Oils generated by Middle Ordovician rocks are found throughout the Mid-Continent and east-central regions of the United States. Gas chromatographic characteristics of these oils include a relatively high abundance of n-alkanes with carbon numbers less than 20, a strong predominance of odd-numbered n-alkanes between C₁₀ and C₂₀, and relatively small amounts of branched and cyclic alkanes. Saturated and aromatic hydrocarbon fractions of 43 Ordovician oils from the Anadarko, Ardmore, Forest City, Illinois, Michigan, Salina-Sedgwick, and Williston basins and the Iowa shelf demonstrate a wide range in carbon isotope composition (^dgr¹³C_sat = -24.9 ^pmil to -33.9 ^pmil, (^dgr¹³C_arom = -24.3 ^pmil to -33.7 ^pmil). Saturated and aromatic hydrocarbons extracted from late Middle Ordovician shales (17 core samples) show ranges in ^dgr¹³C similar to that of the oils.

The wide ranges in ^dgr¹³C for oils and rock extracts reflect a major, positive excursion(s) (6-9 per ^pmil) in organic matter ^dgr¹³C in late Middle Ordovician rocks. This excursion has at least a regional significance in that it can be documented in sections 480 mi (770 km) apart in south-central Kansas and eastern Iowa. The distance may be as much as 930 mi (1,500 km) if the carbon isotope variations observed in Michigan basin Ordovician oils and in organic matter from late Middle Ordovician rocks in southwestern Ontario are related to the same carbon isotope excursion. Organic-matter ^dgr¹³C in core samples from south-central Kansas and eastern Iowa is not directly related to variations in quantity or quality of organic matter, or maceral compositi n. The positive excursion in organic matter ^dgr¹³C is a possible result of increased organic matter productivity and/or preservation.

The parallel shifts in organic and carbonate ^dgr¹³C in core samples from 1 E. M. Greene well, Washington County, Iowa, imply changes in the isotope composition of the ocean-atmosphere carbon reservoir. Differences in the magnitude of the carbon isotope shifts between organic matter (8.8 ^pmil) and carbonate (4.2 ^pmil) in this core suggest a decrease, either locally or regionally, in available dissolved CO₂, possibly a result of high organic-matter productivity and/or limited circulation in the late Middle Ordovician seas.