Stratigraphic Insights into Late Devonian Black Shales of the Illinois and Appalachian Basins: Outcrop to Subsurface Examples

Ovidiu Remus Lazar and Juergen Schieber
Indiana University, Bloomington, IN

Mainly Late Devonian and generally composed of laminated, banded, or bioturbated black shales, and gray bioturbated shales, the New Albany and the Ohio Shales represent major sources and reservoirs of hydrocarbons in the Illinois and Appalachian Basins, respectively. Significant vertical and lateral variability of both shale successions has impinged on stratigraphic correlations within and between the Illinois and Appalachian Basins. Outcrops, drill cores, and gamma-ray logs from both basins were examined to establish a unitary stratigraphic framework for both shale successions.

Examination of roadside exposures located tens of kilometers apart in the Illinois Basin, the Kentucky portion of the Appalachian Basin, and on the Cincinnati Arch between the two basins enabled the recognition of cm-thick lag deposits and sharp contacts that can be related to five laterally extensive erosion surfaces and facilitate the separation of major subdivisions within the New Albany and the Ohio Shales. Erosion surfaces and subdivisions identified in outcrops were next compared and correlated to those recognized in cored intervals from wells drilled in the Illinois Basin. Lastly, erosion surfaces and corresponding shale packages identified in outcrops and drill cores were traced on gamma-ray logs measured in wells drilled through the entire Illinois Basin and the Kentucky portion of the Appalachian Basin. Erosion surfaces traced throughout both basins are sequence boundaries. These boundaries allow for a sequence stratigraphic reassessment of the New Albany and the Ohio Shales, reassessment which enables now a re-evaluation of the mechanisms that led to the deposition of these shale successions.