Application of in-situ anisotropic parameters in 3D seismic velocity analysis for improved pre-drill pore pressure prediction in the onshore Niger Delta basin, Nigeria

Abstract

The inaccurate prediction and characterization of anisotropic properties of hydrocarbon reservoirs during seismic processing have been a major obstacle to optimal time-to-depth conversion, well-to-seismic ties, and a robust fit-for-purpose velocity model for accurate seismic pore pressure (SPP) imaging. Though some levels of exploration successes have been achieved using isotropic velocity models in parts of the Niger Delta Basin, serious challenges however abound at the deeper shale-dominated intervals of the basin because of the intrinsic anisotropic properties of shale. The application of in situ anisotropic parameters in 3D seismic velocity analysis for improved pore pressure prediction in the study area was carried out with the objectives of applying the appropriate anisotropic parameters for optimal processing of seismic velocity field for overpressure studies. Anisotropic parameters for vertical transverse isotropic (VTI) wave-field were estimated from available wire-line logs using standard empirical relationships and were used to correct for shale anisotropy on a 6.0-km offset seismic data. The migrated velocities were further converted to geological depth and calibrated to wells. Isotropic and anisotropic pre-stack depth migrated (preSDM) velocities were inverted to seismic pore pressure prediction. Results of the study revealed that the anisotropic preSDM velocities yielded higher resolution in the vertical and lateral positioning of geological features when compared with the isotropic velocity model. Also, the seismic velocity fields generated from the anisotropic preSDM seismic velocities were observed to optimally tie to the well and accurately imaged the reservoir pore pressures at deeper intervals. These findings, therefore, revealed the appropriateness of the applied anisotropy parameters (−0.05 ≤ δ ≤ 0.05 and 0.08 ≤ η ≤ 0.48) in predicting elasto-thermal properties of the area. These parameters can therefore be incorporated into anisotropic velocity modeling and processing of long offset seismic data for accurate prediction of pore pressure, lithological, and fluid properties of reservoirs in the area.