Geological Applications of the Spectral Gamma-Ray Logging Tool in Kansas

DOVETON, JOHN H., Kansas Geological Survey, Lawrence, KS

The natural gamma-ray spectroscopy tool subdivides the total gamma-ray count measured in subsurface formations into contributions from potassium (%), thorium (ppm), and uranium (ppm). Examples of spectral gamma-ray logs are common in the Chase Group of southwestern Kansas but are also found in a variety of other stratigraphic sections across Kansas. The most common applications are in an improved estimation of shale content based on thorium and potassium and the recognition of localized uranium-rich fractures in otherwise clean carbonates.

The thorium/potassium ratio is a useful general indicator of relative potassium enrichment and allows broad distinctions of illitic and micaceous facies from sandstones and shales richer in kaolinite and/or chlorite. The basal Cretaceous unconformity is sometimes highlighted dramatically by this ratio, where the potassium felspar content of the Permian Cedar Hills Sandstone contrasts with the overlying Cheyenne Sandstone. Thorium-potassium crossplots are helpful in recognizing the pattern of distinctive clay mineral associations.

The thorium/uranium ratio is an excellent indicator of redox potential associated either with the original depositional environment or subsequent diagenesis. Patterns of marine transgression and regression are readily interpreted from spectral gamma-ray logs run in Permian and Cretaceous sections. In other cases, low Th/U ratios appear to reflect diagenesis, such as in some "hot" regressive limestones in the Lansing Group, where uranium may have been enriched by ancient groundwater regimes.

AAPG Search and Discovery Article #91008©1991 AAPG Mid-Continent Section Meeting, Kansas Geological Society, Wichita Kansas, September 22-24, 1991 (2009)