The Role of Organic Carbon and Phosphorus in the Fixation of Uranium in Mudrocks

Black shales are unique mudrocks that are often enriched in trace metals especially uranium, which can be easily measured in outcrops and in wells by spectral gamma ray spectrometry (SGR). Such geochemical techniques are commonly used to establish correlations and/or facies variations in mudrocks which may appear to be the same visually. Previous studies have shown that uranium can often be strongly correlated with total organic carbon, presumably due to scavenging of the U by the organic compounds. However, there are variations in the strength of this correlation, and the correlation does not always exist. One reason for these variations could be that phosphate, which can also be present in black shales in high concentrations, can also be a host for uranium.

The Excello is a Pennsylvanian (Desmoinesian) cyclothemic black shale, whose extent reaches from Illinois to Oklahoma passing through Kansas and Missouri. Though not a traditional source rock, the Excello is well defined stratigraphically and lithologically which makes it a perfect candidate to test the relationship between variations in uranium and total organic carbon with respect to depositional environments. The purpose of this study is to test the hypothesis that high concentrations of uranium correlate with high organic carbon concentrations only when phosphate is absent in the rock. Further refinement of our understanding of the relationship between U and organic carbon, serves to strengthen the use of well log interpretation as a proxy for mudrock characterization, as well as basin wide correlations of mudrocks in general.