Removal of Overburden Channel Effects through Channel Velocity Modeling and Prestack Depth Migration for an Oil Field Offshore Abu-Dhabi

摘要

To generate refined depth structure for a very gentle Cretaceous reservoir on an oil field offshore Abu-Dhabi,we conducted channel velocity modelling and prestack depth migration (PSDM).The target reservoir structure on a vintage prestack time migration (PSTM) contains pull-up and push-down artefacts,induced by a couple of overburden channel velocity fills.These artefacts hindered correct representation of gentle depth structure at the target.Detailed interpretation on the PSTM volume delineates overburden channels in three upper Cretaceous formations.One of them,Fiqa channel,generates small pull-up and the others,Halul and Lafan channels generate push-down below each formation,caused by higher and lower velocity channel fills,respectively.These artefacts overlapped at some locations.Analysis of synthetic PSTM stack from forward modelling reminded us that a low-velocity channel produces obvious push- down below the channel.The shape of push-down become smaller but wider with depth and offset.The resulted push-down in full-offset stack also changed with depth.A "true" channel velocity and implementation of PSDM eliminates push-down at all depths and offsets.Bearing in mind the phenomenon confirmed on the forward modelling,we first produced channel velocity model for the target field data.With the assumption that the depth structure at a layer just below channels is flat,time thickness of channel and the amount of push-down at the layer below the channel could determine the channel velocities.Then,we estimated three channel velocities by channel time thicknesses and the amount of pull-up/push-down at the top Mid-Cretaceous shale,Nahr Umr formation that located just below these channels.On the overlapped area,a linear inversion provided each channel velocity.An application of PSDM with the channel velocity model successfully mitigated the artefact at target depth structure.