SGR
SchlumbergergAPISpectral Gamma Ray
Total gamma ray from the spectral (NGT) tool — the sum of potassium, thorium, and uranium contributions. Same units as standard GR, but with the elemental breakdown behind it, letting you strip uranium out to form the cleaner CGR.
- Property
- Gamma Ray
- Property
- Quantity
- API gamma ray
- gAPI
- Typical range
- ~16.5–172.8 gAPI
- gAPI, like standard GR
Logging tool
No logging tool is associated with this curve.
Service company
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View Schlumberger mnemonics →What is the SGR curve?
How it's measured
A scintillation detector records the full gamma energy spectrum; windows around the potassium, thorium, and uranium emission peaks resolve each element, and SGR is their combined total in API units.
How it's used
Shale indication like GR, plus clay typing, source-rock and uranium-spike detection, and fracture or heavy-mineral analysis via the Th/K ratio. It also enables the uranium-free CGR.
Common gotchas
It is statistically noisier than plain GR and needs adequate logging speed for spectral resolution. Uranium mobility means SGR can flag radioactive zones that are not shale — the very reason CGR exists.
In LAS files
Recorded as SGR (or GR from an NGT/HNGS run) in gAPI, usually with POTA, THOR, URAN and CGR alongside.
Also seen with SGR in real logs
Curves that appear alongside SGR in the same wells across the public corpus.