2014 Rocky Mountain Section AAPG Annual Meeting

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Petroleum Occurrences in Cretaceous Age Reservoirs, Northern Bighorn Basin, Montana & Wyoming


Oil and natural gas have been produced from Cretaceous age sandstone and conglomerate reservoirs in more than a dozen fields in the northern Bighorn Basin. Depths of production range from 1,000 feet to 14,500 feet. Lower Cretaceous reservoirs have cumulative oil and gas production of 6 MMBO and 56 BCFG. Upper Cretaceous reservoirs have produced 20 MMBO and 110 BCFG. All of the fields are on structural closures. Production is primarily from three stratigraphic intervals. They are in (A) the Pryor Conglomerate interval of the Cloverly Fm., (B) the Greybull Sandstone interval of the Fall River Fm., and (C) the Frontier Sandstone. The Pryor and Greybull reservoirs are Lower Cretaceous age while the Frontier is Upper Cretaceous. The Pryor is comprised primarily of fluvial deposits. Reservoir sandstones in the lower portion of the Greybull are of fluvial origin while those in the upper portion are thought to be estuarine to shallow marine. The Frontier sandstones are marine deposits. Oils produced from the Lower Cretaceous intervals are in two distinct geochemical groups. On shallow structures along the flanks of the Pryor and Beartooth Mountains the oils are 10 to 22° API gravity and are biodegraded. Geochemical analysis of the low gravity oils indicates they came from a marine carbonate source likely contained in the Permian age Phosphoria Fm. The oil traps containing this biodegraded low gravity oil may have had more than three billion barrels of original oil in place. The Mosser Dome field area in Yellowstone County, Montana houses a giant stratigraphic trap in the Greybull Sandstone once containing as much as 2 billion barrels of Phosphoria sourced original oil in place. Deeper in the basin the oils from Cretaceous rocks are 44–60° API gravity condensates. Gas chromatograph analyses of these oils suggest two separate oil families. The higher gravity condensates from the Lower Cretaceous age Pryor and Greybull are similar to each other and likely had the same source. They are quite different from the distinctive 44° API gravity oil produced from the Upper Cretaceous Frontier Fm. Source rock analyses of eighty-three Lower Cretaceous age core samples from fifteen wells in southern Montana show TOC values as high as 4.56% with all samples being in the early to middle oil generation window. The samples from the basal portion of the Thermopolis Fm. had the highest TOC values. The Thermopolis is more than 350 feet thick. Significant hydrocarbon shows were seen on mud logs and drill stem tests from this black shale interval in Carbon County, Montana. It is the most likely source for the condensates produced from the Pryor and Greybull reservoirs. The oil produced from the Frontier Sandstone is assumed to have been sourced in the underlying Mowry Fm.