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Channel deposits of the lower Wolfcampian Tannehill sand are a major oil-producing interval in west-central Texas. These sand bodies represent a fluvial meander belt that was part of an extensive depositional system which shifted laterally along paleoslope toward the slowly subsiding Midland basin.
Within the producing zone, stratigraphic traps commonly are formed by structural closure caused by differential compaction and updip pinch-outs against clay plugs of former channel thalwegs. Porosities range from 20 to 30%, with permeabilities of 300 to 700 md. Oil columns of approximately 5 to 30 ft (1.5 to 9 m) are reported. The abrupt erosional contacts of the channel deposits with the regionally persistent Stockwether Limestone clearly delineate the lower boundaries of the Tannehill sand. The typical Tannehill E-log signature shows a sharp basal contact, a decrease in SP amplitude upward, and a more serrate curve upward--all characteristic of point-bar sequences. However, an abnormally low resistivity value (2 ohms) is observed in the oil-saturated portions of the sand. Core analy is shows that this abnormal value does not result from high water-saturation levels. Instead, this anomalous feature is probably due to the retention of water in clay lenses found within the sand bodies.
Ultimate recoverable oil in the Tannehill is estimated to average 400 bbl per acre-foot. The relatively shallow depth, 2,600 to 2,700 ft (792 to 823 m), of the Tannehill and the low cost of drilling to reach it create favorable exploration prospects. Recognition and understanding of the resistivity anomaly in the Tannehill sand could be of major economic importance.
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