Two distinct Middle Ordovician oil-prone organic-matter assemblages occur in thermally immature to marginally mature petroleum source rocks from Iowa. The substantially more oil-prone Assemblage A generates the unique "Ordovician Oil" fingerprint, which has been associated with the organic-walled microfossil Gloeocapsamorpha prisca. The second, Assemblage B, generates a more ordinary signature. The two assemblages, which are mixed or interbedded in many Ordovician sediments of North America, explain the variations in oils derived from Ordovician source rocks. These mixtures also aid in interpretation of the chemistry of G. prisca.

Core samples examined are from three wells in Iowa penetrating the primarily carbonate sequence from the Guttenberg Member of the Decorah Formation down to the St. Peter Sandstone. Microscopy, Rock-Eval pyrolysis, and capillary gas chromatography on source rock bitumen extracts and nearby oils were used to evaluate the two assemblages. Solvent extracts from rocks containing predominantly Assemblage A have the diagnostic Ordovician gas chromatographic signatures with an odd preference in C₁₁-C₁₉ normal alkanes, low amounts of heavier normal alkanes, and virtual absence of isoprenoids including pristane and phytane. Although Assemblage A occurs only in very thin beds, it is potentially so oil generative that thicknesses in inches may be significant. Extracts from r cks containing predominantly Assemblage B show reduced odd preference, a full profile of normal alkanes, and conspicuous occurrence of isoprenoids including pristane and phytane.

These samples from Iowa are representative of Middle Ordovician source rocks found in most Paleozoic basins of North America, as well as Baltic Europe and Australia. Therefore, the conclusions drawn from these Middle Ordovician rock samples from Iowa provide a basis to explain variations in Ordovician-derived oils found in many other areas.