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The AAPG/Datapages Combined Publications Database

GCAGS Transactions


Gulf Coast Association of Geological Societies Transactions
Vol. 23 (1973), Pages 74-83

Modern Well-Site Evaluation of Texas Eocene Wilcox

Alvin L. Schultz (1)


Present exploration efforts in Texas Eocene Wilcox are being conducted in areas with varied depositional environments and associated significant changes in sedimentary facies. The facies changes that most affect quantitative log interpretation are: sediment size; sorting; and composition. Variable lithologic fabric and the wide range of connate waters present in the various intervals can cause difficult and unreliable interpretation when applying conventional methods for Sw and productivity analysis.

This paper shows how supplemental methods are used to (1) locate zones of interest; (2) gain an idea of whether hydrocarbon production can be expected; (3) provide porosity and saturation information; (4) identify gas zones; (5) show changes in shale content.

Zones of interest are detected by comparison of the "Rxo/Rt Quick Look" curve to the SP curve. Changes in connate water and/or shaliness do not weaken this method. Zones so located are then analyzed by using the Dual Induction data to verify productivity.

Further, an Ro curve obtained from a Density-(or Sonic)-derived Formation Factor curve supplements the Rxo/RtQL. This curve can verify the constants needed for water saturation calculations. In the pay zones an overlay of this information on the logarithmic Dual Induction can be scaled to display both porosity and water saturation.

This simultaneous display for visual analysis, available on the basic resistivity log, provides a convenient way to compare the quality of the zones of interest. Combinations of porosity-type logs, such as CNL/FDC or Sonic/FDC, are very useful for identifying type of hydrocarbon and/or degree of shaliness

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