Thursday, November 27, 2014

Velocity Anomalies Beneath Reefs

  Carbonate reefs are important reservoir rocks in many part of the world. Reefs are associated with rapid lateral facies and associated velocity changes, and are so complex that it its difficult to generalize about their detailed attributes. Reef limestone may have higher velocity than the adjacent facies (e.g., back- and forereef shales) ; or they may have a lower velocity where surrounded, for example, by denser limestones and dolomites. Both situations will give rise to velocity anomalies on a time section. Where the reef has a higher interval velocity than the adjacent strata, a velocity pull-up will develop. If the reef is draped by younger sediments, the pull up could suggest that the overall positive feature is a deep-rooted structure (i.e, prereef in age) and may lead to a pessimistically false geological interpretation. Alternatively, where the reef limestone has lower interval velocity than the surrounding sediments, the velocity anomaly results in a push-down, and the reef base could appear as a low on seismic time section. This is identical to the shale push-down and the distortion on the time section is open to misinterpretation.

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