Petroleum Geochemistry of the Upper Ordovician Maquoketa Group Source Rocks, Illinois Basin, USA
The Illinois Basin covers ~60,000 mi2 predominantly in Illinois and extending into western Indiana and northwestern Kentucky. Four petroleum systems have been proposed: the Precambrian to Cambrian; the Ordovician; the Devonian-Mississippian New Albany; and Pennsylvanian Coals. Previous studies have identified three oil families in the basin, with most oil having been generated from the New Albany, but some oils derived from the Ordovician Galena and Maquoketa. Ordovician units in the Michigan Basin (Utica/Collingwood) and western Appalachian Basin (Utica/Point Pleasant), have been identified as good petroleum source units. The Point Pleasant has also been drilled as an unconventional target. With current burial depths between 5000–6500’ in the southern part of the Illinois Basin and modeled paleodepths more than 10,000’, the Maquoketa Shale may be a potential unconventional resource play if organic-rich. This study examines the Maquoketa in 45 wells for organic richness, kerogen type, and thermal maturity. In general, the Maquoketa has poor to fair present-day total organic content (TOC) over much of the basin based upon analysis of available cuttings. Measurements on core sample yield higher TOC on average. Where they can be identified, the Brainard, Fort Atkinson and Scales members have similar organic richness. In some wells however, the Scales has two to three times higher TOC. There are intervals within several wells with measured TOC >1 wt.%, and some >3 wt.%. Original TOC was substantially higher and is sufficient to have generated the Ordovician petroleum discovered in the basin. Preliminary correlations suggest that these intervals are thin and do not appear to be continuous between wells. Thermal maturity tends to follow the depth contours of the basin. The New Albany reaches oil generation maturity only in the southern part of the Illinois Basin. The Maquoketa has passed through the oil window over a larger area and in some regions has reached the gas generation window. While care must be taken for source rock pyrolysis data determined from old and dried cuttings samples, based upon the low volatile hydrocarbon content most oil generated in the Maquoketa has likely been expelled. The low residual hydrocarbon content, restricted area of adequate thermal maturity and intermittent organic richness indicates that the Maquoketa has a significantly elevated geochemical risk for unconventional petroleum production.
AAPG Datapages/Search and Discovery Article #90373 © 2019 AAPG Eastern Section Meeting, Energy from the Heartland, Columbus, Ohio, October 12-16, 2019