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<br />Calculation of water saturation from Sigma: <br />Sw = 5'loe - Fina + ( ma - hc) +Vsh (~ma - Fsh) <br />0(~w - etc) <br />Where: E log <br />Ema <br />Ehc <br />Ew <br />Vsh <br />0 <br />Gamma Ray Log <br />is log derived value <br />is the matrix value <br />is sigma hydrocarbon value <br />is the sigma formation water value <br />is the volume of shale and if there is no shale (as in clean <br />cazbonates) the equation is simplified further. <br />is the porosity <br />The Gamma Ray Log is ideally suited for the identification of reservoir rocks as <br />distinguished from shale. It can be recorded in cased holes, conductive or non-conductive <br />borehole fluids or air /gas filled boreholes. <br />The gamma ray instrument measures the natural radioactivity of the formations. Natural <br />radiation of unstable elements consist primarily of alpha, beta, gamma rays, but it is <br />practical to measure only the gamma rays in the wellbore. <br />Some rocks are naturally radioactive by virtue of the disseminated, unstable elements they <br />contain. In passing through matter gamma rays experience successive Compton - <br />scattering collisions, with the atoms of the formation, losing energy with every collision. <br />Finally after the gamma ray has lost enough energy it is absorbed via the photoelectric <br />effect. With the Pe effect low energy gamma rays are completely absorbed by atoms of <br />the formation material, resulting in the ejection of electrons from the absorbing atoms. <br />The amount of absorption varies with formation density. Two formations having the some <br />amount of radioactive material per unit volume but having different densities will show on <br />the gamma ray log as different radioactivity levels with the less dense formation appearing <br />to be more radioactive. <br />Most of the 65 unstable nuclides exist in nature so razely that they will be omitted from <br />this text. Those of significant abundance aze I) Uranium series, 2) Thorium series and 3) <br />The potassium 40 isotope. These elements contribute a major portion of the natural <br />22 <br />