The Atlantic margin of Angola lies within the heart of the West Africa salt province, extending from the north of Gabon to the volcanic Walvis ridge in Namibia. The Angolan margin comprises three major basins; the Lower Congo, Kwanza-Benguela, and Namibe basins, where Mesozoic sedimentary wedges, including salt, extend into the present-day onshore.
The geological history of the Lower Congo, Kwanza-Benguela, and Namibe basins share structural and stratigraphic characteristics typical for Late Jurassic to Early Cretaceous rifting of the proto-Atlantic. In broad terms, the petroleum systems of Angola can be split into two main systems, pre/syn rift and post rift.
The pre/syn rift is characterized by a phase of extensional rift tectonics that ended with the deposition of transitional phase carbonates and the Loeme Formation salt. Three main syn-rift/presalt reservoir rocks have been identified:
- Pre rift Lucula and syn rift Red Cuvo/Lower Cangulo Formation alluvial sandstones
- Berremian aged Toca/Grey Cuvo/Upper Cangulo Formation lacustrine carbonates
- Aptian Chela Formation and equivalents fluvial sandstones
The main source rock within the syn-rift/presalt sections is the Barremian Bucomazi Formation and deeper syn-rift sediments, which charges both pre and postsalt traps. These thick sequences of fluviatile-lacustrine shale dominated sediments were deposited in narrow, deep lakes with anoxic bottom waters.
The most important seal for the syn-rift/presalt sections is the Loeme Formation salt, with the Bucomazi Formation shales acting as a secondary syn-rift seal.
The post rift sees a shift from restricted/evaporitic facies to open marine lithologies. The transitional phase is characterized by extensive erosion and deposition of sandstone, silts, and dolomites comprising the Chela Formation source rock, overlain by thick evaporites. Reservoirs are commonly found in four main units:
- Late Cretaceous Iabe Formation deep marine sandstones
- Paleogene Landana/Quifangondo Formation deep marine sandstones
- Neogene Malembo Formation deep marine sandstones
The Iabe and Malembo formations seal post-rift reservoirs across all three basins. In the Lower Congo Basin, deltaic facies of the Cirques Formation are trapped by faulted basement blocks.
Major source rocks, reservoirs, and seals of the three major basins are outlined in the stratigraphic chart below. Click on the image for a larger, downloadable version also containing a map of the three major basins.
As PGS expands into frontier areas, exciting opportunities across offshore Angola are to be explored with high-quality datasets. Key basin opportunities are presented in the following sections.
The Overlooked Present-day Shelf of the Kwanza Basin
The present-day shelf of the Kwanza Basin has been overlooked in recent exploration cycles. Of the 23 wells on the shelf, 18 of these were drilled prior to 1993. Two exploration wells were drilled on 3D data, with the remainder on 2D data of various 1980’s vintages. Modern GeoStreamer data acquisition on the shelf reveals considerable uplift from vintage data (see the slider below). Improved understanding of the salt presence and complexity is key to evaluation in this area.
Analysis of well results demonstrates that the key elements of a petroleum system are present. Presalt carbonate reservoirs have porosities of up to 27%, with dolomitization providing significant secondary porosities. In the Albian section, carbonate rafts provide numerous untested leads and prospects analogous to the producing fields of the Sendji carbonate play in the Lower Congo Basin.
Comparison between MC2D PSDM GeoStreamer data (2012) and MC3D Kwanza Shelf Fast Track PSDM showing significant imaging uplift.
Opportunities for Super-giant Fields in the Undrilled Namibe Basin
The frontier Namibe Basin 2019 license round offered acreage to explorers for the first time. Onshore outcrops of the presalt succession, along with oil seeps geochemically identified as originating from a presalt source rock, provide a tantalizing hint of the untested hydrocarbon potential of the Namibe Basin.
PGS acquired a regional 2D survey over the Namibe Basin in 2011, providing oil companies with the first indication of thick presalt sections in multiple rift basins, overlain by laterally extensive salt deposits. PGS subsequently acquired a MC3D survey in 2014. This confirmed the extent of the salt, showing excellent illumination of postsalt Albian rafts underpinning Upper Cretaceous and Tertiary deep marine clastics. Both the pre- and postsalt sections provide the opportunity for super-giant fields of the future.
New MultiClient 3D acquisition in 2020 will cover blocks 29, 30, 44, and 45 to provide a continuation of PGS’ footprint in the Namibe Basin, extending south into Namibian waters. With high-resolution seismic data, that derisks petroleum systems and plays, it is only a matter of time until the first discovery is made.
The Overlooked Tertiary of the Deepwater Kwanza Basin
Exploration of Kwanza deepwater and ultra-deepwater has predominantly targeted presalt prospects with only one known Upper Cretaceous target, the Semba Field in Block 24. Progradational units of the Turonian-Coniacian Itombe Formation (equivalent to the Iabe Formation) contain sandstones interbedded with limestone and dolomite that show varying reservoir properties. Considerable remaining potential is therefore expected within the postsalt succession, which exhibits extensive mass flow sand deposits in seismic data, not dissimilar to those of the Lower Congo Basin.
Of the Early Cretaceous, a lacustrine carbonate shoal facies deposited along the Lower Congo Basin margin was encountered in multiple wells in the Kwanza Basin and found to have good reservoir properties. These carbonates, which are equivalent to the Toca Formation, are trapped by basement faulted structures.
Other potential reservoir units include fluvial and lacustrine intervals of the Grey Cuvo Formation (equivalent to the Chela Sandstone of the Congo Basin), trapped by basement faulting and stratigraphically sealed by interbedded marine shales or overlying salt and Miocene Quifangondo Formation and Cirques Formation equivalent sandstones. These are comprised of stacked turbidite channel sandstones with excellent reservoir properties.
The Undrilled Presalt of the Deepwater Lower Congo Basin
The Lower Congo Basin extends from southern Gabon, through Congo into Angolan waters. The earliest exploration wells of the Lower Congo Basin targeted both presalt and postsalt plays of the shallower waters with considerable exploration success. As exploration ventured to deeper waters, the focus turned to the postsalt, particularly the Miocene clastic succession which, with over 100 discoveries, is the most prevalent target within the basin. These sandstones form stacked turbidite channels, deltas, and deep marine fan complexes that are sealed by overlying/interbedded claystones juxtaposed by diapiric salt.
The post-rift Albian sandstone reservoirs of the Pinda Formation were deposited in sabkha to shallow-marine environments during the initial flooding of the basin. These cyclic sandstones, dolomites, conglomerates, and evaporite units are sealed by overlying carbonates and marine shales, and structurally within roll-over anticlines. The Late Cretaceous Iabe Formation contains numerous sandstone reservoirs, primarily trapped within roll-over anticlines, combined with growth-faults and sealed by marine shales and proximal diapiric salt.
Beyond the present-day shelf, the Albian and presalt intervals of the Lower Congo Basin are undrilled with little exploration of the Late Cretaceous. Oil company focus is expected to extend to these plays in the Lower Congo Basin in the coming years, with considerable remaining potential anticipated.