SPECTRAL GAMMA RAY LOGS AS A PROXY FOR TOC IN THE HEEBNER SHALE: PALEOENVIRONMENTAL CONSTRAINTS

While it is recognized that Upper Pennsylvanian cyclothemic black shales contain high amounts of total organic carbon (TOC), there is lateral variation across the midcontinent in the concentrations found in individual outcrops. This variation reflects the changes in depositional environment—that is, not all black shales form in deep, anoxic marine basins. Similarly, the total gamma ray intensity is known to be “high” in black shales, as compared to other sedimentary rock types. However, because the total gamma ray signal is the sum of intensities from three elements: uranium, thorium and potassium. Each of these is fixed in sediments by different processes, and thus their concentrations and contribution to the total gamma ray signal will vary with depositional environment. The Upper Pennsylvanian Heebner shale was deposited over a wide area in the Virgilian. The stratigraphy and sedimentology of the Heebner and the over- and underlying units suggests that local depositional conditions potentially varied from relatively near-shore to deep basin.

Here, we have undertaken an outcrop based study of gamma-ray spectrometry to delineate depositional environment, as well as to investigate the relationship between gamma-ray intensity and the concentration of TOC. Our four sites range from northern OK to northern MO. Total gamma ray intensity varies between the sites, with the most nearshore site exhibiting the lowest intensity. TOC concentrations vary with depositional environment: low closer to shore, and higher in the most anoxic part of the basin. Two sites however, have relatively similar total gamma ray intensity, but these are driven by variations in different elements. In these cases, TOC shows very different correlations with total gamma ray intensity. However, if the spectral gamma ray log is separated into its components, correlations between TOC and U or Th vary with depositional environment. U-TOC are best correlated in environments that were the most anoxic, while Th-TOC are correlated in more nearshore, and presumable more terrestrially-influenced, environments.

Gamma ray logging has long been used as a proxy to predict TOC during well logging, with varying results. This study indicates that spectral gamma ray logging may be a better proxy for TOC—not just the concentration but source type.