Relevant Velocities in Seismic Prospecting and High Resolution Petroleum Exploration in the Niger Delta and Adjoining in-Land Sedimentary Basins, Nigeria

SUMMARY The algorithm of finding relevant velocities and applying appropriate velocity types to static and dynamic corrections, as well as to data processing culminate into optimum Signal/Noise ratio of data, and subsequent accurate interpretation of subsurface structures. Velocity is an important tool in petroleum exploration, and specifically in seismic prospecting. It is a parameter that measures the total distances travelled per given time (in seconds or milliseconds). In seismic exploration, the velocities are measured as the waves travel from the energy source point through the layers of the subsurface. The wave returns to the surface on encountering a reflector or impedance and is detected by the geophones or hydrophones. Seismic velocities include weathering velocities and refractor velocities which underlie the former. They are associated with the near-surface components of the earth. These velocities are essential in the determination or computation of static and dynamic corrections. They also have indications of changes in lithology, porosity, pore fluids, relative bulk-densities, density contrast across lithologic boundaries, and age of sediment. These velocities among others fall into spectrum of velocities comprising, average velocities, root-mean-square (rms) velocities, interval velocities, stacking velocities and semblance velocities, which are profoundly applicable in seismic data processing and subsequent interpretation. Velocity distribution in the subsurface region is derived from stacking velocities. These are obtained from common-depth point (CDP) stacks. The Root-mean-square (rms) velocities are obtained from stacking velocities. The interval velocity values are derived or computed from the rms velocities. These derivations are contained in the velocity analysis algorithm that evaluates and identifies significant lithologic layers, and also provides the amount of normal move-out which should be removed to maximize the stacking of primary events or reflections. A velocity spectrum is generated that brings out level planes of constant coherency. This reveals the location of peaks on the coherency surface that correspond to primary reflections, characterized by identical velocities. These peaks are then suitably joined to obtain an average rms velocity versus time function display. The most popular statistical measures for coherency attribute are cross-correlation and semblance. The contemporary industry practice of multichannel coherence measurement is carried out using the semblance analysis method. This produces a velocity function that maps out increasing magnitude of velocities with increasing depths, which also defines velocities at given depths. Niger Delta and adjoining sedimentary basins were of age Eocene to Pliocene characterized by sediments that have matured from plenty organic deposits. In the subsurface, the layered deposits have yielded to effect of gravity and external forces which distort and modify the layered strata into faulted substrata with favourable structures for entrapment of hydrocarbons.