syn-orogenic Laramide basin fill. Recent production from the Washakie Basin has demonstrated the viability of the Fort Union Formation as a productive gas reservoir, especially with improved horizontal drilling technology. This begs the question: are there other potentially analogous Fort Union reservoirs that have been overlooked elsewhere in the eastern Greater Green River Basin? In the case of the Washakie Basin, wet gas is produced from the China Butte Member of the Fort Union Formation. This basal member has numerous coal seams interbedded with lenticular sandstones. Gas is believed to be derived in situ, as well as from the deeper Cretaceous-age formations. Production is from approximately 3,048 m (10,000 ft) TVD. Burial history curve analyses and vitrinite reflectance extrapolation suggests 975 m (3,200 ft) of Neogene erosion, reflecting condensate generation at less than 4,023 m (13,200 ft) burial depth (geothermal gradients in this region are not elevated). Regional correlations of the China Butte Member show the succession of coals thickens into the Great Divide Basin, where no Fort Union production is occurring and no drill stem tests are publicly available. Mud logs from wells drilled into the deeper Cretaceous formations show methane gas spikes associated with the China Butte Member, but this coal-rich interval is at maximum depths of approximately 914 to 1,829 m (3,000 to 6,000 ft) TVD. Extrapolation of vitrinite reflectance results suggests 1,676 to 2,103 m (5,500 to 6,900 ft) of Neogene erosion in the Great Divide Basin, placing the China Butte Member at maximum burial depths just shy of those required for in-situ condensate generation in the Washakie Basin. Furthermore, vitrinite reflectance measured from a handful of Fort Union Formation samples in the Great Divide Basin record values approximately 0.4 to 0.7% Ro, significantly less than the >1.2% values from the Washakie Basin. Preliminary data suggest that although Fort Union Formation coals may not have reached maximum burial depths sufficient for condensate generation in the Great Divide Basin, this coal-rich interval may be methane saturated, at least in places, and could be worth a second look.