Velocity Model Building for Depth Imaging of Carbonate Reservoirs to the Deep Salt Incorporating Walkaway VSP and Gravity-Magnetic Data From OBC Survey Offshore Abu Dhabi

Abstract Use of depth imaging is increasing day by day in green as well as in brown fields. Velocity model is constructed by integration of multiple available datasets and techniques. Depth imaging based on a high-resolution velocity model constrained by wells resulted in improved image of the overburden as well as carbonate reservoirs to the deep salt using 3D OBC seismic, borehole seismic and Gravity-Magnetic data from two adjacent offshore fields in Abu Dhabi. Multi-Wave Inversion (MWI) utilizing direct arrivals, surface waves and two-way time surfaces was applied to obtain high-resolution velocity model in the near surface validated by checkshots and sonic logs. Near surface velocity inversions were detected by MWI which improved overall gather flatness in the near surface. The current stress regime as well as the network of faults resulted in HTI anisotropy in the dome area, which is visible on the multi-azimuthal Walkaway VSP travel-time residuals from the observed and modelled data. HTI is also visible on azimuthal move-out in Common Offset Common Azimuth (COCA) gathers. HTI tomography was tested, giving equally flat gathers achieved by azimuthal move-out correction. A Gravity-Magnetic survey covering the area was inverted and integrated for the deep salt and basement model. The basement was constrained by the results from magnetic depth estimation. Density model of deep salt sediments and basement was enhanced by Gravity-Magnetic inversion. The resulting density model was converted to velocity which was then incorporated in the final velocity model followed by tomographic update. Final imaging provided a better stacking response and improved gather flatness on the salt dome. High-resolution velocity model provided improved imaging, well ties and depth conversion. Improved AVO/AVA response helped patrial angles stacks for improved reservoir characterization. Improved imaging in the deep section was also achieved.