Gamma ray logging
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Gamma ray logging is a method of using natural gamma radiation to characterize the rock or sediment in a borehole. It is sometimes used in mineral exploration and water-well drilling, but most commonly for formation evaluation in oil- and gas-well drilling. Different types of rock emit different amounts and different types of natural gamma radiation. In particular, shales usually emit more gamma rays than other sedimentary rocks, such as sandstone, gypsum, salt, coal, dolomite, or limestone. Shales emit more gamma radiation because radioactive potassium is a common component in their clay content, and because the cation exchange capacity of clay causes them to adsorb uranium and thorium. Some sandstones and carbonates can also have high radioaction due to precipitated radioactive material.
This difference in radioactivity between shales and sandstones/carbonate rocks allows the gamma tool to distinguish between shales and nonshales.
The gamma ray log, like other types of well logging, is done by lowering an instrument down the hole and recording gamma radiation at each depth. In the United States, the device most commonly records measurements at 1/2-foot intervals. Gamma radiation is usually recorded in API units, a measurement originated by the petroleum industry. Gamma logs are affected by the diameter of the borehole and the properties of the fluid filling the borehole, but because gamma logs are most often used in a qualitative was, corrections are usually not necessary.
Three elements and their decay chains are responsible for the radiation emitted by rock: potassium, thorium and uranium. Shales often contain potassium as part of their clay content, and tend to absorb uranium and thorium as well. A common gamma-ray log records the total radiation, and cannot distinguish between the radioactive elements.
An advantage of the gamma log over some other types of well logs is that it works through the steel and cement walls of cased boreholes. Although concrete and steel absorb some of the gamma radiation, enough travels through the steel and cement to allow qualitative determinations.
Sometimes non-shales also have elevated levels of gamma radiation. Sandstone can contain uranium mineralization, potassium feldspar, clay filling, or rock fragments that cause it to have higher-than usual gamma readings. Coal and dolomite may contain absorbed uranium. Evaporite deposits may contain potassium minerals such as carnallite.
[edit] Spectral Logging
Some specialized gamma radiation logging distinguishes the three component decay chains (potassium, uranium, and thorium) by the wavelengths of their characteristic gamma emissions.
The characteristic gamma ray line that is associated with each component:
- Potassium : Gamma Ray Energy 1.46 MeV
- Thorium Series: Gamma Ray Energy 2.62 MeV
- Uranium-Radium Series: Gamma Ray Energy 1.76 MeV
[edit] Use in Mineral Exploration
Gamma ray logs are also used in mineral exploration, especially exploration for uranium and potassium salts.
