Summary

The use of interval velocities derived from surface seismic has been an important tool to predict pore pressure at drilling locations in the Nile Delta, and to delineate the distribution of overpressure in the basin. Large accumulations of gas are trapped within overpressured reservoirs within Pliocene and Miocene sandstones. Combined with pressure data from exploration wells, a possible hydrocarbon system can be postulated for both shallow and deep objectives. Both stacking velocities supplied by contractors from regional 2D lines and a 3D survey, and interval velocities derived from depth migration before stack (MBS) processing have been used to estimate pore pressure in the basin. The success of estimating pore pressure from either source of seismic velocity information depends upon reflection quality, acquisition parameters including the cable length, velocity anisotropy within the rock section, and geologic structures that may cause out of plane reflections. Despite the many sources of possible error, reasonable pressure predictions have been derived from surface seismic data using proprietary computer programs. Pore pressure is calculated and presented as a color overlay at every trace along a seismic line, or of a 3D velocity volume. The pore pressure calculation is based on a normal compaction trend and an equation relating the normal, or expected velocity, and the observed velocity. Both the normal compaction trend and the pressure equation are exponential relationships written in terms of effective stress.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas