Brazils offshore potential extends beyond Campos basin

Robert G. Becker
Gustavson Associates Inc.
Boulder, Colo.

The country of Brazil contains one of the worlds largest areas of sedimentary basins (Fig. 1 [39892 bytes]). Although only a fraction of Brazils basinal area is offshore, over three-quarters of the countrys undiscovered reserves are anticipated to be found there.

Considering the continental shelf and slope areas, a total of some 1 million sq km of sedimentary basin is found offshore in water as deep as 2,000 m.

Some 16 basins or offshore areas are classified as Mesozoic-Tertiary age, Atlantic margin basins that developed in response to the divergent margin along the east coast of South America. Ten of these basins have had discoveries of hydrocarbons, most of which are in commercial quantities either on land or offshore.

The future of Brazils petroleum industry will depend heavily on the exploitation of the offshore basins and in deep water along its Atlantic coastal margin.

E&D history

Brazil first began offshore petroleum exploration in the 1950s in the form of isolated geophysical surveys.

In the late 1960s systematic seismic, gravity, and magnetic surveys were initiated on the continental shelf and slope areas.

The first offshore well was drilled in 1968 and the first discovery was made in the same year in the Sergipe-Alagoas basin. The first offshore production began at Enchova field in the Campos basin in 1977.

Exploration in the Campos basin and more specifically at the Marlim complex and Albacora field have placed Brazil (and its state-owned oil company Petrobras) at the forefront of deepwater exploration and development technology. To date Petrobras has drilled about 700 wells offshore, resulting in discovery of some three dozen fields.

Reserves, production

Brazil has considerable proved hydrocarbon reserves estimated at 3.6 billion bbl of oil and 4.8 tcf of gas.

Petrobras the last 40 years has discovered 13 billion bbl of oil equivalent recoverable and has produced more than 4.1 billion BOE.

Estimates of the countrys potential undiscovered re- coverable resources exceed 16.2 billion BOE. Nearly 80% of this potential is believed to exist in offshore, with 63% of the total (onshore and offshore) potential in water deeper than 400 m. Almost half Brazils total known reserves (proven, probable, and possible) exist in the Campos basin.

Hydrocarbon production in Brazil stands at about 700,000 b/d of oil and more than 700 MMcfd of gas. The oil produced is less than 55% of the countrys daily consumption of 1.3 million b/d.

About 85% of the countrys gas production is fully utilized, but this still represents only a small part of its total energy consumption. About 70% of the countrys total hydrocarbon production is from offshore fields located primarily in the Campos basin.

Geopolitics, potential

Despite its great hydrocarbon potential it is unlikely Brazil will meet its goal of obtaining self-sufficiency by late this decade.

The Brazilian government in June 1995 nearly passed a constitutional amendment that would have ended the state-owned Petrobras monopoly on the countrys petroleum industry. In spite of the fact that the lower house of the Brazilian congress twice voted to end the companys monopoly, President Fernardo Enrique Cardoso decided to exclude Petrobras from the group of government entities proposed to be privatized.

Interests favoring the status quo continue to prevail over those who want Petrobras privatization advance. Because of this, Petrobras may continue to reel under the effects of an unstable economy, crippling labor disputes, and a burgeoning corporate debt that measures in the tens of billions of U.S. dollars.

Despite the recent reversal in the move to privatize Petrobras, it seems inevitable that the country will be forced in the near future to seek outside investment (if not outright participation) in its petroleum industry. Both government and Petrobras officials have stated recently that this is the only way for the country to reach its goal of oil self-sufficiency within 10 years.

Contrarians to this position cite Petrobras excellent exploratory drilling success rates (based on 3D seismic data), which over the past 9 years have averaged well over 60%. During this same time period the volume of discovered oil-in-place has nearly doubled (Fig. 2 [33779 bytes]).

Excellent potential remains for more hydrocarbon discoveries, particularly in offshore areas. Nearly 12.8 billion bbl, or 80% of the countrys 16.2 billion BOE (recoverable) undiscovered potential is believed to exist offshore, with more than half of this or 7.2 billion bbl expected to be found in water deeper than 1,000 m (Fig. 3 [25783 bytes]).

