CGR
SchlumbergergAPIComputed Gamma Ray
The uranium-free gamma ray — potassium plus thorium only. Because uranium is mobile and unrelated to clay, CGR is often a cleaner shale indicator than total GR in uranium-rich or source-rock intervals.
- Property
- Gamma Ray Minus Uranium
- Gamma Ray
- Quantity
- API gamma ray
- gAPI
- Typical range
- ~5.4–96.1 gAPI
- gAPI, uranium-stripped
Logging tool
No logging tool is associated with this curve.
Service company
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How it's measured
Computed from the spectral tool by summing only the potassium and thorium contributions and excluding the uranium window — hence gamma ray minus uranium.
How it's used
A more reliable clay and shale-volume indicator where uranium spikes contaminate GR (organic-rich source rocks, phosphates, fractured or mineralized zones). The gap between GR and CGR highlights uranium.
Common gotchas
It is only available from spectral (NGT-type) logs and inherits the spectral tool's statistical noise. In uranium-free rock CGR and GR overlie — the value is precisely where they diverge.
In LAS files
Recorded as CGR in gAPI, alongside SGR (total GR) and the K/Th/U curves. Compare CGR to GR to spot uranium.
Also seen with CGR in real logs
Curves that appear alongside CGR in the same wells across the public corpus.