Regional structure of offshore southeastern Brazil

This view shows geometry, thickness, and complex distribution patterns of the minibasins within the salt province and the major depocenters of the post-salt section.

Despite these successes, the region is still largely under-explored. Upper Cretaceous and Tertiary reservoirs have been the main focus of exploration to date, but future exploration drilling may also involve the probing of deeper objectives, reaching petroleum systems of the syn-rift section within these basins.

This article is based on the interpretation of 85,000 line-km of 2D regional seismic. Data from 58 publicly available wells were also integrated into this study, as well as gravity and magnetic data. In addition, 2D depth migrations of selected lines were useful in revealing complex sub-salt structures. Seven key sequence boundaries were interpreted: Seabed, Blue Marker or Intra-Oligocene time equivalent, Near Top Cretaceous, Near Top Albian, Top Salt, Base Salt, and Top Basement. From these boundaries, key isotimes were constructed: Seabed to Top Salt, Salt, and Base Salt to Top Basement. The resulting maps show the structural expression of these three basins. Among the benefits of this new data is the clearer definition and distribution of the subsalt syn-rift strata, which contain the primary source rocks.

Espírito Santo basin

The stratigraphic column of the Espírito Santo basin comprises a rift section of Neocomian and Barremian strata, mostly clastics interspersed with volcanics. An erosional break-up unconformity caps the rift section, which is overlain by a transitional sequence of Aptian clastics and evaporites. These are buried under a restricted marine sequence that includes clastics and limestones. A very thick Tertiary age carbonate and clastic sequence accumulated on the Abrolhos platform. Progradation occurs around the Abrolhos Bank on the northern margin of the basin. In addition, there are prograding strata from shelf to deepwater in the southern part of the basin.

Top of salt viewed from the south. Red represents shallow salt/salt welds, and blues represent deep salt/salt welds.

Major structural elements of the Espírito Santo basin include two main fault hinge lines: one on the shelf, the pre-Aptian hinge line, and one close to shelf break near the Vitória High. These hinge lines control the deposition of the Late Cretaceous sequences. The shelf sequences are characterized by a syn-rift trough on the platform that is isolated from the deepwater rift sequences by faulting near the slope.

Many of the piercing deepwater salt features rise to the sea floor. The offshore part of the Espírito Santo basin is under-explored, with the deepwater and ultra-deepwater portions of the basin in the south essentially undrilled. These areas display similar features to those of the northern Campos basin, with a variety of turbidite plays in the Tertiary and Upper Cretaceous sections. In the deepwater region, the seismic shows reflector truncations against salt walls. The mini-basins are often floored by welded salt, which allows easier hydrocarbon migration from pre-salt, syn-rift sources.

Campos basin

Basement lithology consists of basaltic extrusion above pre-Cambrian crystalline rocks. The sedimentary section above basement is characterized by the influences of three tectono-stratigraphic mega-sequences: rift, transition, and drift. The rift mega-sequence is composed of alluvial, fluvial, lacustrine, and carbonate coquina sediments. The Pre-Alagoas Unconformity marks the upper boundary of this mega-sequence. The transitional mega-sequence comprises lacustrine and lagoonal deposits, sealed by restricted marine evaporates - mostly halite. However, some anhydrite also occurs landward.

The open marine drift mega-sequence encompasses neritic and bathyal environments of Albian to Cenomanian age. Sediments of this age change from clean limestone at the base of the formation and landward, altering to marls and shales toward the top of the formation, becoming more shaly seaward. Still in the drift phase, open marine sediments were deposited during Turonian time, which continues to the present. Beyond the shelf, bathyal and abyssal clastics occur in deepwater environments. On the shelf there is still a significant amount of carbonate development.

The Lower Cretaceous, organic-rich, lacustrine Lagoa Feia shale is the major source rock for the entire basin within the syn-rift section. This occurs above the basement, which comprises predominantly north-south to northeast-southwest trending faults. The Base Salt to Basement isotime map provides a key indicator for high-volume hydrocarbon source rock regions. Thicknesses of more than 1,000 m (approximately 0.5 sec two-way-time) are common. Locally, source-rock wedges may thicken to more than 3,000 m in the deeper rift half-grabens.

The syn-rift section appears relatively thinner beneath the massive salt province, at distances more than 100 km from the present shelf edge. The syn-rift section seismic character comprises continuous high-amplitude seismic events, contrasting with the low amplitude, low continuity seismic character of the underlying basement and overlying salt.

Basement highs surrounded by thick syn-rift section may be areas containing prospective leads. These "bald" basement highs are common in the deepwater fields of the Campos basin. The giant Rôncador field is situated over a basement high adjacent to thick syn-rift section downdip.

The salt province occurs over a large extent of the Campos basin. Salt is thin throughout the shelf area, with occasional small pillows developing along the footwalls of listric faults.

The massive salt province is situated beyond the shelf edge. Salt diapirism is such that 100 km from the shelf edge, large salt walls have developed. The large salt walls are roughly parallel to the shelf edge and commonly have salt thickness to 4,200 m (2 sec two-way-time). Well-developed mini-basins occur between the salt walls. A salt escarpment, marking the edge of the salt province, trends north-northeast.

Post-salt structures are dominated by salt tectonics, and resulting structures and faults generally trend northeast-southwest. Turtle structures are common and can produce very attractive leads.

