Gamma Spectrometry and Geochemical Investigation of the Mississippian (Chesterian) Fayetteville Shale and Imo Shale, Arkoma Basin, Arkansas

Abstract

The primary goal of this study was to integrate gamma-ray spectrometry, geochemistry and lithofacies distributions to interpret sediment source and depositional settings of the Fayetteville Shale and The Imo Shale in northern Arkansas. This study of the Mississippian (Chesterian) Fayetteville and Imo shales suggests that the black lower Fayetteville shale is organically rich, fossiliferous and contains laterally continuous micritic limestone beds. The black shale is radioactive and has an average TOC content of 4 wt.%. The rhythmic upper Fayetteville shale is an alternating succession of limestone and black to dark-gray shale that is organically rich, fossiliferous, pyritic, radioactive and has an average TOC content of 4.5 wt.%. The Fayetteville Shale at Marshall is relatively low-clay content and interpreted as relatively deeper-marine and anoxic as evidenced by a higher uranium content compared to thorium. The Fayetteville at Spring Valley is organically rich, fossiliferous, pyritic and clay-rich with an average TOC content of 3.5 wt.% It is characterized by an influx of terrestrially derived materials. The U:Th ratio is interpreted to indicate oxic/suboxic proximal deposition. The Imo Shale at Peyton Creek is subdivided into four units using lithology and total gamma-ray. The Imo Shale is relatively clay rich, fossiliferous, radioactive and organically rich with average TOC content of 3.0 wt.%. The Imo Shale is interpreted as an open-shallow marine, oxic/suboxic deposit. The changes in U:Th ratio and TOC across the Fayetteville Shale demonstrate that it contains two shoaling-upward sequences: the lower one terminating at the base of the highly radioactive upper Fayetteville Shale; the upper one culminating with the onset of Pitkin deposition.