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Gamma ray (GR) logs measure the natural radioactivity in formations and can be used for identifying lithologies and for correlating zones. Shale-free sandstones and carbonates have low concentrations of radioactive material and give low gamma ray readings. As shale content increases, the gamma ray log response increases because of the concentration of radioactive material in shale. However, clean sandstone (i.e., with low shale content) might also produce a high gamma ray response if the sandstone contains potassium feldspars, micas, glauconite, or uranium-rich waters.

In zones where the geologist is aware of the presence of potassium feldspars, micas, or glauconite, a spectral gamma ray log can be run in place of the standard the gamma ray log. The spectral gamma ray log records not only the number of gamma rays emitted by the formation but also the energy of each, and processes that information into curves representative of the amounts of thorium (Th), potassium (K), and uranium (U) present in the formation.

If a zone has a high potassium content coupled with a high gamma ray log response, the zone might not be shale. Instead, it could be a feldspathic, glauconitic, or micaceous sandstone.

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