Sequence Stratigraphic Framework for the Upper Devonian Ohio Shale in the Appalachian Basin

Abstract

The Upper Devonian Ohio Shale is a prominent marine stratigraphic interval in the northern Appalachian Basin. While extensive subsurface work has been focused on older Devonian shale units within the basin (i.e. Marcellus Shale), the sequence stratigraphic framework of the Ohio Shale interval has been little explored. The focus of this study is to correlate observed cyclicity from the distal intervals of the Ohio Shale in central Ohio with coeval deposits to the east in West Virginia and Pennsylvania, and to establish a sequence stratigraphic model to explain the distribution of organic-rich shales within the basin. As a framework for regional correlations, the highest high-gamma-ray signatures from well logs within the Ohio Shale interval are interpreted as flooding surfaces that can be identified in both distal and medial parts of the basin. Observed cycles of increasing/decreasing gamma-ray values, including cycle coarsening and evidence for unconformity development in shallow shelf deposits to the east, are interpreted to represent changes in relative sea-level during the time of deposition. Initial analysis of core data from Noble County, Ohio, supports observed cyclicity within gamma-ray signature based on changes in lithological character and contact identification. In addition, preliminary analysis of total organic carbon content within the Ohio Shale interval indicates that sediment supply had a primary control on overall organic richness. The overall organic content in these rocks is related to decreased rates of detrital clay deposition during rising sea-level stages. Conversely, low gamma ray values likely represent increases in sediment supply during sea-level lowstands. Further, thermal maturation based on adjusted vitrinite reflectance shows the Ohio Shale to be within the oil window across eastern Ohio. Collectively these relationships allow for sequence stratigraphic interpretations to be derived from the lithologically monotonous Ohio Shale interval that can be correlated with coeval shallow marine/nonmarine stratigraphic units to the east.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018