Three major sedimentary basins underlie the shelf and slope off eastern Brazil, the Santos Basin in the south, the Campos Basin in the center, and the Espirito Santo Basin in the north. Despite the fact that large oil fields have been discovered in deepwater in the Campos Basin, these easterly petroliferous basins remain relatively under-explored.
The offshore sedimentary basins of Brazil encompass an area of more than 1,550,000 sq km, which is subdivided into Cretaceous-Tertiary basins that border the entire coastline (Figueiredo, 1985). Brazil presently produces more than 1 million b/d of oil, with 70% of this daily production from deepwater regions between 400 meters and 2,000 meters water depth.
The evolution of eastern Brazil was controlled by plate tectonic events associated with the early rifting and subsequent drift of Brazil and Africa. The main characteristic events consist of three stages: pre-rift, rift, and post-rift. These events controlled basin stratigraphy and basin infill. Sedimentary sequences are within four major distinct units:
- The lower basal unit (pre-rift sequences) consists mostly of Early Cretaceous continental red shales and conglomerates.
- The synrift sequences consist of Early Cretaceous lacustrine deposits. These are hydrocarbon-rich source-rock shales and fine-grained sandstones.
- The evaporitic-gulf Albian-Aptian transitional sequence consists of widespread deposits of halite, anhydrite, and carbonates above the "break-up unconformity."
- The marine sequence may be subdivided into two sub-sequences: shallow marine platform carbonate deposits and open marine sediments. The large deepwater discoveries are in these reservoirs.
Geostratigraphic section of Santos Basin. A major discovery was announced by Petrobras in Block BS-500 in Upper Cretaceous deepwater turbidites.
East-west trending fracture zones and marginal plateaus characterize the lower continental margin morphology and indent the continental margin basins. The fracture zones intersect the continental margin where they constitute basement highs that form the boundaries of the marginal basins.
The largest marginal plateau off the eastern Brazilian coast is the Sao Paulo Plateau, situated in ultra-deepwater in the Santos Basin. In the Sao Paulo Plateau, the sedimentary cover may exceed 4 km of thickness and massive salt domes and diapirs are abundant.
A fairway of salt layers and salt diapirs runs from Santos Basin-Sao Paulo Plateau northwards to Sergipe-Alagoas Basin. This salt fairway becomes progressively narrower towards the north. Still, in the Santos, Campos and Espirito Santo basins, salt mobilization is rather extensive in deep waters.
During the last 30 years, exploration and development investments were wide open to capital in the North Sea, Sunda Shelf, Gulf of Mexico, and Western Africa. In Brazil, all exploratory activities were essentially controlled by the Petrobras monopoly. The Agencia Nacional do Petroleo (ANP) opened the Brazilian upstream market to competition and has fostered an intense renewal of exploration in the form of farm-out partnerships with Petrobras and longer-term exploratory programs resulting from bid rounds.
By and large, new capital investments are concentrated in the Campos, Santos, and Espirito Santo basins. ANP has also regulated the geophysical data acquisition business and geophysical companies have responded by investing heavily in non-exclusive speculative modern seismic surveys.
With the implementation of the New Oil Law 9478/97 of August 6, 1997, the Brazilian oil industry entered into a new capital investment era. The new law also effectively ended the state monopoly, Petrobras, which has operated since 1954.
In early 1997, Petrobras started negotiations for farm-out partnerships with foreign companies for the purpose of exploration and production (E&P) activities. As of September 1999, 20 E&P joint venture partnerships have been signed with majors and independents. The most important of these are the contracts for the development of the giant deepwater Albacora East Field by Exxon and partners in the North Campos Basin and the E&P contract in Block BC-4 by Texaco and partners. This includes the development of Frade Field.
The implementation of the law 9478/97 also made several on-the-spot adjustments to all future Petrobras exploration activities. Petrobras was granted a three-year transition period, to explore, delineate and evaluate commercially its own preferred concessions. These encompassed less than 12% of the total Brazilian sedimentary area. The evaluation-delineation period was later extended to five years.
