Ian Davison
University of London
Egham, Surrey
Brazil is the fifth largest country in the world, and its sedimentary basins occupy about 4.6 million sq km, half the area of the U.S.
Brazil has outstanding potential for giant hydrocarbon discoveries, such as recently discovered Marlim field. Marlim is probably the largest oil field found in the western world in the last 7 years, with 15 billion bbl in place.
Brazil produces about 650,000 b/d of oil and 630 MMcfd of gas. This is 60% of the country's oil consumption. Proved original recoverable reserves are an estimated 7.8 billion bbl.
Petrobras SA, the world's 13th largest oil company, has a virtual monopoly on exploration and production.
There are a few exceptions, such as the Pecten-owned gas field due on stream at yearend 1991 in the Santos basin.
Petrobras has only 1,500 technical exploration people, which has restricted its capability to evaluate hydrocarbons reserves in such a vast area.
The monopoly situation may change in light of Brazil's ailing economy, increasing foreign debt, and a new president committed to opening his country to foreign investment.
Brazil is entering an exciting phase of exploration, with its own deepwater production technology breaking world records (1,614 ft of water), and seismic data and geological knowledge greatly improved in recent years.
GEOLOGY, HYDROCARBON POTENTIAL
ATLANTIC MARGIN BASINS
The South Atlantic margin of Brazil extends for about 5,000 km and spans 15 separately named sedimentary basins (Fig. 1).
Continental rifting began in the Neocomian in southern Brazil and spread northwards.
The marine drift stage began in Aptian, followed by slow thermal subsidence until the present day. Mature Neocomian to Aptian lacustrine source rocks occur along most of the margin and are believed to source most of the offshore production.
Structural styles vary systematically along the margin but are mainly extensional fault systems. However, several areas (South Bahia, Sergipe-Alagoas, Barreirinhas, and Ceara) have well developed transcurrent/transpressional flower structures and folds, which constitute the largest structural traps still not drilled in shallow water (Fig. 2).
Structural traps are found around abundant Aptian age salt structures in the Campos, Santos, and Espirito Santo basins, with minor evaporate deposits recorded in the Sergipe-Alagoas and Ceara basins.
Stratigraphic traps are becoming very important targets since the major discoveries in the Campos basin, and Petrobras routinely drills stratigraphic traps in deep water.
Large infilled canyons up to 50 km wide and 200 km long developed in the Campos, Espirito Santo, South Bahia, Potiguar, and Sergipe-Alagoas basins from early Cretaceous to Oligocene times. These can form attractive stratigraphic traps, as in the Espirito Santo basin (Fig. 2).
The basins are reviewed from south to north along the Atlantic margin in the following section, with a summary (see table).
The Pelotas basin exhibits large rotated fault blocks that are clearly imaged on seismic data. The main rotated syn-rift section consists of Lower Cretaceous basalts, red shales, and sandstones.
No good source rocks have been identified in the basin. However, a reasonable mature source interval has been encountered on the nearby Santa Catarina high in well 1-SC-2 (1.7-2% total organic carbon level). The basin is considered to have a high risk with moderate to large rewards if a good source and reservoir are found.
The Santos basin is a priority area for Petrobras after its recent success on acreage released by BP.
Petrobras has slated $120 million for exploration this year. This follows the discovery in 1987 of Tubarao field, which contains about 100 million st-tk bbl of 43 gravity oil in place, and in 1990 the Tubarao pole with a possible 150 million st-tk bbl of oil in place.
The source rocks have not been penetrated but are probably the same as those in the Campos basin.
Many large salt structures constitute attractive prospects (Fig. 2), and the deeper section may hold large reserves, although reflection data are very poor at these depths.
The Campos basin is the most attractive basin in Brazil.
It has abundant reservoirs throughout the stratigraphic column, from early Cretaceous fractured basalts to Miocene turbiditic sandstones. Good source rocks are present in Upper Neocomian lacustrine shales, which lie below the Aptian evaporate sequence.
Most of the oil is trapped in Tertiary turbiditic sandstones, which have been preferentially deposited in lows around the Aptian salt structures, The exceptionally high quality seismic data have ensured that Petrobras enjoys a high discovery success rate in this basin (50% in deep waters).
It is still a relatively unexplored basin with good potential for giant discoveries in deep water (deeper than 400 m) and moderately sized finds in more shallow water.
The Espirito Santo basin lies mainly offshore, but most of the oil finds are onshore in shallow sandstones, stratigraphically trapped below the Aptian evaporate seals, where 39 million bbl have been produced to date.
