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We performed organic geochemical analyses of Paleozoic shales and oils from the northern Denver basin to determine oil-source bed relationships. We recognized two general oil types: oil produced from reservoirs of Virgilian and Wolfcampian age in northeastern Colorado and Nebraska, and oil produced form the Lower Permian Lyons Sandstone near the basin axis in Colorado. Low-gravity oil (20° API) produced from the Virgilian-age reservoir at the Amazon field (Nebraska) and a higher gravity oil (37° API) produced from a well near the Amazon field (Wexpro 1-23 Lyngholm) can be distinguished geochemically from the other Virgilian-Wolfcampian oils studied and may be genetically unrelated to them. For comparison, we analyzed oils from the Minnelusa Formation (Permian-Pe nsylvanian) in the Powder River basin. These oils are geochemically unlike any Paleozoic oils analyzed in this study in southeastern Wyoming and Colorado.
We evaluated shale samples of Desmoinesian, Wolfcampian, and Leonardian age for source rock potential. Shales of Leonardian and Wolfcampian age in southwestern Nebraska contain as much as 6% organic carbon and 4,000-7,000 ppm extractable hydrocarbons. However, the Wolfcampian samples analyzed contain hydrocarbons that are enriched in carbon-13 by 3-5 per mil compared to nearby oil occurrences and are, therefore, improbable source rocks for current oil discoveries. Hydrocarbons extracted from one sample of Leonardian shale are isotopically similar to the Virgilian-Wolfcampian oils. Thermally mature shales of Desmoinesian age in northwestern Nebraska containing as much as 18% organic carbon and about 800-8,000 ppm hydrocarbons are considered excellent potential source rocks.
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