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Sub-salt could be sweet

Seismic and drilling technologies could soon unlock new resources in sub-salt strata beneath existing discoveries in the Campos basin, allowing exploration companies to capitalise on existing infrastructure to improve project economics, claims Schlumberger. Tom Nicholls reports

CONCERNS ABOUT Brazil's upstream potential are unwarranted, as even the country's most prolific oil-producing basin has accessible geological horizons that exploration firms have never tackled, says Schlumberger. Sub-salt strata beneath existing discoveries in Campos (as well as in other basins) could contain important oil deposits that technology is putting within commercial reach, the oilfield services firm claims.

These horizons (see box) have not been examined in much detail yet because there were plenty more readily accessible post-salt turbidite reservoirs to develop first and because of the formidable technical challenges involved in this kind of upstream work.

But as exploration moves into ultra-deep water, sub-salt resources will play an increasingly important role in exploration, say Roberto Fainstein, a Rio de Janeiro-based Schlumberger geophysicist, and Jeff Gorski, a Houston-based manager at Schlumberger Oilfield Services (until last month, commercial director of the company's southern Latin America operations).

The next frontier

"The prospects in the ultra-deep water, both above and beneath salt, are the next exploration frontier," says Fainstein. "This may drive exploration efforts in the next five years. We know there is potential.

Petrobras has found many sub-salt accumulations in shallower waters."

Adds Gorski: "There are several stratigraphic levels that were out of exploration reach. The petroleum system works fine. Technology - seismic technology  - is, therefore, the key to development."

Gorski says exploration firms in Brazil are faced with two basic options - looking for elephants in Brazil's vast frontier basins (an approach that has produced limited results since upstream liberalisation in the late 1990s) or harnessing technology to squeeze further potential out of areas where exploration has already taken place. The second alternative, made possible as a result of advances in seismic and drilling techniques, would enable explorers to take advantage of existing infrastructure in the highly exploited Campos basin, which would improve the economics of potential projects, he says.

This kind of cost saving could prove vital for many developments, as even relatively large finds offshore Brazil can be of marginal commercial value. Most exploration is taking place in deep and ultra-deep water, meaning finding and development costs are high. Also, much of the oil being found is high viscosity, making it difficult and expensive to produce and less valuable in the market. On top of that, Brazil does not yet have the refining capacity to cope with significant volumes of high-viscosity oil, so the cost of transporting crude to overseas refineries must also be taken into account.

The quantity and quality of seismic available to exploration and production (E&P) companies has increased dramatically in recent years. With 225,000 km of 2-D seismic and 25,000 square km of 3-D seismic, Schlumberger has built up a strong position in Brazil's seismic information market. It claims to have 90% of the country's state-of-the-art 2-D data market and 35% of newly acquired 3-D data.

Longer streamers (8 km for 2-D acquisition and 6 km for 3-D, compared with under 3 km in the past) have improved imaging quality. In addition, technical advances are making sub-salt imaging possible for the first time. "Our technology - pre-stack time migration and pre-stack depth migrated processed seismic sections - enable imaging beneath the salt, which was nearly impossible before," says Fainstein, who designed all of Schlumberger's 3-D and 2-D surveys for Brazil.

"The modern 2-D seismic technology employed offshore Brazil includes accurate navigation data through the use of GPS [Global Positioning System] and a longer streamer, and has yielded the improved velocity information critical to the imaging of the sub-salt sequences," says Gorski. Clearer-resolution images of post-salt horizons allow E&P firms to optimise drilling routes above and through the salt, he adds.

Successful studies

The company claims the risk/reward studies it carried out before investing heavily in speculative surveys have paid off. Gorski points out that the three most important discoveries - in blocks BS-500 and BS-4 in Santos and Campos' block BC-60, which could contain 0.6bn barrels of recoverable oil - have been within the areas covered by its 3-D surveys.

Deep-water Campos' high prospectivity has been explained by adequate maturation and timing of hydrocarbons migration, through salt windows, into widely distributed turbidite reservoirs that range from the early Cretaceous to Miocene, says Fainstein. He adds that the same could be the case for recent discoveries in the deep-water area of the northern Santos basin and should be applied for prospects over the numerous deep- and ultra-deep-water blocks of the continental margin.

