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A common gamma-ray log records the total radiation and cannot distinguish between the radioactive elements, while a spectral gamma ray log (see below) can. For standard gamma-ray logs, the measured value of gamma-ray radiation is calculated from concentration of uranium in ppm, thorium in ppm, and potassium in weight percent: e.g., GR API = 8 ...
The gamma ray log is useful in holes drilled with air or with oil based muds, as these wells have no SP voltage. Even in water-based muds, the gamma ray and SP logs are often run together. They comprise a check on each other and can indicate unusual shale sections which may either not be radioactive, or may have an abnormal ionic chemistry.
Gamma-ray measurement has the following applications: Well to well correlation: gamma-ray log fluctuates with changes in formation mineralogy. As such, gamma-ray logs from different wells within the same field or region can be very useful for correlation purposes, because similar formations show similar features.
Measurement while drilling can be cost-effective in exploration wells, particularly in areas of the Gulf of Mexico where wells are drilled in areas of salt diapirs. The resistivity log will detect penetration into salt, and early detection prevents salt damage to bentonite drilling mud. [citation needed]
This bombards the formation with high energy gamma rays. The attenuation of these gamma rays gives an accurate measure of formation density; this has been a standard oilfield tool since 1965. Another source is americium berylium (Am-Be) neutron source used in evaluation of the porosity of the formation, and this has been used since 1950. In a ...
Well logging, also known as borehole logging is the practice of making a detailed record (a well log) of the geologic formations penetrated by a borehole.The log may be based either on visual inspection of samples brought to the surface (geological logs) or on physical measurements made by instruments lowered into the hole (geophysical logs).
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The SP tool is one of the simplest tools and is generally run as standard when logging a hole, along with the gamma ray. SP data can be used to find: Depths of permeable formations; The boundaries of these formations; Correlation of formations when compared with data from other analogue wells; Values for the formation-water resistivity