Deepwater Geohazard Analysis using Prestack Inversion

De Kok, R., Dutta, N., Khan, M., Mallick, S.

71st SEG Meeting, San Antonio USA, 10-14 September 2001.



Drilling for deepwater targets is associated with high cost and risk, while margins of commercial operations are small. Therefore it is imperative to control costs through accurate well planning and reliable anticipation of geohazards. Both in shallow and in deeper zones, P- and S- velocities were determined using seismic pre-stack waveform inversion. Shallow waterflow (SWF) layers in the deepwater Gulf of Mexico were identified through the associated high ratios of P- to S- velocities and verified with well control. A new rock-model based approach especially suited for deepwater pore pressure imaging was applied to predict the presence of deeper over-pressured zones. For the deeper zones, the predicted velocities were verified with log measurements. Predicted and measured pore pressure gradients were found to agree within half a pound per gallon equivalent mudweight. The shear velocity and the extracted shear modulus, proved to be excellent indicators of low effective stresses, corresponding to over-pressured formations.