1. n. [Formation Evaluation]
An element with an atomic number of 92. The 238U isotope is radioactive and decays with a half-life of 4.4 * 109 years through a series of intermediate isotopes to a stable isotope of lead. The intermediate isotopes emit a wide range of gamma rays, the most prominent being that of bismuth, 214Bi. It is assumed that formations are in secular equilibrium; that is, the relative proportions of parent and daughter isotopes remain constant, and the measured spectrum is directly related to the amount of 238U. The concentration in the Earth?s crust is about 4 ppm by weight. Uranium-bearing minerals are rare. Uranium is a soluble trace element that is transported easily and can be precipitated far from its source. It is most frequently found in carbonates and organic materials. A log of uranium is presented in parts per million, ppm. It is useful for detecting organic material, but is otherwise considered not useful for quantitative evaluation. In the corrected gamma ray log, the uranium contribution is removed to differentiate carbonates from shales.