GR
SchlumbergergAPIGamma Ray
Total natural gamma radiation from the formation, measured in API units. The most-used shale indicator in well log analysis — high values suggest shale or radioactive minerals; low values suggest clean sandstones, carbonates, or evaporites.
- Property
- Gamma Ray
- Property
- Quantity
- API gamma ray
- gAPI
- Typical range
- ~17.4–124.2 gAPI
- in clastic basins
Logging tool
No logging tool is associated with this curve.
Service company
Browse the full Schlumberger mnemonic dictionary, including legacy and current tool families.
View Schlumberger mnemonics →What is the GR curve?
How it's measured
The NGT measures naturally-emitted gamma radiation as the tool moves up the borehole. Counts are calibrated against the API standard — a test pit at the University of Houston with known radioactivity — and reported as gAPI.
How it's used
GR is the workhorse shale indicator in petrophysics. Combined with neutron-density crossplots, it drives lithology zonation, net-to-gross calculations, and reservoir characterization workflows.
Common gotchas
Hot sands (potassium-feldspar-rich), uranium spikes in source rock, and volcanic ash beds can all elevate GR without indicating shale. Use the spectral GR (CGR vs SGR) to disambiguate when available.
In LAS files
Most vendors record GR as the first or second curve after depth. Common variants include GR_RAW, GR_EDTC, SGR, and GAM — all of which PetroCurve maps to the standard GR on import.
Also known as
Variants of GR seen in real LAS files — PetroCurve maps each of these on import.
Based on 78,239,386 samples in the public corpus.
Also seen with GR in real logs
Curves that appear alongside GR in the same wells across the public corpus.