ANALYSIS OF THE URANIUM/TOTAL ORGANIC CARBON (TOC) RELATIONSHIP IN THE ORGANIC-RICH OHIO SHALE OF EASTERN KENTUCKY

The gamma-ray tool is used to infer the organic carbon content in shales due to the interaction between uranium (U) and organic carbon. However, the Cleveland Member of the Ohio Shale Group of eastern Kentucky within the Appalachian Basin displays a moderately low gamma-ray response relative to the Lower Huron member of the same group, despite both having similar total organic carbon (TOC) content. There is a strong correlation for many black shales between U and TOC due to similar depositional and geochemical processes, however that correlation does not seem to be present within the Cleveland Shale interval. To investigate this conundrum, we will use trace metal geochemistry of U and supporting molybdenum (Mo) in relation to TOC in order to determine the degree of restriction and oxidation at the time of deposition. We hypothesize that this discrepancy between U concentration and organic carbon accumulation is a result of the depletion in trace metals, especially U, as a result of re-oxidation due to a shifting pycnocline on the basin margin. In this proposed study, U and TOC inventory of the Cleveland Shale and Lower Huron members of the Devonian Ohio Shale Group of Eastern Kentucky will be evaluated to determine the depositional environment and reason for depletion of U.

The U/TOC ratio in organic-rich shales is influenced by many factors such as the primary uranium content of the water body and the sedimentation rate at the time of deposition. The Devonian black shales of North America provide a remarkable study area for this investigation. Results from this project will give insight into depositional conditions as well as processes that occurred post-deposition on the southern rim of the Appalachian Basin. Additionally, this study has implications for predictions of the extent of source rocks in the subsurface when gamma-logs do not detect radioactivity. Though this investigation is primarily focused on the Devonian Ohio Shale Group, the methods and ideas discussed throughout this project can be applied to shales of different age and locality.