The study area is located in the south east of Abu Dhabi, covering approximately 4,000 km2. During several prospectivity studies carried out since 2012, high exploration potential was identified for Lower Cretaceous stratigraphic traps. To resolve the Aptian subtle trap configuration and lateral seals, it was decided in 2017 to acquire a 2,146 km2, high resolution 3D seismic cube. The paper will describe the planning, acquisition, processing and current interpretation results of this seismic campaign. Prior to the 3D seismic acquisition, a parameter design study was conducted to assess the requirements for the seismic acquisition and to insure that potential stratigraphic traps can be imaged in the newly planned 3D seismic cube. The processing was conducted on a zipper by zipper basis, allowing the processing of the acquired data while the remaining acquisition was still ongoing. It was conducted in a south to north direction, allowing the interpretation of the southern most zipper first which has greatest potential according to the existing prospectivety studies. Initial interpretation results confirm the challenge to interpret actual clinoform shapes at the Aptian level. Furthermore, multiples initially obstructed the seismic interpretation. Additional processing was carried out to reduce the impact of multiple energy on the seismic events. It was observed that noise reduction was acheived after the integration of each zipper, as the velocity model was further improved with each step. Albeit with these challenges, attribute extraction has been proven to be highly valuable in defining clinoform trends due to the high quality of the acquired seismic. Based on the seismic interpretation results, an exploration campaign to unlock the potential of the Aptian and deeper prospectivity in the East of the study area has been planned. For the first time in ADNOC and the emirate of Abu Dhabi, a 3D seismic survey was designed, acquired, processed and interpreted to target stratigraphic traps as the main objective. To accelerate the exploration campaign, the 3D seismic data was processed while the acquisition was ongoing. With this strategy, an 18-month turnaround from start of acquisition in December 2017 to the spud of first exploration well in May 2019 was achieved.

Acquiring surface seismic data can be challenging in areas of intense human activities, due to presence of infrastructures (roads, houses, rigs), often leaving large gaps in the fold of coverage that can span over several kilometers. Modern interpolation algorithms can interpolate up to a certain extent, but quality of reconstructed seismic data diminishes as the acquisition gap increases. This is where vintage seismic acquisition can aid processing and imaging, especially if previous acquisition did not face the same surface obstacles. In this paper we will present how the legacy seismic survey has helped to fill in the data gaps of the new acquisition and produced improved seismic image. The new acquisition survey is part of the Mega 3D onshore effort undertaken by ADNOC, characterized by dense shot and receiver spacing with focus on full azimuth and broadband. Due to surface infrastructures, data could not be completely acquired leaving sizable gap in the target area. However, a legacy seismic acquisition undertaken in 2014 had access to such gap zones, as infrastructures were not present at the time. Legacy seismic data has been previously processed and imaged, however simple post-imaging merge would not be adequate as two datasets were processed using different workflows and imaging was done using different velocity models. In order to synchronize the two datasets, we have processed them in parallel. Data matching and merging were done before regularization. It has been regularized to radial geometry using 5D Matching Pursuit with Fourier Interpolation (MPFI). This has provided 12 well sampled azimuth sectors that went through surface consistent processing, multiple attenuation, and residual noise attenuation. Near surface model was built using data-driven image-based static (DIBS) while reflection tomography was used to build the anisotropic velocity model. Imaging was done using Pre-Stack Kirchhoff Depth Migration. Processing legacy survey from the beginning has helped to improve signal to noise ratio which assisted with data merging to not degrade the quality of the end image. Building one near surface model allowed both datasets to match well in time domain. Bringing datasets to the same level was an important condition before matching and merging. Amplitude and phase analysis have shown that both surveys are aligned quite well with minimal difference. Only the portion of the legacy survey that covers the gap was used in the regularization, allowing MPFI to reconstruct missing data. Regularized data went through surface multiple attenuation and further noise attenuation as preconditioning for migration. Final image that is created using both datasets has allowed target to be imaged better.

One of the target formations of the studied field is a clastic reservoir of the Middle Cretaceous. This particular formation is comprised of multiple sand units (bars, channels), with varying thickness and lateral discontinuity, separated by thin shale intervals. Due to data limitation and complex depositional environment, the delineation and reservoir characterization of this clastic sequence is relatively complex. This paper describes the workflow used to extract information about the acoustic properties (acoustic impedance) of the target formation and infer additional knowledge of the reservoir properties (porosity) using a model-based deterministic post-stack seismic inversion. The area of interest (AOI) is covered by a recent high quality 3D seismic survey which has been processed through true-relative amplitude pre-stack time migration. Few wells containing monopole sonic, density and gamma-ray logs were available in the AOI. A basic feasibility study comprising rock physics, 1D well-log based seismic forward modeling, seismic-to-well tie and wavelet extraction was carried out prior to running model-based acoustic impedance inversion. The input seismic volume was post-stack pre-conditioned prior to inversion in order to enhance the usable frequency bandwidth and to reduce residual noise. Several techniques (with and without wells) were used to extract the seismic wavelet. A range of background (low-frequency) models were constructed using different methods and their impact on the inversion was appraised with the assistance of blind wells and classical inversion diagnostics. The resulting average map and cross-sections of acoustic impedance (AI) enabled to locate the areas of high porosity (low AI) within the target formation. Consequently, in addition to seismic interpretation, we could optimize the location of the candidate well within the high porosity area. The generated AI helped in refining the seismic interpretation of this clastic reservoir as well. Despite the complexity and the unpredictability of the Clastic reservoir depositional environment, Post-stack seismic inversion was a powerful tool that helped to locate the areas of good reservoir quality and ultimately to optimize the location of the candidate well.

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.