SPECTRAL ANALYSIS OF THE FAYETTEVILLE SHALE AND THE IMO FORMATION, NORTHERN ARKANSAS, INFERENCES REGARDING RADIONUCLIDE CONCENTRATIONS, TOC AND SHALE-GAS SOURCES

More than 300 gamma-ray spectrometry measurements were collected from the Mississippian Hindsville, Fayetteville, Pitkin and Imo formations, northern Arkansas, and analyzed to provide insight into radionuclide accumulation and concentration of organic matter in these important natural-gas-bearing rocks. The Fayetteville Shale contains a black shale at the base that represents transgression and flooding of the eroded pre-Fayetteville surface. At Spring Valley, AR, the basal black-organic-rich unit transitions upward to gray shale, U and TOC concentrations decrease upward, total API gamma-ray decreases, and Th shows a slight increase. The upward-decreasing gamma-ray values are attributed to an increase in terrigenous sediments that diluted the U concentration. A positive correlation between U and TOC supports a marine source for organic carbon. Gamma-ray spectrometry of the Marshall, AR outcrop confirms two primary depositional cycles in the upper part of the Fayetteville Shale. The lower cycle contains black shale overlain by gray shale with thin-bedded limestone and culminates in limestone. U and Th decrease upward across the cycle, as do API gamma-ray and TOC, a pattern generated as the siliciclastics are diluted by carbonate. The upper cycle at Marshall begins with black shale and is similar to the lower cycle in that U, Th, TOC and API gamma-ray decrease upward as carbonate content increases. This cycle is in transitional contact with the overlying Pitkin limestone. The uppermost Pitkin limestone and the Imo Formation were analyzed at Peyton Creek where 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, TOC and U concentrations are low and gamma-ray correlates to Th rather than U, a result of dilution by terrigenous sediments. Across the Imo, TOC and U positively correlate, suggesting a marine source for organic carbon. Our 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.