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Geology _ Geophysics in Oil Exploration

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Unformatted text preview: timations in porosity. 59 Geology & Geophysics in Oil Exploration Mahmoud Sroor Geology & Geophysics in Oil Exploration Gamma Ray Tools: Total Gamma Ray: (GR) The gamma ray log measures the total natural gamma radiation emanating from a formation. This gamma radiation originates from potassium-40 and the isotopes of the Uranium-Radium and Thorium series. The gamma ray log is commonly given the symbol GR. Its main use is the discrimination of shales by their high radioactivity. Note that shales, organic rich shales and volcanic ash show the highest gamma ray values, and halite, anhydrite, coal, clean sandstones, dolomite and limestone have low gamma ray values. Care must be taken not to generalize these rules too much. For example a clean sandstone may contain feldspars (Arkose sandstones), micas (micaceous sandstones) or both (greywacke), or Glauconite, or heavy minerals, any of which will give the sandstone higher gamma ray values than would be expected from a clean sandstone. Gamma ray may come from the drilling mud itself (some drilling muds are very radioactive). Calibration: The gamma ray log is reported in pseudo-units called API units. The API unit is defined empirically by calibration to a reference well at the University of Houston. This reference well is an artificial one that is composed of large blocks of rock of accurately known radioactivity ranging from very low radioactivity to very large radioactivity. Spectral Gamma Ray: (SGR) The spectral gamma ray log measures the natural gamma radiation emanating from a formation split into contributions from each of the major radio-isotopic sources. The spectral gamma ray tool uses the same sensor as the total gamma ray tool. The output from the sensor is fed into a multi-channel analyzer that calculates the amount of radiation coming from the energies associated with each of the major peaks. This is done by measuring the gamma ray count rate for 3 energy windows around the energies 1.46 MeV for potassium-40, 1.76 MeV for the uranium-radium series, and 2.62 MeV for the thorium series. These readings represent the gamma ray radioactivity from each of these sources. Their sum should be the same as the total gamma ray value measured by the total gamma ray tool. Calibration: The spectral gamma ray tool is calibrated using 4 sources of accurately known composition, one each containing only K40, U238, and Th232, and one containing a mixture. Each of the sources is pl...
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