The complexity of natural reservoir rocks precludes a mathematical treatment in developing a definite relationship between the formation resistivity factor and porosity. Consequently, the main approach has been the empirical analysis of laboratory studies on samples or models of the different rock types by various investigators, who proposed numerous values for the coefficient "a" and exponent "m" in the generalized form of the Archie's equation, F = a x phi^-m The values for "a" and "m" have been related to certain petrophysical characteristics like tortuosity, degree of cementation, type of the pore system, permeability, etc. A great variability of "a" and "m" values has been observed, and therefore, this is a considerable source of error in assuming values for these parameters for the computation of water saturations from logs. At the same time, it is usually difficult and uneconomical to establish the values in each and every area by extensive laboratory work. A new variable "m" approach is found to be more useful in practice in the case of soft granular formations, for which the "Humble Relation" has received wide acceptance. The proposed relationship is statistically reasonable and gives very close values of the formation factor and, in some cases, even closer to the measured values on the samples analyzed by Winsauer, Shearin, Masson and Williams in the development of the "Humble Relation".

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