Sub-salt Modelling in 3D - Integration of Seismic, Well and Gravity Data Validated by Drilling

Offshore sub-salt seismic imaging along the West African margin is a challenge in many areas, and with complex salt geometry, seismic depth imaging alone faces certain limitations. In an effort to further de-risk structures sub-salt, we integrated with gravity data in 3D. We detail that the incorporation of an independent geophysical parameter, such as density can effectively de-risk these difficult targets.

Coincident gravity anomalies and seismic base salt closures were observed in this locality. The gravity anomalies are larger in amplitude and not equivalent to any possible post-salt structure, independent of density. In short, the scenario gravity modelling suggests that these are indeed sub-salt structures, most likely localised basement or some other high density closures – which is consistent with the seismic imaging.

Additional modelling of the various velocity cubes used for sub-salt seismic imaging in 3D using gravity have been used to globally select the ‘best’ velocity model to better constrain deep seismic imaging. Note that one of these targets has since been drilled, intersecting gas/condensate in sandstone reservoirs validating the sub-salt interpretation and the petroleum system in this area. Additional sub-salt structures identified using this methodology are currently being evaluated by new seismic, gravity and magnetic acquisition.