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The Pre-salt is a sequence of sedimentary rocks formed more than 100 million years ago, in the geographic space created by the separation of the ancient Continent of Gondwana. More specifically, it was formed by the separation of the current American and African continents, a process that started about 150 million years ago. Initially, great depressions were formed between the two continents, giving rise to large lakes. Over millions of years, the rocks that would give rise to the pre-salt region's oil were deposited there. Since all the rivers on the continents that separated flew to the lower regions, large amounts of organic matter were deposited there.
As the continents strayed, the organic materials accumulating in the new space were being covered by the waters of the Atlantic Ocean, which was then forming. That was when the layer of salt, which is currently about 2,000 meters thick, started being formed. This layer of salt was deposited on the accumulated organic matter, holding it there for millions of years until thermochemical processes turned the organic layer into hydrocarbons (oil and natural gas).