Gamma Ray Logging is most commonly used for formation evaluation in drilling. It is a method of measuring naturally occurring gamma radiation to characterize sediment or rock in a borehole. Different types of rock emit different amounts and different spectra of natural gamma radiation. In particular, shales usually emit more gamma rays than other sedimentary rocks, because radioactive potassium is a common component in their clay content and because the cation exchange capacity of clay causes them to absorb uranium and thorium. This difference between in radioactivity between shales and sandstone/carbonate rocks allows the gamma tool to distinguish between shales and non-shales.
A log of the total natural radioactivity, measured in API units is recorded in relation to sensor depth. This data is presented in the form of a log and LAS file to the geologist for evaluation and are used to make real-time directional drilling decisions. The gamma ray log was the first nuclear well log and was introduced in the late 1930s.