Structural trapping occurs around the many salt structures, and large buried Cretaceous canyons provide attractive stratigraphic traps offshore.
Most of the large postsalt structural traps have been tested, but there are large untested structures below the salt. However, exploration at these levels has been hampered by lack of good seismic resolution.
Almada, Jequitinhonha, and Cumuruxatiba are enticing basins that have produced several sub-commercial finds.
Many very large transpressional structural closures have been identified in both deep and shallow water (Fig. 2). These basins are poorly known and have complex structures, which makes correlations very difficult.
It would seem that finding good quality reservoirs may be a problem. However, potential for discoveries in very large structures is still high, especially in deep water.
Sergipe-Alagoas is a highly productive basin on- and offshore (Table 1). Reservoirs occur throughout the stratigraphic column with most oil contained in the post-rift sequence of the basin.
Onshore the basin has good potential for small subtle traps in areas where production facilities are already installed. Offshore, many rotated extensional fault blocks, thrust/fold closures, and possible stratigraphic traps in Tertiary fans have been identified in deep water.
This basin presents a moderate risk and moderate reward potential onshore and in shallow water (less than 200 m) and a high risk-high reward potential in deeper water.
The Potiguar basin has been intensively explored since the first offshore discovery in 1973.
The onshore area consists of a failed Lower Cretaceous asymmetric continental rift blanketed by post-rift sediments produced by the adjacent Atlantic rifting. It has not been adequately tested at deeper levels, where stratigraphic traps may be expected, and the offshore area is still relatively unexplored.
Onshore Potiguar is a relatively mature basin with a very high discovery success rate and low exploration costs and is considered to have a low-risk with moderate potential reward.
The offshore area has a higher risk with little documentation available on possible rewards.
Piaui-Ceara has the northernmost oil production on the Atlantic margin, with oil accumulations throughout the stratigraphic column,
Important dextral transcurrent movements have produced numerous flower structures, folds, and possible aligned mud diapirs, although thin evaporates locally occur in this basin. Due to structural complexities, the seismic data are usually poor.
The Mundau subbasin appears to have the best potential for new discoveries as the other subbasins seem to lack a good source rock. Most of the obvious structural traps have been drilled in shallow waters, but deepwater plays in Tertiary turbidities are attractive.
The Barreirinhas basin is situated at the western termination of the complex Romanche transform/fracture zone and contains many very large positive flower and inversion structures.
Sub-commercial oil and good shows have been found in several wells. Unfortunately, reservoir sections appear to be of poor quality and highly segmented due to the large amount of deformation that has occurred in the basin.
Seismic data quality has also been impaired by the intense deformation, and correlation of reflectors is difficult. This makes the basin a high risk proposition, but there are large potential rewards in shallow water.
Para-Maranhao has produced sub-commercial oil from rollover anticlines associated with listric faults affecting late Cretaceous clastics and Tertiary carbonates.
Two wells have produced from fractured Tertiary carbonates, with one well producing 710,000 bbl of oil using a Swops. Only 14 wells have been drilled in the basin, and it is difficult to evaluate its potential. Good reservoirs and adequate seals seem to be the vital factors missing so far.
Much of Foz de Amazonas basin is covered with a thick Cenozoic deposits of the Amazonas cone delta. Approximately 22 wells have been drilled in the cone area, with some sub-commercial oil production and a small gas find (Pirapema, 35 bcf).
The Cassipore graben lies at the extreme north of Brazil's Atlantic margin, where only a handful of wells have been drilled, some of which have produced sub-commercial hydrocarbons.
Very large rotated fault block closures are present in this area, and the basin has good potential for moderate to large discoveries, although it is geographically remote.
MESOZOIC INTRACONTINENTAL RIFTS
The Takutu basin is considered to have a high potential for small finds but it is relatively remote and exploration will be more expensive.
This basin has only been drilled by two wells in Brazil and two in Guyana. One of the Guyanese wells proved sub-commercial oil in fractured basalts with reasonable flow rates that eventually depleted to 70 b/d.
The basin is shale prone, and the problem will be to find good reservoir, although undrilled fan-shaped structures are imaged on the borders of the basin.
Petrobras' policy on the Marajo rift was an exception with regard to foreign companies, as the national company granted an exploration license to Texaco over a large part of the basin without itself having much prior knowledge.
Texaco's seismic data subsequently revealed large rotated fault block prospects below a thick post-rift sedimentary sequence.
