Narrow-spaced resistivity-measuring systems used in electric logging of wells respond mostly to the resistivities of the mud in the well bore and that of the mud filtrate invaded zone. After the necessary corrections are made for hole size, electrode spacing, and mud resistivity, one obtains the true resistivity of the invaded zone. The latter is a function of mud resistivity and of its characteristics as well as of the porosity of the formation and of other lithological factors such as the cementation and saturation exponents. These exponent values are reasonably characteristic of well defined reservoir rocks. If reasonable assumptions are made on the resistivity of connate water, the degree of its flushing by mud filtrate invasion, and on residual oil and/or gas, porosity may be calculated at various levels in the well bore.

Limitations must necessarily be imposed on the approach since the presently accepted mathematical petrophysical expressions apply only to intergranular porosity and uniformly distributed fissures and vugs.

An application of the approach to the interpretation of the limestone sonde (LS 32″) illustrates the method.

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