The Pre-salt Succession of the Kwanza Basin: New Insights into Hydrocarbon Prospectivity

The Kwanza Basin, northern Angola is relatively underexplored compared to other sedimentary basins along the West African margin north of the Walvis Ridge. Exploration success in the basin has been fairly limited since the 1960s but the major recent pre-salt deepwater discoveries in the conjugate Santos and Campos Basins have stimulated renewed interest in the Kwanza Basin. The greatest potential occurs within the thick pre-salt succession but appraisal is hampered by limited well penetrations and poor resolution of seismic data beneath the thick Aptian salt horizon.

Early Berriasian syn-rift sediments were deposited in N-S trending depocentres where potential reservoir quality fluvial sandstones developed. From structural activation histories, a framework of active uplifts was built for palaeogeographic reconstructions and the distribution of the syn-rift facies predicted. As rifting continued, deep lakes developed in a NE-SW trend in hanging wall depocentres fringed with reservoir quality lake margin carbonates and fluvio-deltaic sandstones. Analogues of the stromatolitic, algal and lake margin facies seen in the conjugate Campos and Santos Basins may be developed, although the extent of syn-rift carbonates is unknown. Reservoir quality within the limestones may have been later enhanced by sub-aerial exposure and diagenesis.

Good Barremian pre-salt syn-rift potential source rocks are documented within the Lukunga Shale Formation, Maculungo Member and Falcão Formation. The TOC and HI data for the upper Lukunga Shale suggest dilution may have been more important than preservation in determining the source potential. Where dilution was apparently lower, in the more calcareous Falcão Formation and Maculongo Member, the sediments are as rich as those seen in the Bucomazi Formation. Based on analogues on the West African and Brazilian margins, oil-prone late Berriasian-Hauterivian lacustrine source rocks were also probably developed in the deep offshore grabens. Up to ~1,000 m of mature oil-prone possible Neocomian source rock may occur within these structures.

Seals, traps and migration are a low risk within the pre-salt system with the presence of the thick salt layer, intra-formational seals, a mixture of structural, stratigraphic and combination traps, short migration pathways and the existence of carrier beds.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California