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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
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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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