Shale index from gamma ray spectrometry

Input(s)

$C_{T h_{l o g}}$: Log Response for Thorium Curve (ppm)

$C_{T h_{m i n}}$: Log Response in Zone with Min Radioactivity of Thorium (ppm)

$C_{T h_{s l}:}$: Log Response of Thorium for Shale (ppm)

$C_{K_{\text {log }}:}$: Log Response of Potassium Curve (ppm)

$C_{K_{\text {min }}}$: Log Response in Zone with Min Radioactivity of Potassium (ppm)

$C_{K_{s h}:}: \quad \log$ Response of Potassium for Shale (ppm)

$\gamma_{\left(u f f_{l o g}\right.}$: Log Response for Uranium Free Curve (API units)

$\gamma_{\left(u f_{m i n}\right.}$: Log Response in Zone with Min Radioactivity of Uranium (API units)

$\gamma_{\left(u f_{s h}\right.}$: Log Response of Uranium for Shale (API units)

Output(s)

$I_{(s h)_{T h}}$: Shale Index for Thorium (dimensionless)

$I_{(s h)_{K}}$: Shale Index for Potassium (dimensionless)

$I_{(s h)_{U f}}$: Shale Index for Uranium (dimensionless)

Formula(s)

$$ \begin{gathered} I_{(s h)_{T h}}=\frac{\left[C_{T h_{l o g}}-C_{T h_{\min }}\right]}{\left[C_{T h_{s h}}-C_{T h_{\min }}\right]} \\ I_{(s h)_{K}}=\frac{C_{K_{l o g}}-C_{K_{\min }}}{C_{K_{s h}}-C_{K_{\min }}} \\ I_{(s h)_{U f}}=\frac{\left[\gamma_{(u f)_{l o g}}-\gamma_{(u f)_{\min }}\right]}{\left[\gamma_{(u f)_{s h}}-\gamma_{(u f)_{\min }}\right]} \end{gathered} $$

Reference(s)

Bassiouni, Z., 1994, Theory, Measurement, and Interpretation of Well Logs. SPE Textbook Series Vol. 4. Chapter 7, Page: 156.