In total, Brazil has over 20 offshore sedimentary basins, but only Campos has been consistently explored. Well density elsewhere is extremely low. Yet even Campos remains under-explored, compared with other regions, such as the Gulf of Mexico (GoM).

Gorski says Brazil's growing collection of modern 2-D and 3-D speculative seismic data adds "a powerful database" with which to map the Upper Cretaceous and Tertiary turbidite prospects in deep-water regions, including Campos. These are deep-sea sediments generally encountered within structures constructed by the eastern margin salt wall.

Because of the economics of deep-water development, seismic exploration in Brazil's continental margin will, in future, aim at the precise imaging of hydrocarbons plays in the pre-salt as well as post-salt sequences, says Gorski. The data enable accurate imaging of the salt structures and of the salt fairways of the outer continental slope and plateaus.

The modern data have clearly imaged the syn-rift structures beneath the salt - consequently enabling the mapping of all possible migration routes from source rock to reservoirs. In addition, the structural interpretation of these data may unravel new plays within the syn-rift.

However, in terms of pure results, it is hard to deny that Brazil is proving disappointing. Hopes that Angolan-scale success would quickly be replicated look unfounded. But it is not all bad news. Petrobras recently announced a potentially very large find in the northern part of the Campos basin. Shell is evaluating a 300m-0.5bn barrel heavy-oil prospect in Santos. Other accumulations have been identified and the general view is that it is much too early to write the country off as an attractive exploration zone.

That is the conclusion of a recent report by the UK consultancy firm, Wood Mackenzie, which notes that cumulative Brazilian discoveries are at roughly the same level as in the GoM, but that well density offshore Brazil is much lower. On the negative side, however, it points out the lack of huge discoveries since the late 1990s, implying a diminishing probability of making more of a similar size.

 


The geology

THERE ARE three major sedimentary basins in the shelf and slope off eastern Brazil - Santos to the south, Campos in the middle and Espírito Santo to the north. These petroliferous basins, even Campos, are under-explored, writes Roberto Fainstein, a Schlumberger geophysicist.

The evolution of these three sedimentary basins was controlled by plate tectonic events associated with the early rifting and subsequent drift of Brazil and Africa. The main constructive characteristics of the sedimentation in the offshore basins off eastern Brazil consist of three stages - pre-rift, rift and post-rift.

These events controlled basin stratigraphy and basin infill.

The stratigraphic columns of these basins are generally similar, with lacustrine sediments of the rift stage being covered by evaporites (salt deposits) and carbonates of Aptian and Albian age, and then covered by an overburden of open marine sediments. The stratigraphy of eastern Brazil's continental margin basins, therefore, reflects the evolutionary phases of continental separation and marginal basin accretionary processes during the continental drift of Brazil and Africa. As such, sedimentary sequences may be grouped within four major distinct units:

The lower basal unit, which corresponds to pre-rift sequences and consists mostly of continental red shales and conglomerates; 
The syn-rift sequences, which consist of lacustrine deposits.

These are hydrocarbons-rich, source-rock shales, interspersed with stacked coarse-to-fine-grain sandstones; 
The evaporitic-gulf sequence, deposited during the Albian-Aptian time of restricted ocean circulation south of the equatorial south Atlantic. These are widespread deposits of salt and carbonates above the so-called break-up unconformity; and 

The marine sequence that may be further divided into two sub-sequences - the first is predominantly shallow marine platform carbonate deposits and the second, open marine sediments. The large discoveries in deep water off eastern Brazil are in turbidite reservoirs of this post-salt open marine sequence.

Several petroleum systems occur within Brazil's marginal basins.

The most important is the sub-salt Lagoa Feia formation, associated with the lacustrine syn-rift source rocks. Reservoirs and seals occur within the syn-rift, transitional and marine sediments.

The fairway of massive salt layers and salt diapirs offshore eastern Brazil runs in deep-water Santos, Campos and Espírito Santo.

Massive salt domes occur in Santos' São Paulo plateau. Salt diapirs cause the formation of hydrocarbons-prospective structures along the entire eastern margin. Most turbidite reservoirs are encountered in higher post-salt structures.

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