| FZI = \frac{RQI}{\phi_r} |
where
RQI | Rock Quality Index |
\phi_r | normalised porosity |
It has much weaker dependence on effective porosity and shaliness and can be often considered constant within a given lithofacies which makes it a very efficient tool for lithofacies analysis
The constant FZI means that permeability strictly follows Cozeny-Karman permeability @model.
See also
Petroleum Industry / Upstream / Subsurface E&P Disciplines / Petrophysics