Offshore basins

Campos is by far Brazils richest and most prolific offshore basin. An estimated 22 billion bbl in place, or about 4.5 billion bbl recoverable, have already been discovered there.

Marlim and Albacora giant fields are the most notable along with Barracuda, East Albacora, Caratinga, East Marlim, and Guarajuba. Water depths for these fields range to about 1,100 m. Roughly 400,000 b/d of oil and 315 MMcfd are produced in the 100,000 sq km basin. Reserves are found in highly porous (about 30%) Late Cretaceous-Tertiary turbidites and platform carbonates.

Reservoirs are commonly found at drilling depths of about 3,000 m and are sourced by rift-sequence Lower Cretaceous lacustrine shales. Combination structural/stratigraphic traps are typical in the Campos basin.

The best potential for future large discoveries in the Campos basin will be in waters deeper than 1,000 m. Huge seismic anomalies were delineated in the post-salt stratigraphic sequence in progressively deeper waters of the Campos basin. This, in addition to progressively favorable source rock and reservoir rock sequences, attracted Petrobras geoscientists to these deeper water areas.

The results have been very encouraging in that exploratory drilling based on 2D and more recently 3D seismic data has obtained success rates as high as 68% in water over 400 m deep, leading to discovery of some 18.5 billion bbl in place at Marlim and Albacora fields in the Campos basin alone.

Petrobras current record for a deepwater completion is the 7-MRL-4D-RJS (Marlim No. 4), in 1,027 m of water. Completed in March 1994, the well averaged an initial 6,466 b/d of oil and 4.45 MMcfd of gas. Production (lifting) costs for this project have been put at $3.50/bbl.

The Recncavo basin is second to the Campos basin in production. This largely onshore basin, which produces more than 75,000 b/d, was Brazils principal oil province until the giant Campos basin discoveries of the mid-1980s.

The small offshore part of the Recncavo, and the offshore Camamu, Almada, Jequitinhonha, and Cumuruxatiba basins are sometimes collectively referred to as the Bahia basins. These basins all have rift, evaporite, carbonate platforms and transgressive-regressive sequences.

Although only small fields have been discovered to date in the offshore Recncavo basin (including the 20 million bbl Cassarongonga and 8.5 million bbl Cid Entre fields), the very favorable geologic conditions and historical production of the onshore Recncavo portion suggest that the Bahia basins have good potential for future offshore discoveries.

The third most productive basin in Brazil is the 34,000 sq km Sergipe-Alagoas basin near the countrys eastern tip (Fig. 1[39892 bytes]). The basin produces more than 65,000 b/d from land and offshore fields.

More than 4 billion bbl of oil and 440 bcf of gas reserves have been discovered in this basin, most of which are onshore. In the offshore, excellent quality post-rift turbidite reservoirs have been found in relatively shallow water. The basin is characterized by a narrow band of deep water in which the 1-SES-92 wildcat became a significant oil discovery in 1987.

Hydrocarbons are be- lieved to be sourced primarily from mature, Aptian age bituminous shales of the rift-sequence. Traps associated with tensional fault blocks are common. The overall potential of this basin is considered to be good.

Just north of the Campos basin is the Espirito Santo basin, which produces some 20,000 b/d from onshore and offshore fields. Good to excellent quality source rocks and reservoir rocks, similar to those in Campos, are found in this basin.

Large shallow and deepwater areas of the offshore part of Espirito Santo remain undrilled. Gravity faults, rollover anticlines, and salt domes constitute the major geologic structures in the basin. In light of the above this basin has good potential.

The Potiguar basin northwest of Brazils eastern tip is another promising area with a history of commercial oil production. Potiguars re- serves are about 324 million bbl of oil and nearly 400 bcf of gas. The largest field, Ubarana, is in about 50 m of water. A number of other small fields exist offshore, but most of the basins production has been from its maturely-explored onshore area. Production is from fault traps in continental and transitional sandstones and post-rift turbidites.

Future offshore potential is considered fair in the lightly-explored deepwater area.

Oil accumulations have been found throughout the stratigraphic sequence of the Ceara basin west of the Potiguar basin. Production of over 17,000 b/d has been achieved from six fields located onshore and offshore.

