The Geochemistry of the Pennsylvanian Excello Shale
Black shales are important to the petroleum industry as both source rocks and as subsurface marker beds. Black shales are enriched in trace metals, especially uranium, which can be easily measured in outcrops and in wells by spectral gamma-ray spectrometry (SGR). Previous studies have shown that uranium can be correlated with total organic carbon (TOC), 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. The observed variations could be caused by phosphate, which is often enriched in black shales and strongly scavenges U. The Excello Shale is a Pennsylvanian (Desmoinesian) cyclothemic black shale, which is found from Iowa to Oklahoma. Though not a traditional source rock, the Excello is well defined stratigraphically and lithologically, which makes a perfect candidate to test variations in uranium and total organic carbon with respect to depositional environments (i.e. distance to shore). This study tests the hypothesis that uranium correlates with TOC only when phosphate is absent in the rock.
The overall objectives of this study are to determine the uranium concentration and its relationship to TOC, the source of the organic matter (i.e., terrestrial versus marine) and phosphate concentration. The study includes outcrop data from southeastern Kansas and northeastern Oklahoma. The methods used include gas chromatography (to fingerprint the organic carbon source by determining the composition of hydrocarbons in extracted bitumen), geochemical characterization of phosphorus speciation, and outcrop gamma-ray spectrometry. The data gathered will be used to determine the correlation of uranium with total organic carbon and phosphate concentrations in the shale. This will produce a model to predict shale behavior in the subsurface using well log data, and depositional changed based on geochemical parameters. The further from shore the higher the U, closer to shore the higher the API and lower the U.
The findings from this study will help improve well log based basin correlations, and identification of source rocks in the subsurface. This model may provide tools to help to identify new plays and improve high resolution subsurface correlations. Understanding the relationship between U, organic carbon and phosphorus, will ultimately strengthen the use of SGR logs as a proxy for TOC, and in their use in subsurface correlations.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009