SPECTRAL GAMMA RAY LOGS AS PALEOENVIRONMENTAL INDICATORS IN CARBONIFEROUS BLACK SHALES

Spectral gamma-ray logging profiles in organic-rich shales commonly contain intervals in which readings exceed 150 API units. These intervals of elevated gamma-ray, or “hot streaks”, are considered stratigraphic condensed sections; relatively long periods of continuous geological time amalgamated in and represented by a relatively a thin slice of stratigraphic section. Such gamma-ray profiles represent a combined response from U, K and Th in the shale, and the spectra can be deconvolved and related to the concentrations of each of these elements. These data can be plotted on ternary diagrams, and the resulting trends can be related to depositional environment. Variations in U concentrations are thought to indicate starved sedimentation conditions, typically associated with deep-water settings, while Th concentrations are indicative of continentally derived material. However, it remains unclear how spectral gamma- ray readings obtained in outcrop—and by extension in subsurface well logs—relate to measured concentrations of these elements, as well as other geochemical parameters, such as organic carbon concentrations.

Pennsylvanian (Upper Carboniferous) cyclothemic black shales from the midcontinent of North American are classic sequences that have been the source of much study and debate concerning the paleoenvironment in which they were deposited. Hypotheses range from shallow, nearshore settings to deepest water settings such that the black shales represent maximum sea-level high stand. High-resolution sampling of the Missourian Hushpuckney Shale and the application of several sedimentological, petrographic and geochemical proxies indicate that a complex interplay between sediment supply, degree of anoxia, proximity to hydrothermal vents and the organic matter type control metal enrichments. Within this framework, we are constructing spectral gamma-ray logs in several outcrop locations to calibrate these results with measured concentrations. The integration of gamma-ray logging results with other geochemical indicators provides a potentially powerful geographic proxy to further constrain paleoceanographic conditions, and to correlate them over large distances in the subsurface.