In order to determine the water saturation of a shaly formation, use is made of a formula expressing the relationship between the true resistivity and the formation parameters affecting this resistivity. These include water resistivity, clay resistivity, porosity, clay content, and water saturation. Satisfactory results are usually obtained with a formula of the type: where "m" and "n" are respectively the cementation factor and saturation exponent, and exponent "c" is usually taken equal to 1, although larger values, up to 2, are sometimes used. However, a tendency to overestimate S, values with this type of formula has been observed in cases where the ratio Rcl,& was low and clay content was high. In these conditions, more satisfactory results are obtained with the formula: The formula can frequently be used with reasonable accuracy by setting d equal to one. In this simpler form the equation is more convenient for manual interpretation.

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