I've come across this question in a Petrophysics exercise regarding the Gamma Ray well logging. And I don't know how to solve it.

This is the "believed" AFAIK session

Natural radioactivity on a sedimentary environment is mainly due to a Potassium isotope (K40), responsible for around 20% of total radiation, with a lower although reasonable contribution from Thorium (Th232) and a variable Uranium (U235) amount. Shalestones are rich in K40 and also has metallic ions as Th232 and U235.

For a sandstone, however, radioactivity can arise either from mineral content on the matrix or, the fluid content belonging to the formation or mainly from shale contamination.

End of the AFAIK session

Someone suggested that younger the rocks, lesser the diagenesis and thus more presence of K40 bearing elements, like feldspar and mica. This results in a radioactivity not originated from the shale, so tertiary rocks would show more radioactivity. Someone else pointed out that older doesn't mean more under diagenesis, which opposes to the first reasoning.

  • $\begingroup$ I know nothing about it. But perhaps you can provide some background on what you do regarding radioactivity in related topics, as since it's an exercise, you probably have learned something about it? $\endgroup$ Nov 13, 2018 at 2:34
  • $\begingroup$ Assuming equal/uniform deposition of the same radioactive material during both periods, simply based on radioactive half lives, material from the older period (cretaceous) will have less radioactive material than material from the younger (tertiary) period. If deposition of radioactive material is not uniform over both periods, or different types of radioactive material are deposited during each period, the answer becomes complex. $\endgroup$
    – Fred
    Nov 13, 2018 at 17:38
  • 1
    $\begingroup$ Thanks for the hint @JeopardyTempest! Done it already. :) $\endgroup$ Nov 13, 2018 at 18:24


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