Organic geochemistry and carbon isotopic compositions of strata in the Upper Ordovician Maquoketa Group and oils in Ordovician reservoirs from the Illinois Basin

During Late Ordovician time a shallow, epicontinental sea covered the region of the Illinois basin. This intracratonic North American basin was positioned approximately 10 to 20 degrees south of the equator and was bordered to the east by the Taconic highlands, to the southwest by the Ozark uplift, and to the north by the Transcontinental Arch. The Upper Ordovician Maquoketa Group comprises a thick sequence of siliciclastic sediment that prograded westward from the eastern Taconic highlands of the Appalachian region across Indiana, Illinois, and Iowa. Organic matter in the Maquoketa Group records a shallow marine ecosystem in existence prior to establishment of a diverse terrestrial flora.; Geochemical data were collected for 10 cores of the Maquoketa Group from locations in Indiana, Illinois, and Iowa. Stratigraphic variability of isotopic and geochemical data are described in detail for shales from three cores located along a 400 km NW-SE transect in the northern Illinois basin. The three cores are from the Everhart 1 Swartz of Owen County, Indiana; the New Jersey Zinc J-9 Parrish of Fulton County, Illinois; and the Cominco SS-4A Saunders of Jackson County, Iowa. Carbon isotopic analyses, elemental analyses (C, H), and Rock-Eval pyrolysis data were evaluated for initial assessment of source-rock quality, thermal maturity, and depositional environment.; Gas chromatography and gas chromatography-mass spectrometry were used to characterize the molecular compositions of bitumens from shale and to classify oils from Ordovician reservoirs. The origin of these oils is defined by carbon isotopic analysis of individual n-alkanes and by distributions of aromatic hydrocarbons.; Pyrolysis-gas chromatography of bulk organic matter combined with compound-specific isotopic analysis of bitumen revealed complex mixtures of organic-matter sources in Ordovician strata and enabled assessment of paleoenvironmental conditions during accumulation of the Maquoketa. In particular, significant enrichment of {dollar}sp{lcub}13{rcub}{dollar}C in aryl isoprenoids and the presence of tetramethylbenzene in pyrolytic products of Maquoketa kerogen indicate a contribution of organic matter from green sulfur bacteria. The presence of anoxygenic, photosynthetic, green sulfur bacteria suggests that anoxic conditions extended into the photic zone during accumulation of Maquoketa strata.