Gamma Ray Log
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.
Figures & Tables
Basic well log analysis
This publication is a general introduction to common openhole logging measurements, both wire line and MWD/LWD, and the interpretation of those measurements to determine the traditional analytical goals of porosity, fluid saturation, and lithology/mineralogy. It is arranged by the interpretation goals of the data, rather than by the underlying physics of the measurements. The appendix files contain digital versions of the data from the case studies, a summary guide to the measurements and their interpretation, and a simple spreadsheet containing some of the more common interpretation algorithms. This Second Edition of Basic Well Log Analysis delivers a great impact on training and self-training along with superior workbook exercises, newer measurements, borehole imaging, and nuclear magnetic resonance in separate chapters, all directed to provide a guide through the lengthy and sometimes ambiguous terminology of well logging and petrophysics. It provides readers with interpretation examples (and solutions) so that the techniques described here can be practiced.