Velocity modelling and depth conversion uncertainty analysis of onshore reservoirs in the Niger Delta basin

Abstract

Depth uncertainty is one of the major uncertainties associated with  hydrocarbon field development. This uncertainty mostly arises due to the complexity of the subsurface, paucity of data, time-to-depth conversion, seismic picks, fault positioning and well ties. These uncertainties explain the non-uniqueness of models built and can have a significant impact on fluid contact and hydrocarbon in-place evaluation. To manage depth  uncertainty, The  Polynomial and Vo_K method were adopted to build   velocity models for depth conversion and residual analysis for several  reservoir levels to determine the method that will give the best depth  residuals. Depth conversion residual analysis result of both velocity models for the reservoirs studied gave average depth residual of less than 50ft for reservoir levels below 9000ft. As the depth increases, the polynomial  method derived average residual becomes unreliable with depth uncertainty of over 100ft for the deeper MOT reservoir, compared to 11. 65ft of the Vo_K  method for the same reservoir. This was expected at depth since the polymonial method adopts average velocities while the Vo_K method uses instantaneous velocity. Hence, the latter is expected to give a better result at great depth during depth conversion and should be preferably employed for velocity modeling and depth conversion study of reservoir in the Niger delta Basin.

Key Words: Depth-Conversion, Velocity-Modelling, Polynomial-Function, Vo_K -Function, Niger- Delta.