is a method of measuring naturally occurring gamma radiation to characterize the rock or sediment in a borehole or drill hole. It is a wireline logging method used in mining, mineral exploration, water-well drilling, for formation evaluation in oil and gas well drilling and for other related purposes.
The gamma-ray log, like other types of well logging, is done by lowering an instrument down the drill hole and recording gamma radiation variation with 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 rays attenuate according to the diameter of the borehole mainly because of the properties of the fluid filling the borehole, but because gamma logs are generally used in a qualitative way, amplitude corrections are usually not necessary.
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 for qualitative determinations.
Gamma-ray tools record naturally occurring
gamma-rays in the formations adjacent to
the wellbore. This nuclear measurement indicates the radioactive content of the formations. Effective in any environment, gamma
ray tools are the standard device used for the
correlation of logs in cased and open holes.
In some places, non-shales exhibit elevated levels of gamma radiation. 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. Coal and dolomite may contain absorbed uranium. Evaporite deposits may contain potassium minerals such as sylvite and carnallite. When this is the case, spectral gamma-ray logging should be done to identify the source of these anomalies.
Applications
Depth determination
Depth correlation within the well
and between wells
Lithology identification
Qualitative evaluation of shaliness
Qualitative evaluation of radioactive mineral deposits
Cased hole perforating depth
control
Positioning for open-hole sampling
tools
