SPE Member Abstract There is a long-held paradigm in the California oil industry that wireline log evaluation does not "work" in Monterey Formation lithologies. This study demonstrates that it is possible to calculate accurate oil saturation from wireline log data in the diatomite reservoir at Lost Hills field, California. The ability to calculate simple but accurate oil saturation is important because it allows field management teams to map pay, plan development and waterflood programs, and make estimates of reserves more accurate than those based on core information alone. Core data from eight wells were correlated with modern resistivity and porosity logs, incorporating more than 2000 ft of reservoir section. Porosity was determined from bulk density and water saturation was determined using the Archie equation. Crossplots of corrected core oil saturation versus Archie oil saturation (1-Sw) confirm the accuracy of the algorithm. Improvements in the accuracy and precision of the calculated oil saturation will require more detailed reservoir characterization to take into account lithologic variation. Introduction Development of the Monterey Formation diatomite reservoir at Lost Hills field began with the discovery well in 1910 and continues today. Since logs were first run in the Monterey Formation, geologists and engineers trying to interpret them were puzzled because the usual oil indicators, high resistivity and a distinctive reservoir rock log signature, were either not present or inconsistent. Not only were oil productive zones difficult to identify, but porosity calculated from the bulk density log using measured grain and fluid densities was generally too low compared to core porosity. Consequently, the logs were used qualitatively and only for correlation and completion planning. Few successful attempts were made to relate log response to oil saturation. P. 369

The recent advances in pulsed-neutron technology for oil field applications are discussed. The progress includes applying the technology to a number of formation evaluation and production logging problems, use of Monte Carlo modeling to understand response in complex field-like conditions, and development of interpretation algorithms in non-calibrated conditions. The remaining challenges in current applications and a number of emerging or proposed measurements using the technology are described.

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