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Department of Geology, The University of Texas at Arlington, Texas 76019 ([email protected])

Abstract: A Regional Study of Sequences in the New Albany Shale of the Southeastern Illinois Basin (Indiana) with Gamma Ray Logs and Well Cores

The New Albany Shale of southern Indiana is a middle to late Devonian Black Shale unit that constitutes an important hydrocarbon source rock in the Illinois Basin. Going from east to west, the New Albany Shale thickens and changes its lithologic characteristics. These changes reflect the gradual deepening from the shallow water regions on the Cincinnati Arch to the deeper water regions of the Illinois Basin.

In outcrop studies from Tennessee and central Kentucky, recognition of widespread erosion surfaces allowed a sequence stratigraphic subdivision of this black shale succession. Gamma Ray logs from southern Indiana show that these subdivisions can be carried into the subsurface west of the New Albany outcrop belt. Systematic tracing of these sequences through the Illinois basin may in the future allow substantial refinement in the understanding of the depositional history of these rocks.

The observed variability between adjacent gamma ray logs is attributed to the erosional truncation at the top of individual shale packages. Additional variability is introduced due to the fact that some shale packages that are present in western Indiana have been completely lost to erosion in eastern Indiana and Kentucky. The transgressive base of individual sequences typically coincides with an increase in gamma ray intensity.

Future study of these shales will be the basis of making a better connection between the conformable sequences of the Illinois Basin interior and the discordant sequences of the Cincinnati arch region.

AAPG Search and Discovery Article #90921©1999 AAPG Mid-Continent Section Meeting, Wichita, Kansas