Using Cyclothemic Black Shales to Create a Proxy Calculation for Borehole Toc Estimates

Abstract

Black shales are important to the petroleum industry as both source rocks and subsurface marker beds. Black shales are enriched in trace metals, especially uranium (U), which is easily measured in outcrops and wells by spectral gamma-ray spectrometry (SGR). Previous studies have shown that U can be correlated with total organic carbon (TOC), presumably due to scavenging of the U by the organic compounds. Observed variations could be caused by the presence of phosphate, which is often enriched in black shales and strongly scavenges U. The Excello Shale is a Pennsylvanian (Desmoinesian) black shale, found from Iowa to Oklahoma. The Excello is well defined stratigraphically and lithologically, which makes a perfect candidate to test variations in U and TOC with respect to depositional environments. This study tests the hypothesis that that presence of phosphate can also control uranium concentrations in black shales in different depositional environments, rather than total organic carbon. The objectives of this study are to determine the uranium concentration and its relationship to TOC, and phosphate speciation and concentrations. The study includes outcrop data from southeastern Kansas and northeastern Oklahoma. The methods include phosphorus speciation, SGR and coulometric titration. Data was used to determine the correlation of U, TOC and phosphate concentrations in the shale. Data was used to model a predicted shale behavior in the subsurface using well log data, and depositional changes based on geochemical parameters. The findings from this study improve well log based basin correlations, and identification of sub surface source rocks. Understanding these variables will help improve SGR proxy calculations for TOC.