Shaliness

The shales contain much higher concentration of radioactive minerals comparing to clean sands and carbonates.

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 facies 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 summairzes some popular shaliness models:

The graphic image of different shale volume models is brought on Fig. 1.

Typical GR values in API are summarized on the table below:

References