Geochemical Indicators of Depositional Environment and Source-Rock Potential for the Upper Ordovician Maquoketa Group, Illinois Basin

Abstract

Two depositional cycles are recognized within the Upper Ordovician Maquoketa Group of the Illinois basin in a core from the New Jersey Zinc 1 Parrish well of Fulton County, Illinois. Organic carbon (Corg), total sulfur, 13C content of the organic carbon (^dgr13Corg ), hydrogen and oxygen indices (HI and OI) from Rock-Eval pyrolysis, and yields of extractable organic matter (EOM) vary through the cycles. Dark-brown to black, laminated shales are present in the lower portion of each cycle and have high values of Corg (1.0-3.0%), HI (500-1000 mg hydrocarbon [HC]/g total organic carbon[TOC]), and EOM (500-2500 ppm), and more negative ^dgr13Corg values (^dgr13Corg = -30 to -30.5^ mil). Gray to greenish-gray, bioturbated shales are present in the upper portion of each cycle and have low values of Corg (<1.0%), HI (<500 mg HC/g TOC), and EOM (<500 ppm), and more positive ^dgr13Corg values (-28.5 to -29.5^pmil) compared to the laminated shales. Although thermally immature to marginally mature in this core, the laminated shales represent potential source rocks for petroleum because they contain good to excellent quantities of oil-prone organic matter and are more deeply buried in other areas of the basin. Kerogen elemental data and Rock-Eval data suggest that the source of organic matter in the Maquoketa was uniform, with the notable exception of graptolite-rich layers. Distributions of saturated hydrocarbons for Maquoketa samples resemble those derived from amorphous organic matter. The organic-walled microfossil Gloeocapsomorpha prisca was not identified in kerogens of the Maquoketa shale, and its absence is most likely due to an unfavorable depositional environment and/or climate. Variations in bulk geochemical data and carbon isotopic compositions within the Maquoketa Group indicate substantial reworking and degradation of organic matter associated with bioturbation and oxygenated depositional conditions. Organic-rich shale was deposited in an environment where circulation of the water column was sluggish and dissolved-oxygen concentrations were low, resulting in the preservation of organic matter. Organic-poor shale was deposited in an environment where circulation of the water column was strong, dissolved-oxygen concentrations were high, and burrowing organisms thoroughly reworked the sediment, resulting in the degradation of organic matter. A maximum difference in carbon isotopic ratios of 2^pmil between organic-rich and organic-poor units of the Maquoketa roup suggests that heterotrophic reworking of organic matter was important in the depositional environment. The carbon isotopic composition and H/C ratio of kerogen for graptolite-rich shale suggests that organic productivity may have played an intermittent role in the accumulation of organic matter in the Maquoketa shale.