Pelotas Basin

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Coordinates: 32°06′40″S 48°55′50″W / 32.11111°S 48.93056°W / -32.11111; -48.93056
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Pelotas Basin
Bacia de Pelotas, Cuenca de Pelotas
Field(s)
none (2017)

The Pelotas Basin (

states Santa Catarina and Rio Grande do Sul of Brazil and the departments Cerro Largo, Rocha and Treinta y Tres of Uruguay
.

The Pelotas Basin is one of the basins that formed on the present-day South Atlantic margins of South America and Africa due to the break-up of

clastic sediments. Other than the northern neighbours Santos and Campos Basins, the Pelotas Basin lacks a thick layer of salt and the pre-salt layer
pinches out just in the north of the Pelotas Basin stratigraphy.

Within the Brazilian Atlantic margin, the Pelotas Basin is relatively underexplored. Twenty exploration wells have been drilled in the Brazilian portion of the basin with one ultra-deepwater exploration well drilled on the Uruguayan side in 2016. No hydrocarbon accumulations have been proven in the basin thus far.

Etymology

The basin is named after the city of Pelotas, the hometown of Rodi Ávila Medeiros, the geologist who studied the area.[1]

Description

The Pelotas Basin is an approximately 346,000-square-kilometre-large (134,000 sq mi), mostly offshore sedimentary basin, located in the South Atlantic offshore Brazil and Uruguay. It covers the southern Brazilian states of Santa Catarina and Rio Grande do Sul and the Uruguayan departments Cerro Largo, Rocha and Treinta y Tres. About 40,900 square kilometres (15,800 sq mi) of the basin is onshore.[2] The onshore part of the basin in Uruguay is locally called Merín Basin.[3]

The basin is bound by the

Guaíba, Camaquã, Jaguarão/Yaguarón and Cebollatí Rivers.[5]

Tectonic history

The Pelotas Basin formed with the rifting of Brazil and Africa splitting the Río de la Plata Craton from the Kalahari Craton.
The break-up of Pangaea characterised the start of formation of the Pelotas Basin in the South Atlantic, forming at the same time the Namibia Basin in Africa.[6]

The basins of the South Atlantic margin started forming in the

rift basins bordering the present-day South Atlantic on the Brazilian and southwestern African sides. The Pelotas-Namibia spreading commenced in the Hauterivian, around 133 million years ago, and reached the Santos Basin to the north in the Barremian. Seafloor spreading continued northwards to the Campos Basin in the Early Albian, at approximately 112 Ma.[citation needed
]

Five tectonic stages have been identified in the Brazilian basins:[7]

  1. Pre-rift stage – Jurassic to Valanginian
  2. Syn-rift stage – Hauterivian to Late Barremian
  3. Sag stage – Late Barremian to Late Aptian
  4. Post-rift stage – Early to Middle Albian
  5. Drift stage – Late Albian to Holocene

While the basins to the north of Pelotas Basin, the Santos, Campos and Espírito Santo Basins, are characterised by a thick sequence of salt and an accompanying pre-salt layer, evaporites are almost absent in the Pelotas Basin.[8]

Stratigraphy

The basalts of the Serra Geral Formation are exposed in the Fortaleza Canyon of Serra Geral National Park.

The sedimentary succession of the Pelotas Basin is underlain by an extremely deformed (highly stretched, thinned and faulted) continental crust, covered by grabens that can achieve thicknesses of more than 20 kilometres (66,000 ft).[9] The nature of the lower crust below the Pelotas Basin remains uncertain, but by analogy with the Namibian conjugate margin, it may correspond to a high-density igneous crust-mantle interface intruded by the Tristan da Cunha plume.[7] The central onshore boundary of the basin is formed by the Neoproterozoic Pelotas Batholith.[10][11] The stratigraphy of the Pelotas Basin and the Punta del Este Basin to the south had a different history due to the Polônio paleohigh until the Late Maastrichtian.[12]

The basinal sequence starts with the Imbituba Formation, a unit consisting of

lacustrine. The formation is followed by the volcanics of the Curumim Formation, dated at 113 ± 0.1 Ma, and small incursions in the northern area of Pelotas Basin of the Ariri Formation, the evaporites that form the salt layer in the Santos Basin to the north.[16]

The rapid subsidence from the Albian to

transgression that lasted until the Oligocene. As in other areas of the Brazilian continental margin, the Late Turonian is marked by a regional unconformity.[18]

The Late Cretaceous to recent sequence is subdivided into the proximal Cidreira Formation, related to the fluvial progradation of the Rio Grande,[15] and a distal part known as the Imbé Formation, consisting of shales, intercalated with sporadic siltstones and fine-grained sandstones.[17] The transgressive series is interrupted by the presence of various Paleogene hiatuses, that appear also in the regressive Miocene to recent beds.[18] The unconformities have been analysed in detail using 87Sr/86Sr ratios.[19]

