Twenty-three oils produced from reservoirs within the Ordovician Galena Group (Trenton equivalent) and one oil from the Mississippian Ste. Genevieve Limestone in the Illinois and Indiana portions of the Illinois basin are characterized. Two end-member oil groups (1 and 2) and one intermediate group (1A) are identified using conventional carbon isotopic analysis of whole and fractionated oils, gas chromatography (GC) of saturated hydrocarbon fractions, isotope-ratio-monitoring gas chromatography/mass spectrometry (irm-GC/MS) of n-alkanes ranging from C15 to C25, and gas chromatography/mass spectrometry (GC/MS) of the aromatic hydrocarbon fractions.
Group 1 is characterized by high odd-carbon predominance in mid-chain n-alkanes (C15-C19), low abundance of C20+ n-alkanes, and an absence of pristane and phytane. Group 1A is characterized by slightly lower odd-carbon predominance of mid-chain n-alkanes, greater abundance of C20+ n-alkanes compared to group 1, and no pristane and phytane. The carbon isotopic composition of n-alkanes and the distribution of n-alkylarenes for groups 1 and 1A indicate that these oils were generated from strata containing abundant Gloeocapsomorpha prisca. In contrast, group 2 is characterized by low odd-carbon predominance in mid-chain n-alkanes, moderate abundance of C20+ n-alkanes, and the presence of pristane and phytane. The Mississippian-reservoired oil is characterized by low odd-carbon predominance in mid-chain n-alkanes and greater abundances of C20+ n-alkanes, pristane, and phytane compared to group 2. Distributions of isoprenoidal alkylarenes (aryl isoprenoids) for group 2 and the Mississippian-reservoired oil are similar, suggesting that aryl isoprenoids fail to provide a diagnostic characteristic that can be used to precisely correlate oils and source rocks.
Conventional correlations of oil to source rock based on carbon isotopic–type curves and hopane (m/z 191) and sterane (m/z 217) distributions are of limited use in distinguishing Ordovician-reservoired oil groups and determining their origin. Oil to source rock correlations using the distribution and carbon isotopic composition of n-alkanes and the m/z 133 chromatograms of n-alkylarenes show that groups 1 and 1A originated from strata of the Upper Ordovician Galena Group. Group 2 either originated solely from the Upper Ordovician Maquoketa Group or from a mixture of oils generated from the Maquoketa Group and the Galena Group. The Mississippian-reservoired oil most likely originated from the Devonian New Albany Group. The use of GC, irm-GC/MS, and GC/MS illustrates the value of integrated molecular and isotopic approaches for correlating oil groups with source rocks.