Combinations of logs are frequently used to solve for effective porosity and one or more lithologic components. Densilog, Acoustilog, and Epithermal Neutron logs are most commonly used for these purposes. This article briefly describes the theory of these solutions and their attendant conventional cross plots, and presents a new technique that uses the Neutron Lifetime Log as a porosity log. In combination with the Sidewall Epithermal Neutron Log (SWN), a good solution for porosity and shale percentage is obtained. Favorable conditions for the application of this technique are the presence of low formation water salinities or deep invasion with fresh mud filtrate. Two examples of the new technique are included. The first is in a carbonate sequence containing moderate water salinities, the other is a tuffaceous sand-shale series with very log formation water salinities. Comparison with conventional interpretation technique is included. Results using this new method are considered good, especially when compared with the analyses from correlative conventional cross plots. The technique represents a new approach with good prospects for using the Neutron Lifetime Log as a porosity login combination with other porosity tools. It also adds a new dimension to the already demonstrated versatility of the NLL.

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