Kwanza basin

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The Kwanza basin is located both on and offshore in Angola. It extends from Luanda, located in the north, to Cape Santa Maria farther South.[1] It is characterized by a thick basement and crustal-scale domain.[2] It is currently believed that the basin is mature on land and immature at sea. Many companies have completed 2D and 3D seismic surveys and continued to drill multiple exploratory wells.[1]

History of the basin

Tectonic History

Figure 1 A salt map defining the structure of the onshore and offshore parts of the Kwanza basin.[3]

The creation of the basin was initiated by continental rifting between both the American and African plates. It was formed due to the opening of the South Atlantic Ocean in the late Cretaceous period. It has faced 3 phases of tectonic evolution including rifting, transition, and drift phases. The rifting phase began in the southern area of the African continent as it continued to the North from the late Jurassic to the early Cretaceous period. As volcanic activity intensified, the continent began to crack. During the rifting phase, it went through two stages. In the syn-rift stage, the volcanic upwell caused the thinning of the crust, leading to the continental rift. This stage is usually classified according to strong volcanism and advancement of fault zones. Its highest sedimentary period was seen in the post-rift stage following this. During the transition phase, a large ocean basin was created forming the other known half of the Kwanza basin. At this time, the sedimentary thickness was around 600 meters. Lastly, the drift phase incorporated the formation of the Atlantic Ocean around the continent, the creation of oceanic crust, and sea levels began to rise. The basin was then split into two separate regions the Kwanza and Benguela sub-basin. Within the Kwanza basin it consists of the onshore basin and the offshore basin, as seen in Figure 1. Owing to rising sea levels, the African continent tilted causing uplift and erosion. Due to the reactivation of the basement structure, three fold-thrust belts were formed and were surrounded by rift-transfer fault belts.[4]

Depositional History

As river deltas began to develop, sedimentary deposits were carried into deeper water. With these two factors, the accumulation of marine sediments increased along the coast.[1] Carbonate reservoirs developed locally, and shales source rocks accumulated. These carbonate deposits and shale source rocks gathered they were deposited into the post-rift basin. An increase in the salinity accumulated a thick salt along with the spreading of the seafloor. Other deposited carbonates include late Aptian-Albian shallow sea carbonates deposited on the salt. Platform carbonates are in abundance in the basin.[5] The outer Kwanza Basin contains Cenomanian to Maastrichtian marine shales that deposited from deep-water sedimentation. The uplift of the inner Kwanza Basin led to the deposition of sediments into the outer Kwanza Basin.[6]

Primary geological risks and uncertainties

With minimal studies and exploration done within the kwanza basin, information is scarce. Evaporate rocks within the Kwanza Basin are far less developed than many other surrounding basins.[7] Within the basin depression zones, pockmarks, have been identified in relation to the fault of upward migration. With this comes the prediction of overpressure zones. These zones can bring stability issues that can cause difficulty for drilling operations within the offshore part of the basin. These findings have been the result of 3D seismic surveys performed.[8]

Petroleum elements

Figure 2 Shows the period associated with the lithology, formation, and tectonic stage of the inner Kwanza basin. [9]

Reservoir

In the formation, sandstone provides promising reservoirs trapped in various traps and seals through overlying shales.[5] Onshore and offshore reservoirs both carbonate and clastic type. Within the deeper water of the Kwanza basin, cretaceous to miocene turbidite reservoirs can be seen.[10]

Source rock and migration

The inner, post-rift, Kwanza basin is made up of the Cuvo, Lower Cuvo and Maculungo formations. The Cuvo and Maculungo formations are made up of tuffaceous rocks, shales and organic-rich shales. While the Lower Cuvo formation is made up of red sandstone, conglomerate rock, and claystone. In the syn-rift part of the outer Kwanza Basin source rocks can be traced back from the Neocomian to early Barremian periods. These rocks consist of sandstone, shale, organic-rich shale, siltstone, and minor limestone. Hydrocarbon source rocks in the outer Kwanza Basin consist of organic rich shales, abundant in Type I and Type 2 kerogen. The post-rift rocks of the inner Kwanza basin are comprised of two formations. The lower Aptian upper Cuvo formation consists of dolomite, sandstone, and thin coals. While the Upper Cuvo formation is made up of coarse sandstone and conglomerate. In the outer Kwanza basin the post-rift rock is defined by the Upper Cuvo. Which is comprised of siltstone, sandstone, limestone and shale. Figure 2 shows the source rocks of the inner Kwanza basin, the time period, formation and tectonic stage of the rocks. Migration passageways are heavily fault related within the Kwanza basin and act with lateral oil-migration.[11]

