Reservoir evaluations are based on information obtained at the time wells are drilled and completed and on reservoir performance. Important data normally available from open-hole measurements include gas-oil contacts, water saturations, and water-table levels. These parameters changes as a reservoir is produced and the results manifest themselves as changes or a combination of changes in oil produced, water produced, gas-oil ratio, or bottom-hole pressure. Many perplexing reservoir problems could be solved if changes in these parameters could be observed as the reservoir is produced to complement data observed in the bore hole from the results of production. Such measurements in the hands of the reservoir engineer would be a powerful tool for refinement of reservoir evalu tion.

The Neutron Lifetime Log gives a cased-hole measurement of a formation parameter sensitive to the amount and type of fluid in the formations. This parameter, thermal neutron capture cross section, qualitatively distinguishes potentially productive intervals from non-productive intervals through a wide range of well-bore conditions.

A single mathematical expression relates the contributions of the formation fluids in the pore volume and of the rock matrix to the capture cross section of the formation. Under favorable conditions, quantitative water saturations can be determined. Aside from the obvious application of capture cross section for identifying potentially productive intervals behind casing, this measurement has proved most valuable in reservoir analysis.

Well studies are presented indicating results observed from applying cased-hole measurements of capture cross section to reservoir evaluation.

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