"Ordovician-type" oils are found throughout the Mid-Continent and are characterized by strong odd-carbon predominance in the n-C₁₁ to n-C₁₉ alkanes, and relatively small amounts of branched and cyclic, and higher molecular weight normal (> n-C₁₉) alkanes. Detailed organic geochemical comparisons of these oils with extracts of potential source rocks show that in the Forest City basin of northeastern Kansas and southeastern Nebraska, oil source rocks are Middle Ordovician shales of the Simpson Group. For the Keota Dome field, Washington County, Iowa, the oil source rock is the Middle Ordovician Glenwood Shale Member of the Platteville Formation.

Analyses of saturated and aromatic hydrocarbon fractions of "Ordovician-type" oils from the Forest City basin, Keota Dome field, and the Michigan basin show that ^dgr¹³C of the two fractions are similar and that ^dgr¹³ varies over a considerable range, from -32.5 per mil to -25.5 per mil (PDB). This large range in ^dgr¹³C reflects a major shift in the carbon isotope composition of organic matter during the Middle Ordovician. This shift is shown in a 62.5-ft (19 m) interval of core from the Decorah and Platteville Formations in the E. M. Greene 1 well in Washington County, Iowa, where organic carbon ^dgr¹³C changes regularly upward from -32.2 per mil to -22.7 per mil (PDB). The change in organic carbon ^dgr¹³C in this core is not r lated to variations in amount (0.13-41.4% TOC) or type (hydrogen index = 69 to 1,000 mg HC/g TOC) of the marginally mature (T_max = 440 ± 5°C) organic matter. "Ordovician-type" oils in both the Forest City and Michigan basins show variable ^dgr¹³C, suggesting that the ^dgr¹³C shift displayed in the Middle Ordovician rocks of southeastern Iowa is a regional and possibly a global effect, related to changes in the ^dgr¹³C of the ocean-atmosphere carbon reservoir. Isotopic analyses of coexisting carbonate minerals support this interpretation.

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