Velocity modeling of a long‐period static anomaly, West Cameron Block 225, a Gulf of Mexico case history

The local structure at West Cameron Block 225, offshore Louisiana, was originally interpreted as a double‐crested anticlinal feature. Production was established on both crests. A deviated well was later drilled toward the intervening saddle; the well was structurally higher than the supposed crestal wells and found three additional pay sands. The revised interpretation assumed that the structural saddle was a long period static anomaly. This interpretation produced a single anticlinal feature with substantial attic reserves. In drilling the well to tap these attic reserves, many of the expected pay sands were found to be wet or missing. The well was structurally lower relative to the previous wells at nearly all the marker beds. Velocity sag was indeed present; however, only 60 percent of the correction applied to the seismic data could be accounted for in this manner. The stacking velocity profile was modeled for the maximum velocity sag interpretation using two horizons. The modeled profiles exhibit the anticipated fluctuations, but the magnitude of the anomalies is far in excess of the observed values. Therefore, the assumption of a large long‐period static anomaly is inconsistent with the stacking velocity information, and this interpretation must be rejected. This inexpensive technique can be applied in an exploration mode, and its use is not confined to near‐surface anomalies. Accurate estimates of the width and delay of the anomaly can be obtained through iterative modeling.