An Unprecedented Uplift in Seismic Imaging from an Ultra-Shallow Water Field, Offshore Abu Dhabi

Abstract

Offshore Abu Dhabi is characterized by ultra-shallow waters and thinly layered carbonate formations with high velocity and anisotropy contrasts. This produces complex recorded seismic wave-fields, riddled with noise and multiple energy, which are notoriously difficult to process. In a recent reprocessing project over a producing field offshore Abu Dhabi, we applied innovative processing techniques and obtained an unprecedented uplift in the final seismic image. 3D seismic data was acquired over the study area deploying airgun sources and 4-component receivers with ocean bottom cable (OBC) in an orthogonal cross-spread design. In vintage processing PZ summation was used to obtain the upgoing wavefield with the receiver ghost removed. 1D deconvolution was then applied to attenuate receiver-side pegleg multiples. Strong residual multiple energy nonetheless remained in the overburden and caused challenges in interpretation of the data. For the reprocessing, we applied a method known as up-down deconvolution, which is commonly used in deep water with a smooth water bottom. The implementation of up-down deconvolution in this ultrashallow water environment was challenging due to poor trace sampling and insufficient direct arrival energy in the shallow part of the recorded wavefield in the receiver domain. The mostly flat seafloor and horizontally stratified geology allowed us to implement the method in ultra-shallow water in the crossspread domain, with enhanced trace density. Further, we implemented a three-step method to model velocity and anisotropy parameters. We first used long offset seismic data to estimate effective velocity and anisotropy using a bi-spectral analysis method to pick non-hyperbolic moveout of the reflection data. Secondly, we independently measured Thomsen anisotropic parameters at seismic scale using available well logs and walk-away VSP data. These two independent methods provided an opportunity to build a simple layered anisotropy model. In the third step, we used reflection tomography to derive the final velocity and anisotropy models. The final model shows a wide variation of anisotropy within different geological layers of the survey area, where delta varies between -5% to 10% and epsilon varies between 0% to 40%. The shallow section exhibits high epsilon (30 to 40%) and negative delta values. The new seismic images provide a reliable interpretation of overburden structures. The Vertical Transverse isotropy (VTI) model shows consistency with sonic logs. The seismic images display high correlation with well synthetics at reservoir level. The reliability of the anisotropy model is confirmed by comparing a seismic structural depth map with well tops. The incorporation of anisotropy in incidence angle computation allows accurate angle stacks to be produced, which improves the reliability of subsequent AVO analysis. The workflow showcases the first successful implementation of up-down deconvolution in ultra-shallow waters to remove multiples, and reliable estimation of VTI parameters combining seismic and well data offshore Abu Dhabi.