Subtle facies changes are traced vertically and laterally in the upper Monterey and Reef Ridge formations through application of the Multi-well Faciolog (mark of Schlumberger) technique. Electrofacies, representing intervals of similar log response, are identified in a key well by comparison with mud-log, conventional core, and x-ray diffraction analysis from sidewall cores and are retained in a data base. Five subsequent wells lacking detailed core or x-ray data but with similar log suites (bulk density, neutron porosity, gamma ray, and delta time) were compared to the data base and automatically assigned electrofacies. Twelve electrofacies--including diatomite, porcellanite, chert, dolomite, mudstone, and claystone, plus intermediate members--have been identified at the depth accuracy and resolution of petrophysical logs.

The lateral updip diagenetic facies changes from porous, hydrocarbon-productive diatomaceous mudstone to impermeable, low-porosity, non-productive porcellanite are clearly illustrated by the Faciolog cross-sectional display. McGuire et al, documenting the change from mudstone to porcellanite, recognize it as a controlling factor in formation of a stratigraphic trap. Vertical electrofacies associations reflect cyclic paleoclimatic trends and provide sedimentary sequences that aid in well-to-well correlation, field studies, and mapping in otherwise nondescript shales. Lithologic characterization of fine-grained, compositionally variable reservoirs, such as the Monterey Formation and equivalent rocks, is critical in understanding diagenetically altered porosity and, therefore, production

Comparison of average log values for each electrofacies with equivalent Miocene-age coastal basin rocks reveals decreased dolomite and increased terrigenous clay content in the Lost Hills strata. Using the Faciolog technique, combined with x-ray diffraction analysis, allows identification of average log values associated with specific lithologies.

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