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Depth Imaging In Deep Waters: A Case History
- Publisher: European Association of Geoscientists & Engineers
- Source: Conference Proceedings, 6th International Congress of the Brazilian Geophysical Society, Aug 1999, cp-215-00262
Abstract
The paper outlines the methodological approach followed to solve a complex problem of depth imaging in the<br>Nile Delta, offshore Egypt. The original time image was thought as being distorted not only by sea bottom<br>variations (from few hundred to a thousand meters) but also by the presence of large scale (thick Pleistocenic<br>sequence at high velocity) and smaller scale velocity anomalies (low velocity shales and high velocity<br>evaporites).<br>In the study area the possible target Miocene reservoirs lie below a thick sequence of Plio-Pleistocene<br>sediments and are sealed by an evaporitic sequence of varying thickness. Moreover the Miocene sequences<br>have low but almost uniform velocity, thus causing poor reflectivity and hence a poor signal-to-noise ratio.<br>An exploratory well (Well-1) did find an unexpected thick layer of relatively high velocity Pleistocene instead of<br>prognosed low velocity Pliocene. This caused a complete rethinking of the depth conversion approach followed<br>so far. It was decided to use the most up-to-date technologies, like Pre-Stack Depth Migration (PrSDM), to attack<br>the problem. Since there were constraints either in term of budget and time, it was decided to use 2D PrSDM<br>along 22 inlines to determine the velocity field and then perform a full 3D Post-Stack Depth Migration (PoSDM)<br>of the entire volume. This would have allowed a good compromise in terms of cost - benefit ratio. The final<br>depth volume showed quite clearly that a double culmination present in time turned out to be a single<br>culmination, yet to be drilled. A strict integration between depth imagers and the geologists and geophysicists<br>acquainted with the complex area proved to be one of the winning factor in the succesfull completion of the<br>study together with the use of PrSDM tools in the construction of the velocity volume.