Age Formation Lithologies Maximum thickness
Notes
Pleistocene Cidreira Fm. Imbé Fm. Sandstones, shales Shales, siltstones, sandstones
7,000 m (23,000 ft)
[17][20]
Pliocene
Messinian
[21]
Tortonian
[20]
[21]
Serravallian
Langhian
[20]
Chattian
Rupelian
[21]
Bartonian
[21]
Priabonian
Lutetian
[20]
Ypresian
[21]
[20]
Paleocene
Maastrichtian
Campanian
Santonian
Coniacian
Late Turonian
Early Turonian Tramandaí Fm. Atlántida Fm. Sandstones, siltstones, shales Shales
900 m (3,000 ft)
[17][20]
Cenomanian
Late Albian
Early Albian Porto Belo Fm. Limestones, shales
700 m (2,300 ft)
[20]
Ariri Fm. Thin evaporites
800 m (2,600 ft)
[20]
Aptian Curumim Fm. Basalts
[20]
Cassino Fm.
siltstones
1,800 m (5,900 ft)
[20]
Imbituba Fm. Basalts
[20]
Barremian
Serra Geral Fm. Flood basalts
[20]
Hauterivian
Valanginian
Botucatu Fm. Quartzitic sandstones
[20][22]
Berriasian
Tithonian
Early-Late Jurassic
Late Triassic
Permo-Triassic Rio do Rasto Fm.
Red sandstones
[20][23]
Teresina Fm. Fine sandstones and carbonates
[20][24]
Irati Fm.
Black shales and carbonates
[20][25]
Palermo Fm. Siltstones, fine sandstones
[20][26]
Rio Bonito Fm. Sandstones, siltstones, shales
[20][27]
Early-Late Paleozoic
Precambrian Pelotas Batholith, Dom Feliciano Belt Granite, gabbro, diorite, rhyolitic to basaltic dikes
[20][28]

Exploration

The Pelotas Basin is relatively underexplored. The Brazilian part of the basin had a total of twenty exploration wells drilled until 2017. The first eight wells were drilled in the onshore section in the 1950s and 1960s. After acquiring

seismic survey in the 1970s, seven more wells were drilled in the shallow offshore part. Five other offshore wells were drilled between 1995 and 2001. The first and hitherto only well drilled in the Uruguayan part of the basin was drilled in 2016. Hydrocarbon accumulations have yet to be discovered in Pelotas Basin.[4] An exploration area of approximately 15,000 square kilometres (5,800 sq mi) was offered in the Brazil Bidding Round of 2017.[29]

See also

References

  1. ^ "(in Portuguese) Audiência pública explicará nova tentativa de prospecção de petróleo no estado (Public hearing will explain new attempt to prospect for oil in the state)". GaúchaZH. 26 Jul 2013. Retrieved 2019-03-15.
  2. ^ Arnemann Batista, 2015, p.3
  3. ^ De Santa Ana et al., 2004, p.83
  4. ^ a b Conti et al., 2017, p.2
  5. ^ Correa et al., 2017, p.188
  6. ^ Bryant et al., 2012, p.46
  7. ^ a b Contreras, 2011, p.7
  8. ^ Mello et al., 2012, p.3
  9. ^ Zalán, 2017, p.1
  10. ^ a b Arnemann Batista, 2015, p.6
  11. ^ Philipp et al., 2016, p.97
  12. ^ Soto et al., 2011, p.4
  13. ^ Beasley et al., 2010, p.33
  14. ^ Contreras, 2011, p.13
  15. ^ a b Contreras, 2011, p.9
  16. ^ a b Arnemann Batista, 2015, p.7
  17. ^ a b c d Contreras, 2011, p.21
  18. ^ a b Arnemann Batista, 2015, p.8
  19. ^ De Santana et al., 2014, p.33
  20. ^ a b c d e f g h i j k l m n o p q r s Arnemann Batista, 2015, p.9
  21. ^ a b c d e De Santana et al., 2014, p.31
  22. ^ Petrolli & Pimentel, 2014, p.1
  23. ^ Rohn & Rösler, 1990, p.486
  24. ^ Warren et al., 2015, p.111
  25. ^ Da Costa, 2015, p.23
  26. ^ Roisenberg et al., 2008, p.16
  27. ^ Roisenberg et al., 2008, p.15
  28. ^ Philipp, 2016, p.96
  29. ^ Petersohn, 2017, p.19

Bibliography

Brazil general

Pelotas Basin

External links

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