Abstract: Unexpected Results from a Geochemical Approach to Typing and Correlation of Ordovician Oils in the Illinois Basin

John M. Guthrie

A set of oil samples produced from the Ordovician Trenton Limestone in the Illinois and Indiana parts of the Illinois basin have been characterized geochemically. Two oil types are identified using conventional carbon isotopic analysis of the whole and fractionated oil, gas chromatography (GC) of the saturated hydrocarbon fraction, isotope ratio monitoring, gas chromatography, and mass spectrometry (IRM-GC-MS) of n-alkanes ranging from C₁₅ to C₂₅, and gas chromatography-mass spectrometry (GC-MS) of the aromatic hydrocarbon fraction.

Group 1 oils are characterized by strong odd-number carbon predominance in mid-length n-alkanes (C₁₅ to C₁₉), low amounts of C₂₀ + n-alkanes, and absence of pristane and phytane. The isotopic composition of n-alkanes and the m/z 133 chromatograms of n-alkyl arenes found in Group 1 oils indicate that these oils were sourced from Ordovician strata containing abundant Gloeocapsomorpha prisca. In contrast, Group 2 oils are characterized by weak odd-number carbon predominance in mid-length n-alkanes, moderate amounts of C₂₀ + n-alkanes, and minor pristane and phytane. The isotopic composition of n-alkanes and the m/z 133 chromatograms of isoprenoidal alkyl arenes (aryl isoprenoids) indicate a mixture of sources for Group 2 oils.

Conventional oil to source rock correlations using stable carbon isotope type curves and hopane (m/z 191) and sterane (m/z 217) distributions are of limited usefulness for distinguishing between Ordovician oils and for determining their sources. The use of GC, IRM-GC-MS, and GC-MS illustrates the power of using integrated molecular and isotopic approaches for correlating oils with source rocks.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994