Bypassed, incompletely drained, and untapped thin-bed (6-ft or 2-m) reservoirs of North McFaddin field (Tertiary Gulf Coast basin, Texas) form important economic targets for "secondary" natural gas recovery. Undrained reservoir occurrence is undoubtedly related to technological constraints associated with the vertical resolution of electric log (EL) and induction-electric log (IEL) tools used during primary development. High-resolution induction tools and computed log analysis, particularly enhanced porosity processing, are fundamental to thin-bed petrophysical characterization and resource identification. Subsequent to secondary resource identification, conventional EL and IEL suites (typical of mature fields) are used to delineate potentially productive reservoir limits. This involves superposing contour maps of resistivity, relative spontaneous potential (SP), net thickness, and structure, and integrating these maps with data from well tests, wireline-formation tests, and sidewall cores. When combined with engineering analyses, the technique enables resource quantification and facilitates development design. The informally termed 4,200-ft zone no. 5 reservoir serves as an illustrative example. Relative SP, reflecting SP as a fraction of static SP, represents a crudely normalized parameter. In the case of EL data, thinbed true resistivity approximations were determined using the anomalous response of the lateral device. The deep induction curve was used in the case of IEL data. Well log parameters are interpreted as approximations of petrophysical properties. Resistivity reflects gas saturation, and relative SP corresponds to reservoir quality. By determining appropriate parameter cutoff values, the method is transportable and serves as a model to delineate and evaluate similar secondary thin-bed resources common to other mature gas fields, located perhaps farther afield than the Tertiary Gulf Coast.

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