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The pre-salt is an occurrence of oil and gas in ultra-deep waters in the Santos Basin

Image: P-51, the first 100% Brazilian platform by Disclosure Petrobras / ABr CC-BY-3.0

The term pre-salt is used in Brazil to refer to the oil reservoir found off the Brazilian coast. It is an occurrence of hydrocarbons in the ultra-deep waters of the Santos Basin. Its discovery was announced at the end of 2007 and increased the chance of making Brazil one of the biggest oil and gas producers in the world.

According to Petrobras, the pre-salt rocks act as immense reservoirs of gas and oil located under an extensive layer of salt, which extends in the coastal region between the states of Espírito Santo and Santa Catarina, in a strip of about 800 km long by 200 km wide. In this range, the water depth varies from 1,500 to 3,000 meters in depth, and the reservoirs are located under a pile of rocks 3,000 to 4,000 meters thick, located below the seabed.

The area covered by the pre-salt reservoirs is distributed over the sedimentary basins of Santos and Campos, located on the Brazilian continental margin, as can be seen in the image below:

The quality of the oil found, characterized as light (better quality than heavy oil), makes it feasible to reduce imports of the product, although it requires appropriate technological development.

Hydrocarbons

Oil and gas are, chemically speaking, hydrocarbons. Hydrocarbons are organic compounds formed only by hydrogen (H) and carbon (C).

Methane (CH4) is the simplest structured hydrocarbon. Oil, on the other hand, is formed by a complex mixture of hydrocarbons that can present molecules in the form of chains, rings or other structures. This mixture varies according to the origin of the oil, but it is predominantly formed by normal, cyclic and branched paraffins, resins, asphaltenes and aromatics.

• Meet methane gas

How the pre-salt was formed

The pre-salt originated with the accumulation of hydrocarbons formed from organic matter found in the so-called source rocks. This accumulation occurred with the migration of these hydrocarbons to reservoir rocks (which allow the circulation and storage of oil and gas) and sealing rocks (which prevent the escape of oil and gas when they cover the reservoir rocks).

With the removal of the African continent from Brazil, which occurred due to the movement of tectonic plates, a rift was formed. A rift is a type of sedimentary basin delimited by deep faults. This process, called rifting, can evolve into continental disruption and form an ocean. And this was the case of the Brazilian continental margin, in which the formation of pre-salt reservoirs is directly related to tectonic movements, which promoted the rupture of the Gondwana paleocontinent, causing the separation of the South American and African continents - a process that culminated in the opening of the South Atlantic Ocean.

The formation of the Santos and Campos basins began in the Cretaceous period, more than 130 million years ago. The evolution of these basins has been related to four stages: pre-rift (or mainland), rift (or lake), proto-oceanic (or gulf) and drift (or ocean).

The pre-rift stage, or the continent, occurred with the deposition of sediments coming from water currents, winds and rivers, and, in theory, occurred in a large depression in the east-northeast portion of Brazil and west-southwest Africa .

In the rift stage, volcanism occurred approximately 133 million years ago, especially in the region of the current Santos and Campos basins.

The post-rift stage is characterized by the entrance of a sea to the south, controlled by a topographic high constituted by basaltic rocks. The setting at that time was that of a narrow, elongated gulf, very similar to that of the present-day Red Sea, located between northeastern Africa and the Arabian Peninsula.

The continuous sinking of the basin floor, the hot climate, the salinity of the water and the high rates of evaporation allowed the formation of the salt package, which served as a sealant, exerting the effect of burying and overloading, completing the framework of the petroleum system of the pre-salt .

In the drift stage, the separation between the South American and African continents began and the formation of the South Atlantic Ocean began. This stage began about 112-111 million years ago and continues to this day.