47th Annual AAPG-SPE Eastern Section Joint Meeting

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Regional Distribution of Total Organic Carbon and Geochemical Characterization of the Maquoketa Shale Group within the Illinois Basin


Stratigraphy of the Late Ordovician Maquoketa Shale Group in the Illinois Basin is complex and includes numerous reactivation surfaces and disconformities. In general, there are three intervals that can be correlated across the basin. These horizons correspond to the Brainard Shale (a dolomitic, locally organic-rich shale), the Fort Atkinson Limestone (a fossiliferous packstone and grainstone limestone) and the Scales Formation (a thinly interbedded limestone and shale interval). Geochemical sampling of the Maquoketa Shale is limited to regionally restricted areas in the central and southeastern part of the Illinois Basin. As a consequence, the total organic carbon (TOC) distribution is unknown in most parts of the basin. Much of the sampling is limited to upper horizons within the Maquoketa Shale Group. Petrophysical log data is used to calculate TOC from density, neutron, and sonic logs. Research involving the Maquoketa Shale indicates that TOC estimates from petrophysical data correlate to core/cuttings TOC measurements within 0.5 -0.75 wt.% TOC. Using well logs, the TOC distribution of each of the three horizons in the Maquoketa Shale Group were mapped individually to assess whether there is a pattern in the vertical distribution of TOC within the group. Rock extracts from the Maquoketa Shale were analyzed to characterize hydrocarbons produced from this interval. Lithostratigraphy results indicate that the Brainard Shale has a maximum thickness of 150 to 160 ft. in north-central and thins to less than 40 ft. toward southern Illinois. In much of southeastern Illinois TOC content of Brainard Shale ranges between 1.0 to 1.2 wt.% percent, with higher values in the western parts of the basin. The Fort Atkinson Limestone contains TOC ranging between 0.4 to 1.2 wt.% percent. The lower Scales Shale Member has a maximum thickness of about 160 ft. in southeastern Illinois and contains TOC ranging from < 0.50 to 1.4 wt.% in western Indiana. Rock extracts indicate a low odd-carbon predominance in mid-range n-alkanes and a distribution of aryl isoprenoids that range from C13 to C24 (m/z 133) in the Upper Maquoketa Shale Group. Extracts taken from the lower shale interval contain geochemical markers consistent with a source rock rich in G.prisca, an organic-walled microfossil indicating that the Maquoketa Shale contains diverse organofacies.