Anisotropic PP and PSv prestack depth migration of 4C seismic data: Pamberi, Trinidad

Abstract

In November 2004, EOG Resources acquired an ocean‐bottom cable (OBC) 4C swath survey across the Pamberi‐1 well location in the Lower Reverse L block of the Columbus basin, eastern offshore Trinidad. The motivation from EOG for attempting this was because a conventional 3D towed‐streamer survey acquired the previous year failed to adequately image the target reflectors comprising the reservoir under the main growth fault. Details of the P‐ and PSv‐wave processing of this dataset through anisotropic prestack time migration were previously described by Johns et al. (2006) in which it was demonstrated there existed a qualitative correlation between derived parameters and attributes from P and Sv anisotropic migration velocities, overpressure and known regional geology. This observation was quite remarkable considering that only a limited effort to validate/constrain parameters (in this case, velocity fields to the Pamberi‐1 well checkshots) was performed. Under the “Future work” section of the previous publication, it was suggested that further data quality enhancement in preparation for more quantitative rock property classification, calibrated to wells, could only be achieved after prestack depth imaging. In this paper, we present precisely that next phase in the 4C processing, advancing the P‐ and PSv‐wave data through anisotropic prestack depth migration, using cell‐based tomography with a top‐down layered approach. The Pamberi‐1 well was used to constrain the anisotropy in the shallow section, with the deeper spatial trend away from the proximity of the well determined from the anisotropy derived previously in the time processing. Prior to proceeding with the anisotropic depth imaging, the magnitude of shear splitting (or, birefringence) from the presence of azimuthal anisotropy (HTI) is first examined to assess its potential impact on the radial rotated P‐S signal. The shear‐wave splitting analysis revealed a principal angle of polarization that was closely aligned with the regional stress direction delineated by the major faults blocks acting as pressure seals.