Shaliness from GR

The shales contain much higher concentration of radioactive minerals comparing to clean sands and carbonates (see Table 1 below).

This is why the most common way to quantify the shale content is the intensity of the natural gamma-ray (GR) emission.

The fist step is to normalize the actual GR-tool readings  $//<![CDATA[ \begin{array}{l}GR_{log}\end{array} //]]>$ to the reference values in clean rocks   $//<![CDATA[ \begin{array}{l}GR_m\end{array} //]]>$ and pure shales  $//<![CDATA[ \begin{array}{l}GR_{sh}\end{array} //]]>$ which is called Shale Index: 

where  $//<![CDATA[ \begin{array}{l}l\end{array} //]]>$ – along-hole depth.

The model parameters  $//<![CDATA[ \begin{array}{l}GR_{sh}\end{array} //]]>$  and   $//<![CDATA[ \begin{array}{l}GR_m\end{array} //]]>$  are calibrated for each lithofacies individually.

The Shale Index  $//<![CDATA[ \begin{array}{l}I_{GR}\end{array} //]]>$ is varying between 0 (for non-shally rocks) and 1 (for pure shales) but the actual shaliness may behave non-linearly between these extremes (especially for shallow, young reservoirs). 

This can be calibrated based on the available core data.

The table below summarises some popular shaliness models:

See also

Petroleum Industry / Upstream / Subsurface E&P Disciplines / Petrophysics / Reservoir Data Logs (RDL) @model

References