Shale Index

(1)	$\begin{array}{l}\displaystyle I_{GR}(l) = \frac{GR_{log}(l) - GR_m}{GR_{sh} - GR_m}\end{array}$

where

$\begin{array}{l}GR_{log}\end{array}$	GR-tool readings $\begin{array}{l}GR_{log}\end{array}$
$\begin{array}{l}GR_m\end{array}$	the reference values in clean rocks $\begin{array}{l}GR_m\end{array}$
$\begin{array}{l}GR_{sh}\end{array}$	the reference values in pure shales $\begin{array}{l}GR_{sh}\end{array}$

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