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

Abstract

Abstract

Introduction

Offshore sub-salt seismic imaging along the West African margin is a challenge in many areas, with seismic depth imaging alone facing certain limitations. In an effort to further de-risk deep structure, we detail here integration of seismic imaging with gravity data in 3D with methodology and examples. We hope to demonstrate that the incorporation of an independent geophysical parameter, such as density can effectively help to better de-risk deep sub-salt targets. This work and the structure identified has been validated by drilling, with hydrocarbons found – work is ongoing to extend the methodology to new targets in the area to expand this important sub-salt play offshore Gabon.

Methodology

Firstly, 3D forward gravity modelling was performed to build confidence in the sub-salt structures seen, but not confidently imaged in seismic data. Alternative scenario testing by forward modelling and inversion confirmed that the density anomalies could not be justified by shallower variations or other post-salt structures.

Secondly, conversion of a series of seismic velocity cubes used to improve the sub-salt imaging to density cubes for 3D forward gravity calculation and comparison with the observed gravity data. This conversion and comparison was aimed at helping understand which of a number of seismic velocity models was more globally consistent with respect to the observed gravity data. The high density contrast between the salt and carbonates produced a large amplitude gravity signal that can be readily modelled – enabling better discrimination.

Conclusions

Coincident gravity anomalies and seismic base salt closures were observed in this locality. Various efforts were made to justify the gravity anomalies seen as structures other than sub-salt with no success. 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.

Note that the north-west target 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.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016