Value of PSDM Processing on Low Relief Structure in East Onshore Abu Dhabi

Abstract

The Arabian Gulf is prolific of low relief geological structures, however, their definition and imaging present in general a genuine challenge. It is also commonly understood that low relief structures won't benefit from Pre-Stack Depth Migration (PSDM) whereas, Pre-Stack Time Migration (PSTM) is the ultimate required process. Thus, PSTM is frequently applied for the imaging of these low relief structures. Nonetheless, our recent 3D PSDM processing work has demonstrated that this perception is not all the time correct and has proved that PSDM can indeed add significant value to low relief structures. An Anisotropic PSDM (APSDM) workflow was carefully designed and meticulously applied on a very low relief structure located onshore Abu Dhabi –UAE. The main objectives of this 3D Anisotropic PSDM processing were established as follows: Achieve an accurate & clearer depth structure image with higher resolution.Mitigate & address the observed depth uncertainties at the existing wells.Enhance the faults architecture & imaging.Analyze anisotropic velocity & build a reliable velocity model for depth imaging.Interpret azimuth volumes in depth & time domains. The designed processing workflow consisted of the following main five stages: Gathers pre-conditioning and residual noise attenuation adopting the principle of amplitude preservation.Velocity building & updating using available well data, interpreted horizons and applying VTI full azimuth and multi azimuth velocity tomography processes.Depth imaging using Kirchhoff PSDM in OVT (Offset Vector Tile) domain.Post migration processing for residual inter-bed multiples & noise attenuation in addition to azimuthal anisotropy analysis and final depth-tie examination.Post stack processing for acquisition foot print removal and signal to noise ratio (S/N) enhancement. It should be noted that the selection of the optimum processing parameters at all the processing steps was done after the implementation of an intensive testing & rigorous QC/QA procedures. The main results and findings revealed by the 3D Anisotropic PSDM processing and the subsequent 3D seismic data interpretation are summarized as follows: PSDM shows less depth uncertainty compared to PSTM at existing wells. However, based on the results of recent drilling activities which have been conducted after PSDM, depth uncertainty at new well locations still exists.Reliable velocity model was built for depth imaging. This was established after 12 tomography iterations carried out for the isotropic velocity model and four iterations of anisotropy updates and VTI azimuthal velocity tomography.Some low relief structures are better defined in PSDM than PSTM.Seismic continuity of some target levels has been improved.Seismic resolution is degraded due to the limited frequency content.Fault imaging has been improved at some locations.