In 1939, a small oil accumulation discovered near Salvador City in northeastern Brazil started their first phase of oil exploration. Onshore basins were drilled and explored exclusively until the State oil company, Petrobras, drilled the first offshore well in 1968.
Most of the exploration occurred in northern and northeastern basins in the late 1950s through the 90s, discovering 201 fields, over half of these producing less than 1.0 MMboe. A few individual oil fields that contained original reserves over 400 MMboe were found during this period, however oil production never exceeded 200,000 bopd and total oil reserves did not exceed 1.2 Bb. Since then, there has been little activity in these interior basins.
With Petrobras becoming an "offshore, deep water oil company", small to medium independent oil companies can target mature fields and exploration opportunities in Brazil's onshore basins. In fact, the ANP, Brazil's oil and gas leasing and regulatory agency, is trying to attract companies to some of the mature fields by offering them through bid rounds. In addition to areas already drilled, studies are ongoing to evaluate the basin's petroleum systems and better assess their potential.
The Amazon Region - mostly gas
The Paleozoic Amazon River basins cover 850,000 km² across northern Brazil and are filled with up to 6,000 m of sediments. Basement arches separate the 3 primary basins, Acre, Solimões, and the Amazonas. Combined they form an impressive east-west oriented series of basins 2,500 km long and 500 km wide. These basins are unique from the other two major cratonic basins (Parnaiba and Paraná) in that they contain a sequence of rhythmic bedded carbonate and evaporites deposited when these basins were partially cut off from the open ocean.
The Solimões Basin has seen the development of significant reserves of gas and, to a lesser extent, oil. It also has been the most explored of the Amazon River basins. Development of these basins suffers from the remoteness of the area being entirely within the Amazon rain forest and a long distance to market.
The petroleum system for the better known Solimões basin's source rocks are Devonian shales reaching up to 8% TOC and average 6% TOC. Eolian sandstones of the Carboniferous Juruá Formation are the best and may be the only reservoir in the basin with excellent secondary porosity up to 22%. Maturation of source beds is sensitive to the presence and proximity of intrusive bodies. Most of the hydrocarbon generation and migration occurred during Late Triassic to Early Jurassic magmatism. Present traps were formed during a Jurassic wrenching (tectonic) event.
The Amazon Basin has many similarities to the Solimões basin with a similar Devonian source rock and the Pennsylvanian evaporites as cap rocks. However, gas has been found in slightly younger and lenticular reservoir rocks in the Nova Olinda Formation. Also, traps should be different since deformation that led to the wrench fault system in the Solimões basin was not active here. Probable trapping mechanisms are extensional faulting and salt tectonics. Maturation of the organic matter may be the biggest difference as recent studies indicate early maturation of the organic matter through normal subsidence with the Mesozoic magmatism locally accelerating rates of maturation.
Parnaíba Basin - no discoveries
The Parnaíba Basin occupies about 600,000 km² in the northeastern region of Brazil and contains a 3,500 m sedimentary section near the depocenter. The basin is bound on the north by an arch related to the Mesozoic opening of the Equatorial Atlantic Ocean. Rifting broke the connection of the Parnaíba with analogous basins in northwestern Africa.
Between 1951 and 1988, 41 wells were drilled and there have been 7,800 km of seismic obtained with no discoveries to date. The petroleum system starts with organic rich Devonian shales similar to those found in the Amazon River basins. Devonian shallow marine and fluvial sandstones as well as Early Carboniferous fine-grained sandstones are potential reservoir rocks. The structural framework is influenced by the Transbrasiliano fault zone, a continental scale fault zone that runs northeast to southwest for 3,000 km from the continental margin of the Equatorial Atlantic, crosses the interior basins (Parnaíba, Paraná and Pantanal) and into Paraguay. Outside this fault zone, the basin shows little deformation, mostly subtle fault-block tectonics forming fault related closures. Two pulses of magmatic activity occurred during the Mesozoic, both associated with rifting. The first cycle that emplaced both volcanic flows and numerous intrusive sills and a second emplacement of intrusives corresponds with the opening of the South Atlantic.
The source rocks in this basin are in close contact with overlying potential reservoirs, however those source rocks are immature where not altered by igneous rocks. The abundant igneous intrusions in the basin constitute the only heat source that have promoted maturation of the source rocks and generate hydrocarbons. In many of the wells there are oil and gas shows, gas seeps, and surface geochemical anomalies. However, unlike similar Paleozoic basins in Africa that have hydrocarbon reserves (Ghadames, Illizi, and Murzuq), none have been found in the Parnaíba basin possibly due to late trap formation allowing any hydrocarbons that were generated to escape. However, this large area still has room for more exploration.
Paraná Basin - hampered by basalts
The Paraná Basin in southern Brazil covers over 1,000,000 km² and extends into Paraguay, Argentina, and Uruguay for an additional 400,000 km². The area is in the heart of industrialized Brazil close to markets. Yet, thick basalts that cover over two-thirds of the surface while creating scenic wonders, gemstones, and rich soils have hindered exploration in this area. The stratigraphic record of this enormous basin exceeds 7,000 m along the central depocenter beneath the Paraná River. The eastern flank of the basin has been deeply affected by the Mesozoic rifting with uplift removing great thicknesses of Paleozoic rocks.
Over 100 wells have been drilled across this basin leading to the discovery of several Tcf of gas. Most wells encountered some oil and gas shows. Drilling has been somewhat random though, and in places, about 1,700 m of flood basalts obscure the subsurface.
The most important petroleum system, as with the other interior basins, is the Devonian black shales that have sourced mostly gas into Late Carboniferous to Early Permian glacial and coastal sandstones. Similar Devonian aged source rocks are found in the petroleum-producing Chaco basin in northwestern Argentina and Bolivia. A second younger petroleum system consists of Early Permian organic-rich shales and eolian and fluvial sandstones. About 20 known exhumed accumulations consisting of bituminous sandstones are found on the eastern rim of the basin. Wells have encountered numerous shows of 22 to 33 gravity oil.
Challenges and Opportunity
Dr. Nilo Azambuja, Vice President of HRT and expert on Brazil's onshore basins, concludes "The Brazilian interior basins are huge sedimentary areas with little exploration activities compared to similar basins in other parts of the world. They were put aside by Petrobras during the monopoly and now are low priority due to the great exploratory success in deep and ultra-deep waters of Campos, Santos and Espirito Santo basins."
"The exploration technology used in their initial exploratory phase is now out of date," says Nilo. "Companies are constantly looking for new opportunities. Investments using new technologies to explore here could change their status from frontier to production. This happened in a portion of the Solimões basin in the seventies and eighties, with the discoveries of the Urucu and Jurua fields. Urucu has already produced 200 million barrels of light oil/condensate and will start producing the associated gas to Manaus."