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The optimal use of well logs is to measure properties of rocks in a manner that permits valid and reliable inferences about rock type, porosity, permeability, fluid content, and related characteristics. The success of such an endeavor must be evaluated in terms of the known or fully determinable properties of the rocks that have been logged. Reservoir rocks whose actual physical properties differ significantly from those inferred from wireline logs are common.
At some localities in north-central Oklahoma, logs of Red Fork Sandstone (Desmoinesian, Middle Pennsylvanian) show suppressed spontaneous-potential curves, incomplete bed definition, misleadingly low resistivity, and no consistent, direct quantitative correlation between porosity and permeability. Foot-by-foot evaluation of an enigmatical core of the Red Fork by thin-section analysis, scanning-electron microscopy, and X-ray diffraction explained peculiarities in the gamma-ray and spontaneous-potential curves, and contributed to explanation of uncommonly low resistivity. Diagenetic effects and primary and authigenic clay seem to have had strong effects on log signatures. A large proportion of porosity is secondary.
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