Sensitivity Considerations in Building Velocity Models for the Nile Delta’s Messinian
El-Bassiony, Ahmed *1; Eid, Mamdouh 1; Walker, Chris 2; Caselitz, Bertrand 1; ElAmir, Ashraf 2; Hemdan, Mahmoud 2
(1) Depth Imaging, Petroleum Geo Services, Cairo, Egypt. (2) Regional Studies & Prospect Eval., Rashid Petroleum Company, Cairo, Egypt.
The typical offshore Nile delta Messinian section is considered one of the toughest challenges that face the depth imaging of the Pre-Messinian structure. The post Messinian section shows a uniform gradient of sonic velocities. Given a good initial interval velocity model that is in accordance with the well sonic, a minimum mis-tie could be achieved with the top Messinian. However, the shale layers that occur above the top Messinian can be considered a source of anisotropic effects. The anisotropic effect noticed at the top Messinian top could be classified as anelliptical anisotropy, where the Thomsen parameter delta is zero and the whole anisotropic term is reduced to the eta value. The major challenges are introduced when the Messinian section itself is composed of pockets of anhydrite mixed with sediments while other areas include considerable thicknesses of rotated fault blocks with high velocities. The main challenge in this work was to be able to optimize the tomography to pick abrupt velocity changes at the edges of those fault blocks/pockets and to restore the stability of the pre-Messinian structure. A high density depth residual move-out field was picked and small smoothing radii were introduced inside the tomography. The updated model revealed sharp boundaries at the pockets and at the edges of the fault blocks. A crude interpretation was made on the migrated image to isolate the Messinian section from the lower one. A model was then built with an optimized velocity below the Messinian with adequate flattening of the gathers in depth. The sensitivity of perturbing the velocities at the fault block edges were tested by introducing different interpretations at the base of the Messinian. It was obvious that the structure stability of the Pre-Messinian is depending solely on the perturbation of the velocity field inside the fault blocks as well as the perturbation in the surface that separates it from the low velocity shales lying beneath. The optimum velocity model building technique for such complex structures shall therefore make full use of the tomography to optimize the velocity field distribution inside the Messinian and also use the most accurate interpretation to obtain a reliable image for the pre-Messinian. The key for the imaging condition is the sensitivity towards the changes made in both the velocity field and the boundary separating the Messinian from the lower section.
AAPG Search and Discovery Article #90141©2012, GEO-2012, 10th Middle East Geosciences Conference and Exhibition, 4-7 March 2012, Manama, Bahrain