Gamma ray logging

In particular, shales usually emit more gamma rays than other sedimentary rocks, such as sandstone, gypsum, salt, coal, dolomite, or limestone 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.

Gamma radiation is usually recorded in API units, a measurement originated by the petroleum industry.

Three elements and their decay chains are responsible for the radiation emitted by rock: potassium, thorium and uranium.

For this reason, spectral gamma ray is used to provide an individual reading for each element so that anomalous concentrations can be found and properly interpreted.

For instance, sandstones can contain uranium minerals, potassium feldspar, clay filling, or lithic fragments that cause the rock to have higher than usual gamma readings.

Spectral logging is the technique of measuring the spectrum, or number and energy, of gamma rays emitted via natural radioactivity of the rock formation.

These radioactive isotopes each emit gamma rays that have a characteristic energy level measured in MeV.