IVZ - General Discussion, page-992

" I also hope they will run the Spectral Gamma ray tool."

I'd say a spectral GR (NGS) will be run with one of the wireline tool strings.

For the non-geos. Clay minerals contain radioactive forms of the elements potassium, thorium and uranium within the crystal lattices. The gamma radiation emitted by these isotopes is measured by a scintillation counter within the gamma ray tool.

A conventional gamma ray tool measures the total radioactive count rate (in API units) from all the isotopes. A Spectral Gamma Ray tool on the other hand can differentiate between the different radioactive elements (K, Th, U) and outputs the abundance of each in parts per million (ppm).

As noted above where you have a lot of feldspar in a sandstone the potassium in the feldspar can give high gamma ray readings and look like a clay on a non spectral gamma ray log. With a spectral gamma ray log you can look at the relative abundance of Potassium to Thorium and Uranium and tell if the high gamma ray is a response to high clay levels or due to a "hot" sandstone. ie a clay will have high levels of K, U and Th while a hot sand will have high K but low U and Th levels.