s_{wi}(h) = 1 - a \exp \left( -\left(\frac{b}{h-h_{FWL}+d}\right)^c \right)

where model parameters   are calibrated on water saturation from OH logs for each lithofacies or petrotype individually.

The coefficient  is related to FWL correction but usually set the same for all lithofacies / petrotypes of a given reservoir unit.

If core data is abundant then one can build a reasonable correlation of model parameters on porosity  and/or permeability :

a = a(\phi, k_a), \; b = b(\phi, k_a), \; c = c(\phi, k_a)