Brazil's other offshore hydrocarbon basins relatively untested

The Espírito Santo and Santos Basins are nowhere near as well explored, excepting the onshore portion of the former. They share many parameters of stratigraphy, tectonics, and play types with the Campos, which make them highly attractive to the industry. This is reflected in the heavy focus of investment by companies in these basins in the first Open Bid Round, which closed on 16 June 1999.

In the frenzy to secure acreage in these highly attractive areas, it should not be overlooked that the other coastal basins off Brazil remain prospective. Although not nearly as heavily documented in the literature as the "Big Three," sufficient information on these other basins can be found in the public domain to evaluate and rank them, and to determine which, if any, merit further consideration.

Tectonic background

The Brazilian continental margin basins were formed as a result of the breakup of the Gondwana supercontinent and the opening of the South Atlantic Ocean, a process beginning in the Late Jurassic. The opening was diachronous, with rifting and zipper-like separation generally moving from south to north. Separation as far as the Sergipe-Alagoas Basin area had ended by Early Aptian. Tectonic activity then shifted to what became the equatorial coast of Brazil, and separation between Africa and South America was completed (final communication with the North Atlantic) by the end of the Aptian.

This shift in tectonic activity is an important one, because it helps explain the very different tectonic styles encountered in the eastern coastal area (Pelotas to Sergipe-Alagoas Basins) and in the northern one (Potiguar to Foz do Amazonas Basins). While the former basins are characterized by extensional tectonic movements (with critically important halokinesis), the structural architecture along the northern coast has been strongly affected by wrench tectonics.

East Coast basins

Proceeding northward from the Espírito Santo Basin, one encounters a chain of areas collectively known as the Bahia Sul (South Bahia) Basins. The southern (Cumuruxatiba) and central (Jequitinhonha) ones are essentially an extension of the Espírito Santo, with which they share the same stratigraphic nomenclature. The separation into discrete basins is the result of the continental extension of Paleogene seafloor volcanic activity, with the Royal Charlotte Bank separating Jequitinhonha from Cumuruxatiba and the much larger Abrolhos Bank separating Cumuruxatiba from Espírito Santo.

The Abrolhos volcanics (a formation name also used in the Royal Charlotte Bank) are often depicted as a thick mass of basalt, but in reality the unit consists of alternating layers of basalt and sediment, and is locally capped by a substantial carbonate layer. This combination has made imaging of the pre-Tertiary section a challenge.

Nevertheless, the pre-volcanic section, including a distal hydrocarbon kitchen, overlying salt, and the Upper Cretaceous marine section (with potential turbiditic sand flows) are expected to occur undisturbed beneath the bank. The one exception is in the halo zones of the basalt vents. These still need to be defined. Although still requiring more study, the areas covered by volcanics will become a focus of interest in the not-too-distant future.

• Cumuruxatiba/Jequitinhonha Basins: Both the shelf and deepwater areas of the Cumuruxatiba and Jequitinhonha Basins are fundamentally under-explored. Submarine canyons have been recognized, suggesting the presence of submarine fans, which occur in large numbers in the shallow Espírito Santo Basin (and presumably in its deepwater sector). The Belmonte Canyon in Jequitinhonha appears to have cut mainly into carbonates in the Regàência Platform, which would have unfortunate consequences for reservoir potential in the deeper fans.

The Mucuri Canyon in northern Espírito Santo/southern Cumuruxatiba, in eroding into the Sao Mateus and Mucuri sandstones, is more likely to have provided a good potential reservoir unit in deeper water. A distal pre-salt section in the Cumuruxatiba Basin is expected to provide a viable and mature source rock, and salt movement to permit migration of generated hydrocarbons through salt welds and listric faults into turbidite sandstone reservoirs in the post-salt section.

• Camamu-Almada Basin: The northern Bahia Sul Basin, Camamu-Almada, is more closely related to the onshore Recôncavo Basin immediately to its north. The latter was part of the eastern coast spreading complex but, as a failed rift arm since the Early Aptian, exhibits neither Aptian evaporites nor a post-Aptian marine section. Nonetheless its 6,000 meter of rift section were the principal producing area of Brazil prior to the offshore discoveries, with nearly 5 billion bbl of oil and 3 tcf of gas in place, discovered by 1994. The genetic relationship of Camamu-Almada to Recôncavo, with its proven high-quality and mature source rock, makes the former, which has seen very little work performed in its deepwater sector, a very attractive area in which to pursue potentially large targets.
• Sergipe-Alagoas Basin: After Recôncavo, the major focus in Brazil's exploration efforts centered on the onshore basins of Sergipe and Alagoas states. It was here that Petrobras' move offshore was begun and the first offshore discovery (Guaricema) made in 1971. The focus of offshore exploration work has been, and largely continues to be, the relatively narrow shelf area. Very little drilling has been carried out in the very steep slope or deep basin areas. Nonetheless, numerous paleocanyons have been identified and there is justifiable expectation of deepwater reservoirs, substantially larger than what has been found to date in the shelf zone (estimated 120 million boe).

• Potiguar Basin: Turning the corner from the eastern to the equatorial coast brings one to the Potiguar Basin, which has the distinction of offshore hydrocarbon discoveries that pre-date the first successful onshore accumulations. Potiguar is unusual in that its shallow offshore area demonstrates the overprint of WNW-ESE-trending tectonic activity on the SW-NE-trending rift system related to (but separate from) the one which opened the South Atlantic up to Sergipe-Alagoas and Recôncavo (Matos, 1992).