Traps and seals

The majority of the traps on shore and offshore fields are anticlinal. Within the turbidite sandstone it is predicted that stratigraphic and structural traps are present with shale seals. The reservoir seals are both Cretaceous and Tertiary shales. Within the outer Kwanza basin tertiary sandstone represent deposition in turbidite channels along with sandstone in delta and basin floor fan settings. Within these marine shale that has been interbedded acts as the reservoir seals. [10]

Petroleum and facility engineering

Four pre-salt discovery wells were previously drilled in the Kwanza basin. In 1983 the first well was drilled Denden 1, following were Baleia 1, Cameia 1 and Azul 1. Azul 1 being the first deep water pre-salt discovery. Cameia 1 discovered an oil column and sustained 5010 bpd of oil and 14.3 MMcf per day of gas. Following this Cameia 2 was drilled and found another high-quality reservoir.[6] With the current increase in exploration work, 23 exploratory wells have been discovered. The results of exploratory wells show the key elements of the petroleum system. A 75M net oil layer was discovered during the operation of the orca oil field in 2014, which is the largest sub-salt discovery in the Kwanza Basin. Various tests have shown that large carbonate rafts are most likely to be blocked by the shale in the post-salt part of the basin.[11]

Drilling offshore in Angola[12]

Future petroleum potential

The Kwanza petroleum system has a total mean of undiscovered oil of 21,715 MMBO. Porosity and permeabilities within the Kwanza reservoir rocks range from 20-40% porosity and permeability of 1-5 Darcy. Most previous exploration work that has been done focused on the onshore parts of the basin. Today only a few moderate sized oil fields have been discovered in the basin. With the lack of exploration, offshore, brings potential for larger oil fields to be discovered. The further south in the basin the likelihood of hydrocarbon production is limited due to the lack of large amounts of sediment. It is predicted in the southern Kwanza basin there is a lack of sedimentary reservoirs and source rocks have not had enough time for maturity, therefore not been buried deep enough. A geologic model had been created for assessment of oil and gas in the Kwanza basin. The geological model states oil and gas has been generated from lacustrine shale and marine shale. Source rocks are around 3.1 perfect TOC, while some may have TOC contents greater than 20 percent. With hydrocarbon generation starting in the Late Cretaceous and continue on today.[10]

See Also

https://www.geoexpro.com/articles/2015/03/angola-kwanza-basin-exploring-further-and-deeper-for-oil-and-gas

https://www.beg.utexas.edu/agl/animations/AGL96-MM-003

https://www.searchanddiscovery.com/abstracts/html/2018/lisbon.90325/abstracts/29077333.html

References

  1. ↑ 1.0 1.1 1.2 https://academic.oup.com/jge/article/17/6/956/5911472
  2. ↑ https://www.researchgate.net/publication/328630931_Tectonic_evolution_of_the_Kwanza_basin_offshore_Angola
  3. ↑ [1]
  4. ↑ https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/114/10/1222/183795/Structural-segmentation-inversion-and-salt?redirectedFrom=fulltext
  5. ↑ 5.0 5.1 https://www.geoexpro.com/articles/2020/11/unlocking-hydrocarbon-prospectivity-on-the-angola-kwanza-shelf
  6. ↑ 6.0 6.1 [2]
  7. ↑ https://reader.elsevier.com/reader/sd/pii/S1876380408600865?token=AF838B9F80A89344B225488B28153890B383325E04E4A579F33ECA8A04E124D0F22D1A2330CC5144764E6E468B87174E&originRegion=us-east-1&originCreation=20211215053924
  8. ↑ https://ntnuopen.ntnu.no/ntnu-xmlui/handle/11250/240538
  9. ↑ [3]
  10. ↑ 10.0 10.1 10.2 [4]
  11. ↑ 11.0 11.1 [5]
  12. ↑ https://furtherafrica.com/2021/07/23/angola-on-track-for-predicted-oil-sales-in-2021/
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