GEOPHYSICS LETTERS

58 Seismic study to recognize reef reservoirspatterns, which include amplitude, frequency and continuity changes(seismic facies change).Among the seismic criteria as represented on Figs. (7) and (8), that areused in the present study to recognize the carbonate build-ups on theavailable seismic sections are:..a) Outline reflection:The reflection from the top and side boundaries of the build-ups andits shape, which is generally mound-like, is a good criterion to recognizereefs (build-ups). This criterion results from the acoustic impedancecontrast between reefs and the overlying strata.b) Seismic facies change:The seismic facies change between the reefs and the enveloping strataincludes amplitude, frequency and continuity changes. It results from themain difference in the petrographic properties between the reefs and theadjacent strata, which leads to the comparable difference in reflectionconfiguration patterns between the reefs and the adjacent strata.c) Drape in the overlying strata:The reefs are physically less compacted than the normally adjacentstrata, which may be evaporites or intercalations of carbonates andanhydrite. This criterion appears on the seismic sections as drape in theoverlying strata to reefs and this drape decreases up-ward.d) Velocity anomaly:The reefs are characterized by their higher velocities than thevelocities of the adjacent strata which are a lot of higher. This criterionappears on the seismic sections as pulled-up of the underlying strata toreefs. In some cases, if the velocity of the reefs is lower than the velocityof the adjacent strata (may be due to the effect of fluids content withinthe reefs) and in this case the reflections of the underlying strata to reefsare pulled-down.3- Environment, mapping and distribution of reefs:Depositional environments can be predicted from seismic reflectiondata through the interpretation of seismic reflection. Reflectionamplitude contains information of the velocity density contrast,individual interfaces, and on the extent of inter-bedding. Continuity ofreflectors is closely associated with the continuity of bedding. The