Gamma ray log is a type of well log used to detect and measure incoming gamma rays naturally emitted by radioactive formations. The gamma ray log is a passive log, it really does not emit any gamma ray of its own, the tool merely detects or senses the presence of radioactive elements found naturally in certain underground formations.
The radioactive element could be an isotope of Uranium, Thorium, Potassium or a combination of any of them. Radioactive isotopes are unstable and to achieve stability they are constantly “decaying” with the resultant release of gamma rays. This released gamma ray is what is detected by the gamma ray log and can give an indication of the type of rock drilled.
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For example, clean sandstone does not contain any naturally occurring radioactive element whereas shale rock usually contains radioactive elements. Therefore by detecting the presence or absence of gamma rays, gamma ray logs can tell if the rock is sandstone or shale. The very first gamma ray log was used in 1930.
How the Gamma Ray Log Works
Gamma ray logs are obtained by lowering and retrieving the logging tool from the borehole. The logging tool houses gamma ray detectors. Examples of gamma ray detectors are the Geiger-Mueller and scintillation detectors.
Modern day logging tools are usually fitted with scintillation detectors instead of Geiger-Mueller detectors. Scintillation detectors contain sodium iodide (NaI) crystals.
Now, since radioactive isotopes within shaley formations are always emitting gamma rays as they seek stability, the emitted gamma ray strikes the surface of the sodium iodide (NaI) in the detector as soon as the logging tool passes through the hole adjacent the radioactive formation. This striking action on the crystal releases a photon of light which then goes on to strike a photo-cathode.
As the photon of light strikes the photo-cathode, a bunch of electrons are also released. These electrons then move through an electric field and strike another electrode releasing even more electrons. This continues with more and more electrons being released each time creating an electric current as the electrons move through the electrodes.
Finally, this current passes through a resistor to create a voltage pulse. The voltage pulse is what is actually measured to give an indication of the gamma ray emitted by the formation. So in essence, the gamma ray log makes use of an indirect measurement instead of directly recording the gamma ray.
It all starts with the gamma ray striking the crystal, which releases a photon of light which strikes an electrode to release electrons. This continues until one voltage pulse is obtained for every gamma ray detected.
Application of Gamma Ray Logs
Gamma ray logs are primarily useful for identifying lithology or the type of rock. This eventually gives an indication of potentially productive intervals, as well as zones that do not look promising. Primarily, gamma ray logs distinguish between sandstone and shale. Clean sandstone formations are potential hydrocarbon bearing formations whereas shale may not be as promising.
So gamma ray logs in combination with other logs can help decide the interval to be perforated. Clay minerals are usually composed of radioactive elements. Shale is therefore radioactive because of the clay it is made up of.
However, care must be applied when making interpretations because some formations that are actually clean sands may still emit gamma rays. This may be from the migration of water from shaley formations to the sand thereby contaminating the sand with radioactive elements from the parent shale.
If confusion arises when interpreting the log, a spectralog can be run in addition to the gamma ray log. Spectralogs go further to tell what kind of radioactive material is present in the formation. This understanding can now tell if the high gamma ray log reading observed is merely from radioactive contamination or the formation is really shale.