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The Kwanza Basin

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Background

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The Kwanza basin is a sub-basin in the southern region of the Angola Basin located in Western South Africa. The Angola Basin began formation soon after the initial breakup of the super continent of Pangaea in the early Cretaceous period. The Angola Basin is composed of two sub-basins: the Kwanza basin and the Lower Congo basin. The Kwanza Basin can be further divided into the inner and outer Kwanza Basins. The Ametista, Benguela, and Flamingo Platforms were responsible for diving the Kwanza Basin into the inner and outer sections.[1]

 
Figure 1: Map View of the Kwanza Basin

Formation of the Kwanza Basin

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The syn-rift stage of the Kwanza Basin is dominated by series of horst and graben basins, in which fluvial sediments were deposited.[2] The Kwanza Basin was formed during the time of Neocomian rifting of the Atlantic margin in South Africa. This event began at about 140 Ma and ended at about 120 Ma once the continent was completely separated. After continental separation was completed, marine conditions allowed for the precipitation of evaporites in the Kwanza Basin. Since then the basin has been characterized by the deposition of Cenomanian clastic sediments. Also, post-rift deposition is characterized by fluvial and transgressive lagoonal rocks. The Kwanza basin is divided into the inner (onshore) and outer (offshore) sections by a central platform where the Aptian salt layer was thin or absent.[3] The inner Kwanza basin has northward trending basement structures that enclose Interior salt basins. The outer Kwanza is a passive margin where gravity spreading is the main driving force. These two sections of the Kwanza Basin have very differing stratigraphy that occurred at the same time interval. The Basement structures that divide the inner and outer Kwanza basin are: Benguela, Flamingo, and Ametista Platforms.[4] These platforms listed comprise what is known as the Atlantic hinge zone.

Salt Tectonics

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The driving force of salt tectonics in this region is thought to be extension due to gravity. As a result of gravity spreading, down-slope contraction as well as up-slope extension occurs.[5] This is a possible explanation for all of the unique features and folds in this basin. Deep troughs were also formed as salt dissolved, allowing a large space for sediment in-fill. In the Kwanza Basin there are two major types of salt structures. First, there are narrow salt walls which developed as a result of salt-cored folds in the region. Also, broad salt walls formed in this area due to a major uplifting event along the Atlantic coast of South Africa.[5] .

 
Figure 2: Gravity spreading diagram

Source Rocks & Hydrocarbons

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The hydrocarbon generation in the Kwanza Basin began after the deposition of thick salt beds in the Cretaceous period.[6]There are six different source formations in the Kwanza Basin: Infra-Cuvi, Upper Cuvio, Binga Teba, Cunga, and Quifangindo.[7] The Infra-Cuvo (Neocomian-Barremian) formation is characterized by the presence of organic rich lacustrine shales that display Type I kerogen characteristics.[7] The Upper Cuvo (Early Aptian) contains marine/lacustrine shales and it displays both type I & II kerogens.[7] The Upper Cuvo is the most widespread of the six source formations. The Binga formation (Aptian-Albian) consists of shallow, organic rich, micritic shales that display a type Ii kerogen characteristics. The Teba formation (campanian-Maastrichtian) is characterized by marls and marine shales that display type II kerogen characteristics.[7] Finally, the Cunga and Quifangondo formations (Eocene-Miocene) are composed of very similar marine black shale. These formations display type II & III kerogen characteristics.[7] The hydrocarbons in this area are beneath an Aptian-aged evaporite layer located within a lacustrine carbonate reservoir.[1] More exploration is needed to know the exact types of hydrocarbons located below.

Site History

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The Angola Basin, which contains the Kwanza and Lower Congo Basins, was found the the 18th century by Portuguese colonists. Oil seeps and asphalt deposits were discovered and they used it as a material to seal ships from water leakage.[8] Oil production officially began in 1956 in the Benfica oil field. Present day companies, Maersk, Cobalt, and others, found many promising pre-salt discoveries in extremely deep water basins.[8] These deep water reservoirs are the future of the offshore Kwanza Basin oil field.

 
Map view of major oil producing areas in the Kwanza Basin

Production

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The Angola basin has been estimated to have over 8 billion barrels is crude oil reserves.[9] Most of these reserves are located in offshore areas of the Kwanza and Lower Congo Basin. Many new, recent pre-salt discoveries in the Kwanza Basin has led companies to increase drilling in the area. In January 2011, Angola announced that it awarded 11 pre-salt offshore blocks in the Kwanza basin, following a closed licensing round that a few selected IOCs were invited to.[9] IOCs that were awarded blocks include Petrobras, Maersk Oil, Cobalt, BP, Repsol, Total, Eni, ConocoPhillips, and Statoil.[9]

  1. ^ a b "GEO ExPro - Angola, Kwanza Basin: Exploring Further and Deeper". www.geoexpro.com. Retrieved 2016-12-03.
  2. ^ Brownfield, Michael (June 2006). "Geology and Total Petroleum Systems of the West-Central Coastal Province (7203), West Africa" (PDF). U.S. Geological Survey. Retrieved December 1, 2016.
  3. ^ Jackson, Martin (June 10, 2009). "Interplay of Basement Tectonics, Salt Tectonics, and Sedimentation in the Kwanza Basin, Angola" (PDF). University of Texas. Retrieved Dec 1, 2016.
  4. ^ Cazler, Edward (October 27, 2014). "Petroleum Geology of Cameia Field, Deepwater Pre-Salt Kwanza Basin, Angola, West Africa" (PDF). Retrieved Dec 1, 2016.
  5. ^ a b Hudec, Michael (October 2002). "Structural segmentation, inversion, and salt tectonics on a passive margin: Evolution of the Inner Kwanza Basin, Angola" (PDF). Bureau of Economic Geology. Retrieved Dec 1, 2016.
  6. ^ Greenhalgh, Jennifer (September 24, 2012). "Petroleum Plays and Prospectivity in the Kwanza and Benguela Basins of Offshore Angola" (PDF). Retrieved Dec 1, 2016.
  7. ^ a b c d e Fernanda, Rodrigues, Cristina. "An Evaluation of Source Rocks of the Kwanza Basin: A review". {{cite journal}}: Cite journal requires |journal= (help)CS1 maint: multiple names: authors list (link)
  8. ^ a b Koning, Tako (November 30, 2014). "Angola's Oil Industry - A Century of Progress in Exploration and Production" (PDF). Retrieved Dec 1, 2016.
  9. ^ a b c "Country Analysis Brief: Angola" (PDF). U.S. Energy Information Administration. May 18, 2016. Retrieved Dec 1, 2016.
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