Industry Article: Benefits of TTI PSDM for reservoir evaluation offshore West Africa

Prestack depth migration is now routinely applied to improve the structural and stratigraphic imaging of complex structures,but also in cases where dips are gentle.  State-of-the-art migration algorithms, in combination with accurate velocity models that have been calibrated with well data, yield detailed images which can be used for reliable extraction and mapping of reservoir attributes. We present a case study of a Cenozoic field, located offshore West Africa, which was imaged initially using prestack time migration and subsequently using anisotropic prestack depth migration with vertical transversely isotropic and tilted transversely isotropic velocity models.  A turbiditic channel reservoir is located on an anticline structure with dips up to 40 degrees. Imaging with vertical transverse isotropy shows a structure which is too narrow, residual moveout that is difficult to account for, and significant well misties.  Migrating with tilted transverse isotropy, refined by a further tomographic velocity inversion, produces a more realistic image of the structure, in agreement with well data, with better definition of the reservoir. Amplitudes can now be used for improved characterization of the reservoir, uncertainty in fluid contact location is reduced, and future development wells can be located with greater confidence.

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