Enhanced Sub-Salt Imaging of 2D Seismic from the Northern Red Sea, Egypt
Norman C. Allegar1, Robert A Ripple1, Skip Lemanski1, Gamal Abdel Ghafour2, Nabil ElKady2, Sayed Khalil2
(1) BP Egypt, Digla, Maadi, Cairo, Egypt (2) WesternGeco, Cairo, Egypt
Exploration targets in the recently offered Northern Red Sea license blocks were historically base salt highs in the Miocene. The low success rate for this play was attributable to poor seismic data quality. 2D seismic data has been reprocessed utilizing the key technologies of surface related multiple elimination (SRME), pre-stack Kirchhoff time migration, and pre-stack Kirchhoff depth migration. Dramatic improvements in seismic imaging show that the exploration wells missed the Miocene highs or tested eroded highs where the pre-Miocene reservoir section was absent.
The SRME technique substantially improved the data, suppressing strong free-surface related multiples and allowing for better velocity estimation and imaging. Interbed multiple elimination (IME), with the water bottom as the multiple generator, provided minimal improvement. Radon de-multiple added minor improvements. Remaining multiple energy was deemed to be of a diffracted or 3D nature and could not be removed with 2D techniques.
Pre-stack time migration provided reliable velocities and imaging to top salt and proved a good starting point for the depth migration of base salt and the sub-salt section. Tomography, salt flooding and horizon-based substitutions were used to generate the velocity field. Additional improvements to the seismic image were obtained by vertically stacking a series of migrations where the pre-salt section was migrated with slightly different velocity fields. This vertical stacking process reduced remnants of the diffracted multiples and increased the signal-to-noise of the gently dipping underlying geology.
This reprocessing effort contributed greatly to BP’s ability to properly assess the acreage and prepare an appropriate bid application.