Question 6 of 10: Radioisotopes Detected by Spectral Gamma Ray

Question 6: Radioisotopes Detected by Spectral Gamma Ray (3 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Unlike a total (standard) GR tool, which sums all gamma energies into one count rate, a spectral gamma-ray (NGS) tool resolves the energy spectrum into windows characteristic of the three naturally occurring radioactive series/isotopes hosted in reservoir minerals:

• Potassium-40 (&sup4;⁰K). A single-energy (1.46 MeV) gamma emitter hosted mainly in illite/glauconite clays, feldspars, and mica — the dominant signal in most "ordinary" shale and arkosic sand.
• Uranium (²³&sup8;U decay series, via its daughter ²¹&sup4;Bi at 1.76 MeV). Concentrated by adsorption in organic-rich source rock and marine shale (the "black marine shale" of Question 2) and in some fracture/vug fill; also the isotope most affected by later mobilization/redistribution along fractures.
• Thorium (²³²Th decay series, via its daughter ²&sup0;⁵Tl at 2.62 MeV). Hosted in heavy/resistant accessory minerals (zircon, monazite) and kaolinite clay; because Th is immobile (unlike U), the Th/K ratio is a standard clay-typing crossplot.

What makes the decomposition useful is that the three isotopes behave differently in the ground, not merely at different gamma energies. Potassium and thorium are locked into the clay lattice and into detrital accessory minerals, so they track clay content faithfully; uranium is soluble in oxidising pore water and is precipitated where conditions turn reducing, so it accumulates in organic-rich beds and along fracture and stylolite surfaces long after deposition. A total-GR curve sums all three, which is why a uranium-rich streak in an otherwise clean sand or carbonate reads as though it were shale and inflates any V_sh computed from it — exactly the failure mode listed among the assumptions in Question 9(d).

The standard remedy is to recompute the shale index from the "computed gamma ray" (CGR = the potassium plus thorium contribution alone, with uranium stripped out) rather than from the total (SGR) curve. The two ratio crossplots follow: Th/K separates clay types (kaolinite and chlorite plot at high Th/K, illite in the middle, glauconite and feldspar at low Th/K), while a high U/Th ratio flags source rock, phosphate or fracture-related mineralisation. None of this is recoverable from the single number a standard gamma-ray tool reports.