The pre-salt is a sequence of sedimentary rocks formed more than 100 million years ago from the break-up of the ancient continent Gondwana which formed the current continents of South America and Africa.
Large depressions were formed between the two continents, which gave rise to great lakes. In the deeper regions of these lakes, large amounts of organic matter were accumulated, mostly microscopic algae. This organic matter, mixed with sediments, formed the generating rocks of pre-salt oil and gas. After a process which involves high temperatures and pressures, the organic matter was transformed into oil and gas, in a process entitled generation.
In the shallowest parts, in large lacustrine islands, many calcareous shells (coquinas) were deposited and later deposits of stromatolites (types of algae which form calcareous rocks) were accumulated. These two types of deposits compose the main pre-salt reservoirs.
After the deposition of the stromatolite layers, the great lakes were connected to the oceans, changing from lacustrine systems to a restricted marine system, which caused the formation of an extensive gulf. Due to the arid climate prevailing at that time (the Aptian), the intense evaporation of marine water, which invaded these lake depressions, led to the accumulation of salts, which resulted in a thick layer of salt that served as a seal to prevent the oil from escaping and reaching the surface.
Located in an area of approximately 149,000 square kilometers in the territorial sea between the states of Santa Catarina and Espírito Santo, the pre-salt polygon is among the most important discoveries of oil and natural gas in the last years.
The total depth (distance between the surface of the sea and the oil reservoirs below the salt layer) can reach 7,000 meters. The reserves are composed of large accumulations of light oil, which has excellent quality and a high commercial value.
The productivity of the pre-salt wells is high. The daily oil production in the pre-salt increased from an average of 41 thousand barrels per day in 2010 to 1.35 million barrels per day in June 2017. This represents a growth of about 3,290%. Also in June 2017, the oil production in the pre-salt exceeded that of the post-salt, which totaled 1.32 million barrels per day.
More information can be found at http://www.presalpetroleo.gov.br/ppsa/o-pre-sal/caracteristicas.