ABSTRACT: Organic Facies and Petroleum Source Potential of Upper Ordovician Shale in the Illinois Basin

GUTHRIE, JOHN M., and LISA M. PRATT, Indiana University, Bloomington, IN

Regional variations in organic and stable isotopic geochemistry have been documented through the Upper Ordovician Maquoketa Group, Illinois basin. Maquoketa shale occurs about 80 m above the Gloeocapsomorpha prisca zone of the Middle Ordovician Guttenberg Formation in Iowa. In contrast to G. prisca-rich shale, Maquoketa shale extracts have a slight odd-carbon predominance in C15-C19 n-alkanes, substantial quantities of C20+ n-alkanes, and moderate to high abundance of pristane and phytane relative to n-C17 and n-C18.

Based on approximately 200 core samples, organic carbon contents (Corg) in the Maquoketa Group range from 0.1 to 8.7%, with an average value of 1.0%. Samples with Corg less than about 1.0% occur in gray shales. Samples with greater than about 2.0% Corg occur in dark gray to brown, laminated, shales of the Brainard and Scales formations in western Illinois and eastern Iowa.

Values of isotope {13}C for bulk Corg vary between -28.1 to -30.7 o/oo . Samples with low Corg have isotope {13}C values in the range of -28 to -29.5 o/oo and samples with high Corg values have lighter isotope {13}C values (-29.5 to -30.7 o/oo ). Isotopic analyses of individual n-alkanes yield isotope {13}C values that are close to isotope {13}C of bulk Corg for both Maquoketa and G. prisca-rich shale.

Visual kerogen analyses, gas chromatography, and isotope {13}C compositions of bulk organic carbon and individual n-alkanes show no vestige of G. prisca in either the organic poor or organic rich shales of the Maquoketa Group. High Corg and high extractability of laminated Maquoketa shale indicate good petroleum source potential for Upper Ordovician strata in the western and northwestern portion of the Illinois basin.

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