However, contrary to misleadingly optimistic press reports, Texaco's deep well was not encouraging. It encountered mainly a sand prone sequence with little chance of a source and seal.
The Reconcavo-Tucano-Jatoba basin is the oldest producing basin in Brazil.
It is a failed rift that developed in early Cretaceous times during Atlantic spreading.
The mature Reconcavo subbasin has produced some surprising new discoveries in recent years: stratigraphic traps have been found in the Taquipe Canyon in lacustrine turbidities below the fanglomerate wedge on the eastern faulted margin, and in localized channel sandstones.
The South Tucano subbasin is more attractive for gas, with recent discoveries in rollover anticlines.
Petrobras recently drilled the deepest well in the southern hemisphere in the North Tucano subbasin looking for good Paleozoic source rocks, which have been identified in outcrop on the eastern margin of the basin. This stratigraphic test was unsuccessful.
The Jatoba subbasin is generally too shallow for organic maturation, and no good source rocks have been identified.
PALEOZOIC BASINS
The three major Paleozoic basins of Brazil appear to have formed at approximately the same time with important sediment deposition taking place intermittently from Silurian to Tertiary times.
Multiphase thermal and extensional events must have occurred to promote such long depositional histories.
Despite their long history large structural traps are not present in any of the basins, suggesting crustal extensions were small. All three basins are virtually unexplored at deeper levels, and considering their size exploration possibilities are vast.
The Parana basin holds the largest known sub-surface flood basalt sequence, which covers two thirds of the basin area and reaches a thickness of 1.7 km.
Many subcommercial oil discoveries and good shows have been encountered. However, exploration has been difficult because the most prospective parts of the basin lie below the flood basalts and sills, and no large structures appear at shallower levels.
Petrobras has recently renewed its exploration effort in this basin after 12 years of inactivity. Improved seismic acquisition has shown large structural closures below the basalts.
The first deep well in the main basin depocenter began drilling in late 1990. Parana. is considered to have high potential for moderately-sized structures and is still unexplored.
Parnaiba is a large saucer-shaped basin with little structure and has also been a low Petrobras priority until recently. However, good quality mature source rocks, reasonable reservoirs, and the lack of seismic data make this a potentially rewarding yet relatively unknown basin.
The Solimoes basin has production from small oil and gas fields.
The main hydrocarbon trends appear to be well-defined east-northeast/west-northwest oriented en-echelon fold and high angle reverse faults associated with a 1,300 km long dextral transpressional system thatdeveloped in Late Jurassic times.
Petrobras will be investing around $100 million in production facilities around Urucu field in 1991. This will make the basin far more attractive for future exploration, since exploration and production costs have been very high in this remote part of the Amazon rain forest.
Almost nothing is known about the Parecis basin, which is about 1,000 km long.
The presence of a large negative gravity anomaly indicates a possible 6 km of sedimentary fill. Good Carboniferous source rocks and reservoirs have been found at its western end in surface outcrops.
PRECAMBRIAN BASINS
The attractive parts of the Sad Francisco basin consist of gently folded structures produced in late Precambrian times that can be surface mapped and present cheap exploration targets in a relatively accessible area.
The main reservoir targets are fractured carbonates with secondary porosity. Excellent source rocks have been identified in the Middle and Lower Proterozoic rocks.
Source rock thickness reaches 1,960 ft. Organic analyses seem to indicate that most rocks are overmature, although surface gas seeps have been recorded.
Recently discovered seeps have prompted Petrobras to drill several unsuccessful exploratory wells.
LICENSING
Brazil has never had official open licensing rounds, but during the last 22 years ad hoc exploration licenses have been granted to companies that have approached the Brazilian government.
Pecten is the first foreign oil company to acquire a production license in the Santos basin.
CONCLUDING REMARKS
Brazil has potential for many giant hydrocarbon discoveries in deep water (deeper than 400 m) and for many moderately-sized discoveries in both shallow water and onshore.
In view of the reduction in exploration in the last 5 years because of the country's economic crisis, it would seem unlikely that this potential will be fully realized without future foreign investment.
ACKNOWLEDGMENTS
The author thanks P. Szatmari, E.J. Milani, R. Matos, C. Cainelli, L. Magnavita, A.S. Netto, G. Raja Gabaglia, P. Zalan, R. Senna, and A. Zuppi for their kind invitations to participate in technical symposia, field and office work, and especially for their friendship during his stay in Brazil.
Copyright 1991 Oil & Gas Journal. All Rights Reserved.
