Pyrobitumen nodules from the northern flank of the Wichita Mountains, Kiowa County, Oklahoma, contain unusually high concentrations of uranium (2,235 to 10,112 ppm), while the thorium content is low (0.1 to 2.0 ppm). The nodules are surrounded by a halo of gray dolomitic siltstone in a Permian (Hennessey Group) red dolomitic siltstone matrix. Microscopic examination indicates that the nodules consist of at least two phases of distinctly different reflectivity. Surprisingly, the uranium is associated with the low reflectance phase, in which it ranges up to 10% by weight.

Stable carbon isotope ratios of the uraniferous nodules show a consistent decrease from -31.2 ppt in the center of the nodule to -31.6 ppt at the outer edge (all values relative to PDB). This isotopic lightening at the edge of the nodule is also reflected in the surrounding siltstones. The carbon of the carbonate within the gray siltstone immediately surrounding the nodule is always lighter than that in the adjacent red siltstone by 0.3 to 0.8 ppt. Atomic H/C ratios of whole nodules average 0.83, while atomic O/C ratios of the nodules average 0.15, indicating that the nodules are extremely oxidized.

The presence of petroliferous rocks in the subsurface of the study area and zones of reduction along cracks and faults in the red siltstone suggest that the pyrobitumens are secondary,

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that is, alteration products of crude oil. This interpretation is further supported by microscopic examination revealing fracture-infilling by bituminous material. Finally, uranium was provided by ground waters rather than by concentration due to the oil-pyrobitumen transition.

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