Improving fractured basement imaging through WAZ OBC acquisition

The oil-bearing fractured granite basement rocks form a very important and complicated hydrocarbon reservoir in Cuu Long basin offshore Vietnam. However, the poor fractured basement imaging in the conventional tow streamer data makes it hard for detailed interpretation and future well placement. To improve the seismic imaging, the first 3D/4C OBC acquisition over the field was carried out to provide better illumination, better elimination of multiples and broader spectrum with better signal to noise ratio. However, the presence of strong azimuthal anisotropy poses a serious challenge in imaging this OBC data with the full azimuth (FAZ) nature. The steeply dipping fracture imaging can be smeared if the subsurface orthorhombic (ORT) velocity model is not properly derived. In this paper, we present a new orthorhombic velocity model building workflow to estimate the azimuthal anisotropic velocity by incorporating shear wave splitting analysis, well formation microimager (FMI) information and 3D RTM subsurface angle gather based velocity sweeping inside basement. Two geological layers with strong azimuthal anisotropy are identified and incorporated into the final ORT model which results in much shaper imaging not only in shallow classic sediment layers but also in the fractured basement.