The term effective porosity lacks a single or straightforwar definition. Even some of the terms used in its mathematical description (“Vcl” and “Vsh”) have multiple definitions. However, it is most commonly considered to represent the porosity of a rock excluding water bound to clay particles - i.e. what proportion of the rock comprises voids through which fluids - such as water and oil - can freely pass. The effective porosity is of great importance in considering the suitability of rocks as oil reservoir rocks.