OUTCROP CHARACTERIZATION OF MID-CONTINENT PENNSYLVANIAN SHALES THROUGH SPECTRAL GAMMA RAY (SGR) PROFILING AND TOC ANALYSES

Spectral gamma-ray (SGR) 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 to be condensed sections where long periods of continuous geological time are represented by a relatively thin slice of the 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. Variations in U and Th concentrations are thought to indicate deposition in deep marine settings and nearshore settings, respectively. Complementary analysis of TOC values in outcrop samples will allow for the development of quantitative, predictive models linking SGR profiles—i.e., U and Th fluxes—to carbon fluxes in ancient seas. However, to successfully use such models with wireline logs over the area of an entire basin, the validity of TOC-U/Th correlations must be tested over a wide range of conditions.

The Pennsylvanian system is characterized in the mid-continent by cyclic shale and limestone units deposited as an epeiric sea periodically inundated the continent due to the waxing and waning of Gondwanan glaciers. Traditional models indicate that the black shale facies represents deposition in sediment-starved, anoxic, deep marine settings. However, recent work suggests that the regional paleoenvironment may have been more complex, with the black facies in a single unit deposited in a range of environments. In Oklahoma the cyclothem record is overlain by a high degree of variability from autocyclic processes, compared to Kansas, where units record shifts from deep to shallow water that are more clearly linked to glacial/interglacial cyclicity. Thus, these units represent ideal strata within which to test the robustness of a TOC/SGR model. Ultimately, we hope to link the SGR measurements to TOC measurements to develop a predictive tool that can be widely applied to wireline logs.