Over 84 million bbl of oil and 65 bcf of gas reserves have been found. Numerous flower structures, folds, and mud diapirs exist that are the result of rift related and transcurrent movements in the Ceara basin. Rift sequence sandstones and transgressive-regressive turbidite reservoirs are charged with hydrocarbons generated from Cretaceous source rock shales.

Most of the obvious structural traps have been drilled in Cearas shallow waters, but plays still exist in the deepwater part of the offshore basin in Tertiary turbidites. Several large structures, identified by seismic data, have been mapped recently in water up to 2,500 m deep but have not been drilled.

South of the Campos basin is the 130,000 sq km Santos basin. Four offshore oil and gas fields have been discovered in this basin, three in recent years.

Although it lacks a rift sequence, the Santos basin is similar in structural style and stratigraphic section to the nearby Campos. The Santos basin has a thicker evaporite section and widespread, mature Cenomanian through Santonian marine shale source rocks of fair quality. Deepwater potential based on seismic data is unclear, as no wells have been drilled in this part of the basin.

Brazils largest offshore basin, Foz do Amazonas, covers some 140,000 sq km. Much of this basin is underlain by thick Cenozoic deltaic deposits of the Amazonas cone delta; only a couple of dozen exploratory wells have tested this stratigraphic section. Some sub-commercial oil production and a small gas discovery have been the result of this effort.

The bulk of the Foz do Amazonas basin is characterized by Gulf Coast-type growth faults and shale diapirs. A small area to the west, known as the Cassipore area, has an underlying rift sequence.

Several deepwater Paleocene turbidite systems have been mapped in the area, and constitute the newest play of the region. Albian-Cenomanian source rocks have been identified in several wells, but they may be largely gas-prone (biogenic).

Subcommercial accumulations of oil have been found in the moderate-sized Barreirinhas basin, which covers some 45,000 sq km. This basin is situated between the Foz do Amazonas and Ceara basins. It contains many large flower and inverted structures related to the complex Romanche transform/fracture zone located to the east.

About 90 wildcats have been drilled in this basin; possible reservoir sequences appear to be of poor quality and limited areal extent due to extensive deformation within the basin.

Other basins of the Brazilian margin that to date have seen little drilling are the relatively unstructured Pernambuco-Paraiba and Pelotas basins, both of which lack a rift sequence. While four wells have been drilled in the southernmost Pelotas basin, none has encountered hydrocarbons.

Recently, however, a broad sedimentary feature has been delineated by seismic data in the Pelotas in water deeper than 200 m. Additionally, turbidite plays in water more than 1,500 m deep may become future Pelotas basin targets, bolstering its hydrocarbon potential.

The offshore basins of Brazil will play a key role in the countrys push toward energy self-sufficiency. Most of the hydrocarbon potential of the country is believed to exist in the offshore basins. Offshore prospects outnumber those located onshore by a three to one margin (Fig. 4 [26416 bytes]).

Given Petrobras outstanding record for deepwater exploration, drilling, and completions, development of this potential is within reach for Brazil. Nevertheless, the ability of Brazil to provide its citizenry with ample and competitively priced hydrocarbons may depend more on politics than technology.

Bibliography

Barcelos, A., Deepwater activities offshore Brazil: Evolution on drilling technology, Tulsa University Centennial Petroleum Engineering Symposium, SPE 28004, 1994, pp. 495-504.

Franke, Milton R., Discovered and potential petroleum resources-deep offshore Brazil, in proceedings of the 13th World Petroleum Congress, Vol. 2 Exploration and Production, John Wiley & Sons, New York, 1991, pp. 71-73.

Lucchesi, C.F., et al., 3D seismic as an exploration and production tool: Campos basin experience, 27th annual Offshore Technology Conference, Houston, Vol. 1, OTC 7691, 1995, pp. 507-511.

Possato, S., et al., The discovery and appraisal history of two supergiant oil fields-offshore Brazil, 22nd annual Offshore Technology Conference, Houston, Vol. 1, OTC 6268, 1990, pp. 621-634.

Weiner, J., The petroleum potential of offshore Brazil, Gustavson Associates Inc., Vols. 1 & 2, 1988, 330 p.

The Author