An ocean's ghost
Over 100 million years ago, the supercontinent Gondwana was tearing apart. As South America and Africa drifted away from each other, a long, narrow seaway formed—the proto-Atlantic Ocean. In a period of intense heat during the Aptian age (around 120 million years ago), this young ocean experienced extreme evaporation. This process left behind a colossal layer of salt, in some places up to 2,000 meters (6,600 feet) thick. This massive salt deposit would eventually settle over geological formations that today contain a significant portion of the world's oil reserves.
Before the salt was deposited, rift lakes in the separating continents created the perfect conditions for life. Over millions of years, vast quantities of microscopic algae and other organic matter accumulated on the lakebeds, mixing with sediment to form source rocks. As these source rocks were buried deeper, heat and pressure cooked the organic material into high-quality light oil. The thick, impermeable salt layer above acted as a perfect seal, trapping the oil and natural gas in underlying carbonate reservoirs. These reservoirs, now known as the pre-salt, are a time capsule of ancient lacustrine environments, with the oil trapped in rocks formed from fossilized microorganisms and the shells of bivalve mollusks.
Drilling through deep time
The existence of oil beneath the salt was a long-held theory, but the first major discovery did not occur until 2006 in what would be named the Tupi field (later called the Lula field). The announcement of reserves potentially holding over 50 billion barrels of oil transformed Brazil into a major player in global energy. The pre-salt region stretches for 800 kilometers along the Brazilian coast, covering an area of around 150,000 square kilometers.
Extracting this oil is a monumental engineering challenge. The reservoirs lie under more than 2,000 meters of water and are buried beneath 5,000 meters of rock and salt. The total depth from the sea surface to the reservoir can exceed 7,000 meters. Drillers must navigate the extreme depth but also the nature of the salt layer itself, which behaves like a slow-moving plastic under immense pressure and can close off a well during drilling.
Operations rely on advanced technology, including Floating Production, Storage, and Offloading (FPSO) vessels, which are massive ships and floating production centers. The largest of these, the FPSO Almirante Tamandaré, is 182 meters tall. The extracted oil often contains a high concentration of CO2, which is separated on the FPSO and reinjected back into the reservoir to maintain pressure and reduce greenhouse gas emissions.