The sharp lithological contrasts between and within the Wilcox and Frio Formations in south Texas frequently necessitate that different basic assumptions be used in interpreting logs of the formations. Accurate determination of the type, quantity, and rate of production of the interstitial fluids depends upon accurate interpretation of the well logs and the lithology of the rock. Ideally, geologic data should be quantified, digitized, and then integrated with digital well-log data. This allows log interpretation to be based on a detailed knowledge of the rocks. Use of logs allows this detailed knowledge of rocks to be extrapolated vertically for the well or used for older nearby wells where sample data may be unavailable. Detailed geologic data may be derived from small samples (cuttings, sidewall cores, or pieces of conventional core).

Lithological differences, such as those existing between the Wilcox and Frio, can significantly affect the radioactivity and density of the rocks. This, in turn, will influence calculations of shale volume and porosity. Differences in the composition of dispersed clays will affect Sw calculations. The calculation of porosity and water saturation does not identify either the amount or type of fluid production. Special log interpretation techniques, along with the detailed knowledge of the different clay varieties, allow an accurate prediction of the type of expected production.

Examples will be presented from the Wilcox and Frio of south Texas. These will include wells in which over-optimistic log interpretation resulted in repeated unsuccessful stimulation attempts, and examples of bypassed production.

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