GAMMA RAY LOGGING The first gamma ray logs were run by Lane Wells in 1936 Gamma ray (GR) logs measure the natural radioactivity in formations and can be used for identifying lithologies and for correl[.]
GAMMA RAY LOGGING The first gamma ray logs were run by Lane Wells in 1936 Gamma ray (GR) logs measure the natural radioactivity in formations and can be used for identifying lithologies and for correlating zones Many elements are naturally radioactive as a result of basic particle physics Gamma ray logs measures the number of natural gamma rays emitted by the rocks surrounding the tool This is often proportional to the amount of shale in the rocks, but there are other causes of gamma radiation 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 Because radioactive material is concentrated in shale, shale has a high gamma ray reading Shale-free sandstones and carbonates, therefore, usually have 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,….Therefore, 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 It looked similar to an SP log and was easy to use in correlating zones from well to well, gamma ray logs can be used not only for correlation, but also for the determination of shale (clay) volumes It was hailed as a great advance over the SP log because its value does not depend on formation water resistivity., and because the gamma ray log responds to the radioactive nature of the formation rather than the electrical nature, it can be used in cased holes and in open holes containing nonconducting drilling fluids (i.e., oil-based muds or air) Moreover, Radiation is naturally erratic A stationary detector facing a given gamma ray flux will not see a constant stream of gamma rays To obtain a reliable count rate, measuring instruments record the total number of emissions over a period of time, known as the time constant For most gamma ray tools, the time constant is or seconds to obtain a smooth log curve The differences in count rates between one time constant and another are called statistical variations Summary Gamma ray logs are lithology logs that measure the natural radioactivity of a formation Because radioactive material is concentrated in shale, shale has a high gamma ray reading Shale-free sandstones and carbonates, therefore, usually have low gamma ray readings Gamma ray logs are used to identify lithologies, correlate between formations, and calculate volume of shale