On the other hand, the first bidding round conducted by ANP (June 1999) opened the Brazilian market to competition, with new international and national companies bidding for acreage with a 7-9 year evaluation period. It is interesting to note that high bids were concentrated in the blocks of Santos, Campos and Espirito Santo Basin.
The ANP now oversees all activities in the oil business. Since the New Oil Law, the systematic evaluation of the Brazilian oil potential is coordinated solely by the ANP. Among the main foreseen future activities are the continuation of the considerable investments in exploration and development of the deepwater and ultra-deepwater regions in the Campos Basin. A strong increase in exploration activities is also foreseen in deep water of the best risk/reward basins of the Brazilian coast such as Santos and Espirito Santo Basin.
In addition to the Upper Cretaceous and Tertiary reservoirs, future exploration drilling will involve probing of deeper objectives reaching petroleum systems of the synrift in the Campos, Espirito Santo, and Santos basins. At present, 70% of the Brazilian oil production comes from post-salt reservoirs of the Campos Basin.
The other main producing provinces are the onshore Reconcavo Basin and the offshore Sergipe-Alagoas, Potiguar, and Ceara Basins.
Therefore, the opening of the Brazilian basins to partnerships and to competition is stimulating an intensive renewal of the upstream business onshore and offshore. It is interesting to note that only nine of the 26 main sedimentary basins of Brazil have been consistently explored.
As a by-product of the new law, ANP issued a Ministry Law in December 1998 regulating geological and geophysical data acquisition, processing and interpretation. The Portaria No. 188 regulated and stimulated the acquisition of speculative surveys. Applications to ANP for the purpose of acquiring seismic speculative surveys soon followed.
A deepwater strike line over the Espirito Santo Basin. The shadow zones may indicate diapirism over the slope. These are interspersed with volcanics. Gravity and magnetics are needed to fully resolve these features.
Petrobras started offshore exploration in 1967 first with its offshore seismic surveys. Exploratory drilling soon followed. Petrobras' first strike of oil offshore, Guaricema, was in the Sergipe-Alagoas Basin, off northeast Brazil. In the Campos Basin offshore Rio de Janeiro, the first oil strike, Garoupa, was made in 1974 with the drilling of the eighth well, 1-RJS-9 well (Ponte et al, 1977). Oil production started in 1977 and, so far, more than 60 oil and gas fields have been discovered in the Campos Basin, seven of which are giant oil fields, and all are in deepwater.
With its exploration and pro duction operations in the Campos Basin, Petrobras developed a worldwide leadership position in deep water technology. Petrobras has concentrated most of its investment efforts in the Campos Basin off Rio de Janeiro and in the Reconcavo, Sergipe-Alagoas, Potiguar, and Ceara basins in northeast Brazil. The Campos Basin, with its giant deepwater fields, is a world class hydrocarbon province. The deepwater areas of Espirito Santo Basin, Santos Basin and even selected areas of Campos Basin remain under-explored.
Improved seismic technology
New exploration impetus provided by ANP has had a large impact on new seismic data acquisition. As a result, the offshore Brazilian area accounts for a high number of active seismic vessels in the world. Technology employed in these surveys includes accurate navigation data through the use of a global positioning system (GPS), reduced towing noise due to smaller diameter streamers, smaller separation between streamers, longer streamers yielding improved velocity information, increased footprints possible due to improved towing technology and improved real-time data quality checking.
The seismic exploration effort in the eastern Brazilian margin is geared toward accurate imaging of pre-salt and post-salt sequences. The resulting seismic resolution in these sequences has improved substantially with modern technologies that are being employed in the data acquisition, data processing, and data interpretation. Gravity and magnetics are being acquired and interpreted along with the seismic profiles.
Due to the economics in the deepwater environment, all future seismic exploration in the Brazilian continental margin will aim at the precise imaging of hydrocarbon plays in the pre-salt and post-salt sequences. The imaging of these deepwater reservoirs has also been accomplished with new 2D and 3D technology. Improved multiple attenuation and migration algorithms provide better imaging of the whole sedimentary section, hence seismic resolution.
