Gamma Ray Quality Control Reference Manual

Date: 12/10/2014

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A natural gamma ray (GR) tool measures the natural radioactivity of earth formations. The results are primarily used for correlation and shale volume determination because it is the measurement least affected by changes in formation porosity and fluid type. 

Schlumberger Drilling & Measurements (D&M) offers several GR measurements using scintillation detectors. Total count rate detectors record the total number of GRs that impact the detector. They do not discriminate GRs originating from the three naturally occurring radioactive isotopes: thorium (Th), uranium (U), and potassium (K). The resulting total count rate is converted into American Petroleum Institute (API) units to express the GR activity of the formation. This conversion uses a single factor determined experimentally on a master tool during characterization at the standard reference facility for GR well logging. 

Typically, GR logs are presented in gamma ray API units (gAPI). The American Petroleum Institute established this unit, independent of tool count rate, to allow consistency in GR presentation by the various service companies.

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