Improving sub-salt imaging using 3D basin model derived velocities

Abstract

The quality of depth imaging is directly related to the accuracy of the underlying velocity model. In most sub-salt settings, lack of angular illumination severely degrades the resolution and accuracy of velocity information derived from the seismic data itself. A standard approach for building a starting velocity model uses more reliable velocity information outboard of salt which is subsequently extrapolated to populate the sub-salt regions. The shortcoming of this method lies in the assumption that the effective stress observed outboard of salt can be extrapolated beneath salt solely as a function of depth below mudline. Velocities derived from basin model effective stress data show excellent large-scale agreement to seismic velocities. 3D basin modeling represents the most rigorous approach for determining sub-salt effective stresses because it accounts for depositional history, sand distribution and connectivity, variable shale properties, timing of salt emplacement – all of which influence the magnitude and distribution of abnormal pore pressure. In the presented study a calibrated present day effective stress cube from a 3D basin model was used to calculate sub-salt velocities for an exploration well in the Central Gulf of Mexico. The comparison between basin modeling derived velocities and the original velocity field showed remarkable differences, particularly in that the former indicated a far more complex sub-salt velocity distribution than the latter. Utilizing basin model sediment velocities to re-migrate the seismic data set resulted in a more detailed image as well as changed source rock geometries. Where the original image showed a deep de-focus at the source rock level, the new geometry would focus hydrocarbons into the identified trap. Based on the results of the subsequently drilled exploration well, it has to be assumed that the re-migrated data, aided by basin modeling derived velocities, provide a more accurate image of the sub-salt geology.