Seismic Reprocessing Leads to New Breakthroughs——A Successful Case in the ASH Field, A.G. Basin in Egypt

Abstract

ASH oil field located in the east of the AG Basin in Egypt. Lower Cretaceous Alam El Bueib is the main oil producing formation. Due high heterogeneity of the Abu Roash Members succession in addition to the influence of thick limestone of the Upper Cretaceous and the influence of multiple complex faults, the quality of seismic data is very poor. This is mainly manifested in, a, the variation of vertical velocity in lithology changes resulting a significant error in depth migration; b, the fault imaging is not clear due to low S/N ratio, which leads to serious challenge for structure mapping. Therefore, the seismic re-processing was carried out. Two key techniques were carried out to achieve the target, that: uses new well VSP data to adjust the velocity model and CRAM PSDM for re-processing. Adjusting the velocity model: The previous PSDM acquired in 2015 had no well control covering the deeper target of Alam El Bueib Formation but the advantage of using VSP data of ASH-3 well as a well control for building new velocity model of the reprocessed data. CRAM PSDM re-processing: The CRAM (Common Reflection Angle Migration) is a cluster-based imaging system that generates conventional reflection angle gathers without azimuth dependency. Optimal local tapered beams are internally created and imaged to form high-quality image gathers, which can then be used in standard interpretation systems for accurate velocity model building and amplitude inversion (AVA). Applying CRAM technology in depth migration instead of Kirchhoff depth migration, has a great impact on enhance the final seismic image. The CRAM algorithm is using the Ray path migration of the seismic signal instead of using migration aperture as in Kirchhoff. Applying the ray path migration helps in adopting the seismic trace position ultimately enhance the fault definition and S/N ratio at the deeper target levels. As a practical case from the comparable seismic sections, same arbitrary line in different seismic volumes, the reprocessed data showed a high level of improvement in fault definition specially in the north portion of ASH field closer to the major fault which was very poor in the previous data. In the reprocessed CRAM PSDM data, the good amplitude extended towards the north and west portions of the field, which allowed to define the faults through Alam El Bueib horizon and decrease the uncertainty of the new proposed well location in the entire field. (Figure 1) Figure 1 Amplitude comparison between reprocessed and original data, ASH field