Gamma Spectrometry and Geochemical Investigation tf the Mississippian (Chesterian) Fayetteville Shale and Imo Shale, Arkoma Basin, Arkansas
Adetola O. Alase
Department of Geology, Oklahoma State University, Stillwater
More than 250 gamma-ray spectrometry measurements were collected from the Mississippian (Chesterian) Hindsville, Fayetteville, Pitkin and Imo formations, northern Arkansas and analyzed to provide insight into radionuclide buildup and concentration of organic matter in these important natural-gasbearing rocks. 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.
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 darkgray 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 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.
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 contains black shale that transitions upward to gray shale, which is succeeded by sandstone and dark gray shale with thin dark limestone beds. U, Th, TOC and gamma-ray decrease upward from the basal black shale to the sandstone. Above the sandstone, as a result of dilution by terrigenous sediments, TOC and U concentrations decreases and gamma-ray correlates to Th rather than U. Across the Imo, TOC and U positively correlate, suggesting a marine source for organic carbon. The results indicate that API gamma-ray responds to U and Th and consequently may not be a reliable indicator of TOC concentration. However, U correlates positively with TOC across all units and is viewed as a reliable tool for estimating their gas-sourcing potential.
AAPG Search and Discovery Article #90182©2013 AAPG/SEG Student Expo, Houston, Texas, September 16-17, 2013