The association of tuffaceous formations with radionuclide-bearing lignites is well established in Texas and elsewhere in the United States. Radionuclides in or near minable lignite horizons are important due to public and governmental concern over the human and environmental effect of low-level radioactivity.
The primary lignite-bearing formations of Texas include the Eocene Calvert Bluff, Yegua, and Manning Formations. A study of the formations adjacent to these lignite formations reveals that the Manning Formation is the likeliest to contain radionuclide zones in or near the lignite horizons. The overlying tuffaceous Catahoula and Whitsett Formations are the probable source of the radionuclides. The reduced carbonaceous zones of the Manning Formation provided a favorable environment for radionuclide precipitation.
The occurrence of radionuclide zones within oxidized carbonaceous horizons are modeled for the Manning Formation using geophysical logs, cross sections and areal occurrence maps. In-situ borehole assays confirmed a wide range of gamma radiation log responses, and quantification of them revealed the concentrations and specific types of radionuclides present.
Specific exploration techniques are used to model this phenomenon during lignite exploration activities. These techniques include calibrated gamma probes, in-situ borehole assays using advanced high-resolution gamma-ray spectroscopy logging tools, mapping of lignite weathering profiles using close-spaced drilling techniques, geologic interpretation using cross sections, and mapping of the radionuclide zones and sands. The resulting geologic model can be used for environmental impact studies and mine planning.