1. A method of estimating permeability of a rock formation using a lambda parameter, A, representing the size of dynamically connected pores in said rock formation, said method comprising the steps of: a) obtaining core data for a rock formation; b) estimating permeability values for said rock formation from a first, k-Lambda expression and said core data, and when at least a portion of the permeability values derived from said first k-lambda expression does not correlate with measured permeability; then c) estimating said permeability of said rock formation using a second additional k-lambda expression for a portion of said core data to which said first k-lambda expression does not correlate.