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Velocity anomalies large enough to cause severe errors in depth maps may not be recognized from prior drilling experience, from regional velocity control, or from seismic reflection configurations. A chastening example of one such anomaly is an outpost well near the Katy field in Waller County, Texas. This outpost well, the Exxon 1 Sparks, tested Wilcox sands, which were both dry and 500 ft low to seismic prediction from Vibroseis data. The well came in structurally low as a result of incorrect predictions of both near-surface and post-Wilcox velocities. Additional studies indicated that the Sparks well was drilled on the flank of a structure that was later drilled and proved to be the discovery of a new Wilcox field.
Accurate velocities are becoming increasingly important in all phases of oil and gas exploration, particularly in mature exploration areas where prospects are subtle and difficult to find.
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