The Santos Basin encompasses an area of more than 210,000 sq km and extends from Rio de Janeiro to Florianopolis. It is one of the largest prospective offshore basins in the world and is still under-explored. A large deepwater corridor, within the 500-meter and 2,000-meter isobaths, is essentially unexplored.
This basin has been explored since 1970, mostly within the wide continental shelf. Near the coast, the basin is a platform area that includes the basin margin hinge-line system. The down-to-basin fault system strikes parallel to the coastline. In the Santos Basin the "salt wall" is rather massive in deepwater and ultra-deepwater, and may, locally, preclude hydrocarbon migration from source to reservoirs.
The Campos Basin occupies an area of about 100,000 sq km. The basin is bounded to the north by the Vitoria High and to the south by the Cabo Frio Arch. Campos Basin is the main hydrocarbon producing basin off Brazil. In the Campos Basin, an extensive unconformity marks the boundary between the Cretaceous and the Tertiary.
Below this unconformity, a transitional environment lithology of carbonates and salt layers were pierced by wells above the lacustrine synrift section. In the Tertiary overburden, there is the occurrence of a pervasive distribution of thick turbidite sand reservoirs, particularly in deep water.
The structures of Campos Basin are within a northeast-southwest trend of down-to-basin faults. Migration of oil from the synrift Lagoa Feia Formation towards the Upper Cretaceous and Tertiary turbidite reservoirs is accomplished through extensive faulting and breaches through the salt wall.
The offshore extension of the Espirito Santo Basin reaches a width of more than 300 km from the coastline to the continental slope. The total area of the basin exceeds 50,000 sq km of which almost half is occupied by the Abrolhos Bank (Fainstein and Summerhayes, 1982).
The internal structuring of the Espirito Santo Basin and its marginal banks is further complicated by the extensive Eocene lava flows, which are interspersed with the salt domes. Here, resolution of oil and gas plays is supported by synergistic interpretation of seismic, gravity and magnetics.
The structures encountered in the salt basins off eastern Brazil are regionally similar. They are essentially typical of a divergent, rifted, passive margin (Asmus and Ponte, 1973). Seaward tilted tensional features characterize most dip profiles. The normal block faulting pattern is also evident in the basement relief, which consists of an alternating series of graben and horst blocks.
The oil and gas plays are mostly confined to stratigraphic features of the higher horst blocks. Mirror images of these structural features are encountered in several of the West African marginal basins.
Salt diapirism also originates hydrocarbon prospective structures in deepwater and ultra-deepwater. Most remain untested. Salt tectonics influence the structuring of the entire eastern continental margin.
Several distinct petroleum systems occur within the eastern Brazilian marginal basins. They are associated with lacustrine synrift source rocks, restricted marine and transitional environments, and open-marine transgressive sediments. The best source rocks are within the synrift sequence.
Substantial oil reserves have been found recently in deepwater and ultra-deepwater offshore Brazil and Africa. Among the deepwater fields of Brazil are Marimba, Espadarte, Caratinga, Barracuda, Albacora, Marlin and Roncador. All are within the Campos Basin. Estimated recoverable reserves for the three giant oil fields in ultra-deepwater, Albacora, Marlin and Roncador, exceed five billion bbl.
Main oil reservoirs of these deepwater discoveries are either shelf-derived turbiditic sands, encountered in the upper and lower continental slope, or distal marine turbiditic sand bodies transported via channels to the lower slope (Dolivo, 1997; Fainstein et al, 1998). These are generally found in the Upper Cretaceous and Tertiary sequences. It is interesting to note that there are prolific deepwater trends on both sides of the South Atlantic - the Lower Campos Basin and the Lower Congo Basin. The two approximately match continental reconstruction.
The complete list of references is too long to present here and can be obtained from the author at Email: email@example.com.