Spectral Gamma Ray

Three Windows

The natural-gamma-ray spectrometry tool does more than count: it sorts the gamma rays by energy into three windows, the contributions of potassium, uranium, and thorium. The total gamma ray is just their sum, roughly $GR = 16,K + 8,U + 4,Th$ in API for K in percent and U and Th in ppm. Splitting it apart turns one number into three, each telling a different story about the rock.

The Uranium Problem

Potassium and thorium sit in clay, so they are honest shale flags. Uranium is the troublemaker: it also concentrates in organic matter, phosphates, and the residues of fracture-borne fluids, none of which is clay. A clean, organic-rich source rock can light up the gamma ray as hot as a shale on uranium alone. The fix is the computed gamma ray (CGR), the total with the uranium window removed; it tracks only the K and Th in the clay, so it does not call clean organic rock shaly.

Clay Typing with Th/K

The split also fingerprints the minerals. The thorium-to-potassium ratio separates clay types: high Th/K points to kaolinite and chlorite, intermediate values to mixed-layer clays, and low Th/K to illite, mica, and potassium feldspar. Uranium peaks flag source rocks and fractures; potassium highs flag feldspar or micaceous sands. The spectral gamma ray is the difference between knowing a bed is radioactive and knowing why.

References

• Asquith, G. and Krygowski, D. (2004). Basic Well Log Analysis, 2nd ed. AAPG Methods in Exploration 16.
• Schlumberger (1982). Natural Gamma Ray Spectrometry: Essentials of NGS Interpretation.