The effect of lithologic heterogeneity on the seismic velocity distribution of the Nile Delta section of Egypt

الملخص EN

The effect of litho logic heterogeneity of the Tertiary and Quaternary rocks of the Nile Delta of Egypt on the velocity distribution is studied through the relation between root mean square velocity and aver¬age velocity.

Such velocities of a number of wells and a number of seismic lines are calculated and plotted to define the drift between them.

The pro¬duced discrepancy between the root mean square velocity and average vel¬ocity proved to be a measure of the heterogeneity between the sands, clays and shale’s of the considered sequence, as shown from the velocity heterogeneity maps.

Average velocity gradient maps for the tops of El-Wastani, Kafr El- Sheikh and Abu Madi (Pliocene), Qawasim and Side Salem (Miocene) and Qantara (Oligocene) Formations are constructed.

The velocity variation encountered in El-Wastani, Kafr El-Sheikh, Abu Madi and Qawasim For-mations is nearly the same.

This may reflect consistent conditions of sedimentation of these units.

Root mean square velocity profiling is carried out, in which seven of these profiles (mostly pass through the drilled wells) are chosen for illustration in this work.

These profiles reveal the subdivision of the evaluated section into a number of tears (two or three or four) expres¬sing the accompanying changes in the stratigraphy of the shallow section.

The drift between time-depth curves derived from velocity analysis and those derived from well data are also calculated and plotted for the wells: Kafr El-Sheikh-1, Side Salem-1, Qawasim-1, North Bilqas-1 and Abadiya- 1.

Added, the vertical drift of these wells are followed laterally through time deviation maps of the six fore-mentioned rock units.

These time deviations can be used for correcting the rodt mean square velocities of the stacked sections in the areas of no available velocity measurements needed for de¬termining the optimum seismic velocities.