Lazar, Ovidiu Remus1, Juergen Schieber1
(1) Indiana University, Bloomington, IN
ABSTRACT: Gaining New Insights into the Stratigraphy of the New Albany Shale of the Illinois Basin through an Integrated Study of Well Cores and Gamma-Ray Logs
The Middle Devonian to Early Mississippian New Albany Shale represents a major source and reservoir of hydrocarbons in the Illinois Basin. Thirteen members and five formal beds have been defined based on outcrop studies in various parts of the basin. On geophysical logs, however, these subdivisions are notoriously difficult to correlate over large distances. A detailed analysis of the shale stratigraphic variability was performed on east-west, north-south, and northeast-southwest cross sections, using well cores and gamma-ray logs. This allowed us to identify subdivisions that can be traced throughout the basin and can be related to sequence-bounding erosion surfaces recognized in shallower portions of the basin. Millimeter-scale investigations of well cores revealed an abundance of erosion-related features in this shale succession, such as silty, sandy, pyritic, and shelly lags, bone beds, and sharp-based black shale contacts. UV imaging of cores revealed distinct maxima of Tasmanites cysts that reflect reworking during sea level drop as well as sediment starvation during maximum flooding. Thicker lags were interpreted to be the result of basinwide erosion, and in combination with UV maxima due to maximum flooding, allowed the recognition of 12 major subdivisions within the shale succession. Shale packages that are recognized in cores typically can be traced from well to well with gamma-ray logs, once allowance is made for partial erosion at the top of individual packages. Continuation of this study will allow integration of the New Albany Shale into a well-constrained sequence stratigraphic framework for the Devonian black shales of the eastern U.S.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.