Campos turbidite deposits produce excellent reservoirs; the Marlim, Albacora, and Rôncador reservoirs are good examples. Fields comprising large turbidite deposits tend to lie basinward of the present shelf edge. Seismic events analogous to those resulting from known turbidite fields have been identified at greater distances from the shelf and into the mini-basins between salt walls.

Many of the turbidite field oil accumulations are in combination structure/stratigraphic traps, with over 80% of the oil partially trapped stratigraphically. Pre-salt reservoirs also occur in the Campos basin, where hydrocarbon migration takes place over shorter distances.

Santos basin

Early Cretaceous Barremian to Neocomian volcanics, deposited above pre-Cambrian crystalline rocks, form the basement architecture of the Santos basin. The overlying syn-rift sequence consists of conglomerates, fine-grained sandstones, source-rock shales, and coquinas.

The South Atlantic opened from south to north. During Aptian time, rift valleys filled with salt. By end Aptian time, rifting transitioned to passive margin drifting. After deposition of salt as a horizontal layer filling the Aptian rift valleys, a continuous sheet of shallow water carbonates was deposited extending up to 350 km from the present shoreline.

Map showing the thickness and distribution of syn-rift section that contains the major source rocks of these basins.

Mid-to Late Albian carbonate rocks, deposited as a prograding carbonate bank, are thick on the present-day shelf region. The carbonates thin as they prograde seaward into deeper water. Albian carbonate anticlines and rollover fault traps occur where the salt pinches out updip from the salt basin.

After Middle Albian time, basin subsidence exceeded carbonate formation rates. The depositional environment changed from shallow water to deepwater. The section tilted down to the east toward the ocean basin, with turbidite sandstones being deposited on the basin slope. These late Albian deposits are terrigenous clastics, shales and turbidite sandstones that bypassed the shelf, depositing into deepwater down the slope via channels.

These slope fan sediment fairways create a distinct distributary pattern through the thick salt province and influence salt distribution. Strong hydrocarbon indicators are seen in these slope fans on the flank of the Sao Paulo Plateau basement high, where the channels terminate or drape over the offshore plateau high.

This turtle structure formed between two salt highs. Beds terminating against salt form additional traps.

A large fan was deposited in a trough on the slope in the southern half of the Santos basin in response to the Middle Oligocene sea level fall at about 30 Ma.

Late Cretaceous and Early Tertiary volcanic intrusives are also present within the northern and southern Santos basin. Approaching the Cabo Frio High, volcanic episodes occurred during Santonian (about 84 Ma) and also Eocene (about 50 Ma) time, forming mound-like features in the northern Santos basin.

The structure of the basin near the coast consists of a carbonate platform area that includes the basin margin hinge line. This system of down-to-basin faults strikes generally parallel to the coastline. The main exception is the Cabo Frio Fault zone in the central-northern Santos basin. This fault formed as a result of rapid progradation of the Middle Albian to Late Cretaceous fan onto a thick mobile salt sheet. The progradation of the fan caused the salt mass to be displaced from west to east toward deepwater, creating a fault gap with over 50 km of horizontal displacement toward the coast in the Albian carbonate section. Compressional structures and other features associated with salt tectonics are common in the basinward regions. There is also a thick syn-rift source rock section in the abandoned rift basin under the present slope.

The Sao Paulo Plateau, which is associated with a major basement high composed of very large basement fault blocks, is located on the continental slope. It covers an area of 16,000 sq km, with its outer edge essentially forming the eastern margin of the Santos basin. The interpretation of the 2D regional seismic survey identifies rift sedimentation over parts of the Sao Paulo Plateau, indicating it is underlain by continental crust. In Late Albian time, the Sao Paulo Plateau remained a shallow-water marine block while the rest of the basin deepened as thermal subsidence increased.

Widespread massive salt walls occur over a large extent of the Santos basin. Prospects around the salt walls, within the intervening mini-basins, comprise Upper Cretaceous turbidite bed terminations against the salt walls. The new seismic dataset has significantly improved imaging of the well-developed syn-rift section that suggest new plays in prospective reservoirs within the Early Cretaceous Guaratiba Formation, including Barremian-Early Aptian coquinas, and lacustrine sandstones.

In the southern Santos basin, the salt province boundary is seen toward the southern edge of the study area. basinward of this boundary, the top of oceanic basement is well imaged as a highly faulted but relatively flat seismic event that is indicative of oceanic crust.

Untestd play types consist of turbidites truncating beneath the Near Top Cretaceous Unconformity around the flanks of the Sao Paulo Plateau basement high; turbidite pinchouts against salt wall flanks; and syn-rift coquinas, lacustrine sandstones, and fractured basalts draped over or fault-trapped against basement highs.

Conclusions

Regional high-quality datasets, such as those used in this study, allow for an improved understanding of the regional structural setting of this petroliferous, but under-explored, region of offshore Brazil. Improved imaging of the pre-salt syn-rift sections produces more accurate source rock distribution maps as well as better defined pre-salt targets. In addition, the locations of known fields and discoveries in the post-salt section can be put into their regional context such that analog structures and play types can be more readily identified, cataloged, and ranked. Tested and untested plays still offer many attractive targets in this region.

Acknowledgements

The authors would like to thank WesternGeco and TGS Nopec for use and display of the seismic data of the Brasil 1999/2000 program.

References

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