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.