North Coast basins

This resulted in complex structural architecture of the shallow offshore area (Touros and eastern part of the Aracati Platforms), and small individual fields with an estimated total of 300 million boe in place. Of particular interest for further potential are large submarine canyon systems (notably Ubarana). These have cut through the Aptian Açu formation as deeply as the Early Cretaceous Pendáecia formation, which is made up of coarse-grained alluvial fan and braided stream clastics.

The eroded material is expected to be found in deeper water, in slope canyons or at the foot of the continental slope. As no drilling has been carried out in water deeper than 100 meters, the potential for encountering substantial hydrocarbon reserves in turbidite traps is considerable.

• Ceará-Piauí Basin: Immediately to the northwest of Potiguar, the Ceará-Piauí Basin, shows the effects of wrench-faulting on hydrocarbon prospectivity. In those areas where the coast is substantially parallel to the direction of wrench activity (essentially following the trends of the Atlantic transform faults), the tectonic picture is very complex, structures tend to be very small and broken up, and little or no success in exploratory work is recorded.

On the other hand, where the coast forms an angle to the wrenching trend, extensional tectonics appear to play a larger role. It appears that, during breakup, several small pull-apart basins may have formed. Source rock may have been deposited in these basins.

Later movement did not have a negative impact on structures in these basins, preserving their integrity and that of any existing seal. The Mundaú Sub-basin, very similar to Potiguar but separated from it by the Fortaleza High, has provided a similar amount of hydrocarbon in place. To the north and west of this sub-basin, the stratigraphic column is far less complete and the stronger tectonic activity during the Upper Cretaceous and Tertiary is quite obvious. The Mundaú Sub-basin is considered to have similar potential to the Potiguar in mass-flow deposits at the foot of the continental slope, an area that has only seen two wells drilled in water deeper than 200 meters.

• Par'a-Maranhão Basin: The Par'a-Maranhão Basin, located on the eastern side of the mouth of the Amazon River, has seen sporadic exploratory work performed both by Petrobras and some risk contractors, but all in very shallow water depths. Only two non-commercial oil discoveries resulted (Pas-9 and Pas-11). The shallow basin exhibits numerous submarine canyon cuts, most of which appear to cut into the extensive carbonate platform developed there. Some of them have, however, eroded portions of the proximal alluvial fan facies, allowing coarse clastic sediments to be transported into deepwater areas.
• Foz do Amazonas Basin: Last but not least, the Foz do Amazonas Basin remains an attractive if high-risk area. Given its relatively remote location, it has been fairly well explored, but work has generally been concentrated in specific areas, such as the Caciporé Graben. This sub-basin off the coast of the state of Amap is a possible pull-apart feature with mild wrench effects, based on its tectonic position.

A source rock has been identified in the Caciporé, and numerous oil shows were encountered while drilling. The gas from the only completed "field" in the basin, Pirapema, is biogenic. In recent talks in Salt Lake City and Rio de Janeiro, Damuth (1998) has pointed out the potential for encountering reservoir sands in what is often assumed to be the silt/shale pile of the Amazon Cone.

The basin is probably best known and feared for the considerable operational difficulties encountered there. Strong winds, currents, tides, salinity changes, massive discharges of material from silt to tree-trunks, and shallow gas accumulations are some of the challenges presented to operators by the nearby discharge of the Amazon River. The rewards, however, if these obstacles can be surmounted, may be well worth the effort. It should also be noted that the eastern side of the Amazon River mouth, closer to where the Pará River discharges, are far less well explored (especially the deeper sections of the rift grabens) and present considerably less problematic operational challenges.

Conclusions

Although no other basin has been studied and had its results published to the extent that Campos has, there is enough material available in the public domain on other basins to allow a fair review of the comparative merits of all of them. It is certainly clear that the Campos Basin has numerous advantages over other basins. The main ones are:

• A suitable hinterland for sourcing clean, clastic reservoir material and a fluvial system to bring it offshore
• A broad platform area to receive the clastics prior to renewed transport down the continental slope for final deposition in a broad abyssal plain
• A prolific source rock
• A tectonic framework permitting source deposition relatively close to shore
• A substantial evaporite section subjected to flow and deformation, providing structural and stratigraphic trapping mechanisms as well as migration pathways
• A benign geothermal gradient.

Some of these factors occur in other basins, suggesting that while the hydrocarbon volumes may be smaller than those expected to be found in the "Big Three" basins, they may still constitute economically worthwhile targets. Companies interested in investing in exploration and production offshore Brazil have several other basins available, which offer opportunities for finding economically viable hydrocarbon accumulations.

Author

Dr. Henry M. Lieberman is a regional explorationist and Consultant with PGS Reservoir (US) in Houston. Information on further references, comments and queries should be addressed by e-mail to [email protected].

References

Damuth, J., 1998: New Evidence from Sediment Cores for Extensive Sand Distribution in "Mud-Rich" Deep-Sea Fans: Insights Gained from Drilling Amazon Fan during ODP Leg 155; AAPG Convention Extended Abstracts, Vol. 1, A147.

Matos, R.M.D., 1992: The Northeast Brazilian Rift System; Tectonics, Vol. 11.