ABSTRACT: Geochemistry of Oils and Source Rocks from the Ordovician (Champlainian) Galena Group (Trenton) in Illinois

SALMON, G. L., and J. B. RISATTI, Illinois State Geological Survey, Champaign, IL, and J. R. HATCH, U.S. Geological Survey, Denver, CO

Oils produced from reservoirs in the Champlainian Galena Group in Illinois and from the Champlainian Platteville Formation in the Keota Dome field, Washington County, Iowa, are members of a geochemically defined family of oils that occurs worldwide. In North America these oils are found in lower Paleozoic reservoirs in most sedimentary basins east of the Rocky Mountains and north of the Gulf Coast. They are characterized by relatively abundant n-alkanes that show a strong predominance of the odd-carbon-numbered n-alkanes between n-C[10] and n-C[20]. Additionally, the reduced amounts of n-alkanes between n-C[20] and n-C[40] exhibit no odd-even predominance and relatively small amounts (6%) of branched and cyclic alkanes are present. Organic-matter-rich intervals within the Champlainian section are identified as likely source rocks for these oils based primarily on similar n-alkane, saturate and aromatic biomarker distributions, and carbon isotopic compositions of saturated and aromatic hydrocarbon fractions of oil and rock extracts. Compositional variations in oils from the Galena Group and Keota Dome field are related to apparent differences in thermal maturity and to organic-matter isotope {13}C variations in the Champlainian source rocks. With increased thermal maturity, both the carbon preference index of the n-C[10] to n-C20 n-alkanes and the vanadium contents decrease, while the tri-aromatic cracking ratio, the C[20] aromatic sterane ratio, and the oil gravity ( degrees API) increase. This statistically significant correlation between C[20] aromatic sterane ratio and depth for oils is not influenced by the Duquoin monocline. This suggests limited migration distances and/or additional thermal maturation of the oils within the reservoirs.

AAPG Search and Discovery Article #91013©1992 AAPG Eastern Section Meeting, Champaign, Illinois, September 20-22, 1992 (2009)