Velocity anisotropy in the Niger delta sediments derived from geophysical logs. – D.O. Ogagarue, J.O. Ebeniro and C. N. Ehirim.
An investigation of velocity anisotropy was carried out using density and dipole sonic logs comprising compressional and shear interval transit times, acquired between 2000 and 2002 in some well-spaced petroleum wells in the western part of the Niger Delta. The aim of the study was to understand how seismic velocities vary laterally and vertically within the sediments, and obtain a trending of the velocities. The anisotropy was modeled by deriving stiffness constants at some depths in each well and relating them to the anisotropic parameter , and . The study shows that the Niger Delta sediments are clearly anisotropic; the values being higher in shale’s than in sands. Average P-wave anisotropy is concentrated between -3.5% and 5.7% while near-vertical P-wave anisotropy is concentrated between -3.5% and 4.9%. The localized S-wave anisotropy is higher and concentrates between -4.3% and 7.4%. Seismic velocities decrease and increase laterally and vertically respectively, towards the coast. These variations are attributable to the lateral and vertical change in the degrees of compaction coastward and reduction in porosity with depth. Three zones of steep, moderate and slow velocity gradients, respectively, have been identified in the study area. The velocities generally decrease SW towards the coastline and also towards the southern part of the delta. This decrease can be attributed to a decrease in sediment compaction and the presence of synsedimentary structures, clay diapirs and other structural features which become more pronounced coastward.