Application of velocity feature analysis and large-scale variable velocity mapping to a low exploration maturity basin: A case study of the MSGBC Basin in West Africa

With the MSGBC basin in West Africa chosen for a case study, the basin evolution and stratigraphic structure characteristics thereof were elucidated, the macroscopic features of the formation velocity were analyzed, the characteristics of the formation velocity distribution during the basin drift stage were summarized, and the key factors influencing the velocity distribution were analyzed herein.The results show that the MSGBC basin is characterized by a laterally low formation velocity in the northwest deepwater region, which becomes high in the southeast shelf region.In the vertical direction, the formation velocity is characterized by two stages.The following key factors were found to influence the velocity distribution: burial depth and compaction, formation lithology, combination of geological era, and formation thickness.Considering that the MSGBC basin covers a wide area, does not have a succession of three-dimensional (3D) seismic data, has limited and sparse well logging and two-dimensional seismic data, and various 3D velocity model building methods are inapplicable, a zonal well-controlling interpolation and variable-velocity fitting stripping method was proposed to construct the interval velocity model.Based on the seismic interpretation, vertical unitization and lateral zonation were employed to build the velocity model unit.Moreover, well velocity interpolation or the variable velocity fitting method was used to establish the model velocity.Subsequently, after stripping of the calculation, basin-scale variable-velocity mapping was performed.From the results of the large-scale time-depth conversion, a more accurate understanding of the lateral structural and stratigraphic patterns of the basin was achieved, and the hydrocarbon source kitchen of the basin was predicted more reliably.This improved understanding can facilitate a rapid search of structural traps in the deep water and shelf-slope break zones of the basin.This research therefore provides an important reference for large-scale variable velocity mapping in a low exploration maturity basin.