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The mappability of a seismic-stratigraphic trap depends not only on the thickness and the stratigraphic position of the objective unit, but the velocity contrast with nearby beds. Analysis of band-pass filtered sonic logs provides a useful technique for determining the portion of the frequency spectrum that carries the basic stratigraphic information. Depending on local stratigraphy, both high (75 to 125 hz) and low (0 to 10 hz) frequency components may be important in defining the trap. The filtered sonic can be used to predict the seismic mappability of stratigraphic units.
Review of seismic-stratigraphic data over fields in the Gulf Coast indicates that determining the mappability of a feature depends on complete understanding of the trap. At Walker Creek field, Smackover porosity is not resolved with a 60-hz filtered sonic. Because the basic reservoir-seal relation is low frequency, however, the field limits are clearly expressed on real seismic-stratigraphic data. Alternatively, recognition of the pinch-out of the 20-ft (6.1 m) thick Spanish Camp Sand at South Lissie field serves as an excellent example of a trap expressed as a high frequency feature.
These simple examples illustrate clearly that both high and low frequency components are required for successful seismic mapping of Gulf Coast stratigraphic traps.
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