28 Sequence Stratigraphy and Biostratigraphy of Pennsylvanian-Lower Permian Cyclothems in the North American Midcontinent II

Thursday, 12 April 2007
1:40 PM - 5:00 PM

TIMING RELATED TO THE REDOXIMORPHIC PROPERTIES OF IRON AND URANIUM IN WOODFORD SHALE THROUGH QUANTITATIVE MICROPROBE ANALYSIS OF PYRITE NODULES

Paper number: 28-6
Program: THEME SESSIONS
Time: 3:20 PM – 3:40 PM

Authors

Abstract

Previous studies of Woodford Shale have a reported linear relationship and positive correlation between gamma- ray (GR) magnitude expressed in API units and total organic carbon (TOC). Other Woodford studies have reported an inverse linear relationship and inverse correlation between bulk density and TOC. Comparison between API Gamma and TOC in Woodford Shale revealed two populations. One population behaved in a linear fashion with respect to GR and TOC. The second population exhibited amomalous GR. The predictive power of linear equations with respect to TOC was poor for these “hot” samples. Plots of bulk density (BD) vs. TOC in Woodford Shale revealed similar population anomalies. These plots contained a second population of samples in which anomalously high BD lacked predictive power with regard to TOC through linear equations. We hypothesize that reducing conditions produce black shales which can be described by linear equations of GR vs. TOC and API vs. TOC. Further, when environmental conditions shift from reducing to euxinic, excessive accumulations of authigenic uranium and pyrite iron interfere with the linear relationships discussed above.

In this study, we will interpret quantitative analysis of pyrite nodules from Woodford Shale with element maps generated with an electron microprobe. The results of these new tests will be used to interpret timing of geochemical changes relating to precipitation of authigenic Fe and U in the most radioactive black shales. This study is important to many fields because intervals of peak gamma magnitude, used in regional correlation of Woodford Shale in petroleum exploration, correspond to intervals interpreted as maximum flooding surface (MFS) and eustatic high sea level.