The use of Archie's Law to estimate formation factor in low porosity and high shaliness formation is associated with high uncertainties. The core analysis data in this type of formation is normally scattered and shows deviation from the Archie's straight line on the log-log plot of formation factor vs. porosity. The same deviation has also been observed in resistivity log data. Using Archie's Law for this type of formation generates inaccurate petrophysical constant i.e cementation factor, m that might consequently give inexact prediction of water saturation. Recently established mathematical equations modelled the deviation to geometric effect without considering the effect of clay conductivity. In this paper we established an improved mathematical equation to relate this phenomenon to both geometric and clay conductivity effect. This equation was derived from the original theory on excess clay conductivity and is easy to be applied using parameters from well log data. This equation had shown good match with our experimental work data as well as core analysis data from three (3) different regions. © 2019 Elsevier B.V.