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Added: 15th July 2019 by CGG
One of the last underexplored regions in West Africa is the deepwater area of the South Gabon Basin.
Successful exploration offshore Brazil, which prior to the Atlantic Ocean opening was connected to this part of Africa, has sparked renewed interest in deepwater pre-salt plays in the whole offshore southwestern region of Africa, including Gabon, with significant discoveries offshore Angola and Congo.
CGG has acquired and imaged data for our MCNV library throughout this region with 25,000 km² of continuous broadband 3D marine seismic in Gabon alone (an area larger than Wales). Even before the final image was generated the process of building the earth model required to accurately image this data gave significant insights into the geology of the area.
The formation of the South Gabon basin started in Early Cretaceous times with the opening of the South Atlantic rift splitting Africa and South America. River and lake sediments deposited during this time (syn-rift sediments) now form today’s primary hydrocarbon source and reservoir rocks at depths of more than 4000 m.
In later times, arid climate conditions prevailed with high evaporation rates and a thick salt layer was deposited as sea waters progressively made their way through the area. This salt is now heavily deformed due to the gravitational sliding oceanwards of post-rift sediments overlying the salt and forms a major seal across the basin, stopping and trapping hydrocarbons from reaching the surface.
Shallower, younger sediments include remaining pieces of a broken-up post-rift carbonate shelf that are now sliding on the salt. These so-called carbonate rafts have much higher seismic velocities than surrounding sediments. Sand-filled turbidites (deposited by prehistoric seafloor turbidity currents) punctuate the otherwise benign low velocity Tertiary sediments.
Finally a few hundred metres beneath the sea floor where the sediments are very unconsolidated and actively dewatering at present (and as a result have seismic velocities not much higher than water) we see gas pockets and escape conduits. These are often co-incident with the crests of the deeper salt diapirs (indicating the gas probably originates from beneath the salt and the movement of the salt allowed it to escape) and are capped by gas-hydrates which transition from frozen to unfrozen as the temperature increases in the earth with depth.
To capture this level of geological detail in our velocity model for imaging we used a technique known as Full Waveform Inversion (FWI) . FWI is an inversion algorithm based on the wave equation, which is designed to iteratively update an initial starting earth model until the difference between synthetic data generated by forward modelling through the model and the actual recorded seismic data is minimised. FWI is a far more powerful tool than conventional refraction and reflection tomography techniques, which only use the travel-time kinematics of the seismic data, because it is able to use the additional information on velocity, density and other rock properties provided by the amplitude and phase of the seismic waves.
The image shows a depth slice taken through the data at 3700 m for: (a) starting velocity model, (b) FWI velocity model, (c) seismic image migrated with FWI velocity model, and (d) the velocity perturbation or difference betweeen the starting and final models. The inset in (c) shows the approximately 1000 km² displayed relative to the full area of the survey. The oval yellow shape in (c) is missing data that was filled in later.
This depth slice was chosen as being typical for the volume, where we see an increase in resolution and a number of interesting features coming through in the velocity model after FWI. For example, we see prehistoric channels containing high-velocity material that are being very clearly picked out by FWI and, in general, are easier to see in the velocity model than in the seismic itself.