Generating velocity information for pore pressure prediction in the Niger Delta using post-stack inverted seismic amplitude data
Surface prediction of pore pressure on inverted seismic sections has been carried out in this study using a 3-D seismic amplitude data obtained from a field (Field YYY) in the coastal swamp of the onshore Central Niger Delta, Nigeria. In this study, robust velocity information with sufficient vertical resolution has been extracted from 3-D seismic amplitude data to help diagnose the subsurface shale that sometimes suffers from mechanical compaction disequilibrium, resulting in the overpressure that occurs in the Niger Delta. The 3-D amplitude dataset was inverted using post-stack seismic inversion technique that takes flexible advantage of the shale-sand-shale lithology of the geology of the Niger Delta. The seismic dataset was integrated with well data (sonic and density logs) obtained from the same field, enabling building of a high resolution 3-D p-impedance earth model from which the velocity information was generated. The inputs to the 3-D p-impedance earth model building and actual sparse spike deconvolution/inversion include p-sonic, density log, seismic synthetics, time interpreted geologic horizons, estimated wavelets from the well with the same ranges of frequency, amplitude and phase spectra. The seismic synthetics were coherently matched with the recorded seismic gather while the parameters of the inversion were further modified and controlled to ensure that reasonable results were obtained. The time window for the inversion was set between 1350ms and 1950ms. From the results obtained, the inverted Vp clearly diagnosed the shales encasing the reservoirs around the 1460ms and 1900ms horizons. The encasing shales showed relatively higher values of Vp on the inverted Vp cross sections between 3320m/s and 3645m/s. Overpressured zones are usually characterized by low velocities. The observed higher values of Vp thus showed that the encasing shales around the reservoirs have normal pressure or do not contain overpressure, implying that the shales are well compacted by the high effective stress acting on them. Wells drilled through the shales and encased reservoirs will be stable and not prone to blow-out.
Keywords: Pore Pressure, Geopressure, Post-stack, Inversion, Velocity