A gamma-ray (GR) tool counts the natural gamma radiation emitted by the formation next to the borehole. Most of that radiation comes from potassium, thorium and uranium, which tend to concentrate in clay minerals. That is why the curve is read, as a first pass, as a clay indicator: high readings suggest claystone or shale, low readings suggest clean sandstone, carbonate or many basalts.
What it is good for
- Correlation. GR character is repeatable between nearby boreholes, so it is the usual curve for matching intervals from one well to the next.
- Depth matching. Core and other log runs are normally shifted to the GR curve of a reference run.
- A first lithology split. Combined with resistivity and density, it separates the main rock types in a sequence.
Where it misleads
A sandstone rich in potassium feldspar or mica reads high without being clay-rich. Some volcanic rocks are radioactive for mineralogical reasons unrelated to clay. Uranium can concentrate along fractures or with organic matter. Borehole size, mud weight and casing all reduce the count rate and need correction.
The practical rule is that GR is interpreted with the other curves, never alone, and checked against core or cuttings wherever they exist.
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