AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
New 3-D Depth Imaging Reveals Insights into the Evolution and Pre-salt Prospectivity of the Angolan Kwanza Basin, West Africa
(1) WesternGeco Schlumberger, Gatwick, United Kingdom.
(2) New Digit Geoscience, LDA., Luanda, Angola.
In 2006 the discovery of the Tupi field deepwater offshore Brazil proved the presalt play potential. Subsequent presalt discoveries demonstrate this to be truly a world class exploration play. Interest has extended to follow this trend along the conjugate Atlantic margins. The Cretaceous synrift and sag phase of the Kwanza Basin has already yielded hydrocarbons along the basin margin.
However, this basin will not give up its riches easily. Deep drilling through thick salt in deep water will be expensive. Our challenge at the start of this project was to image the presalt beneath the complex salt. Subsalt 3D seismic imaging has advanced rapidly due to the need to image subsalt in the Gulf of Mexico where another set of exciting plays are being developed. In the Kwanza basin we see two additional challenges. First, the presence of the Albian carbonates and second, velocity anisotropy, stronger than that observed in the Gulf of Mexico studies. These are particular issues when sediments are tilted due to movement of the underlying salt. Today we are seeing the benefit of detailed analysis of the overburden as we strip away its effect to reveal the true shape of the top salt and subsequently reveal the depositional environment of the presalt.
Beneath the salt we now interpret a 3D clear image of the Barremian age sag basin that followed the Rifting of the basin. The base of the salt exhibits very little relief giving us confidence in the overlying velocity control. Further this indicates a large relatively flat area during deposition of the salt layers. The deepwater area has almost no well control. The horsts are clearly imaged allowing the boundary at the top of the synrift sediments to be inferred. We observe up to 6 kms of presalt sediments. This agrees with thickness of 5km for presalt sediments of the Santos basin in Brazil. The seismic quality is good and allows us to image the base of the crust to a strong reflection, 13-14km deep, interpreted as the Moho. Within the synrift sediments we map saucer-shaped geometries that probably indicate volcanic sills. This is consistent with a thinning of the crust and intrusion up fault zones where some appear listric detaching at the Moho.
The presence of thick salt and a thin crust suggests petroleum system modelling will play an important role in the future exploration of the basin. Our evidence to date indicates the play does indeed look similar to that proven in Brazil and is truly worthy of further study.