SOUTH ATLANTIC GEOLOGY-1: Geologic insights listed into Africa, Brazil deepwater exploration

March 26, 2001
Triton Energy Corp.'s discovery of Ceiba field off Equatorial Guinea highlights key concepts of the geology and exploration potential of the central and southern Atlantic Ocean region.
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Triton Energy Corp.'s discovery of Ceiba field off Equatorial Guinea highlights key concepts of the geology and exploration potential of the central and southern Atlantic Ocean region (Fig. 1).

Ceiba is in an area neglected by the industry but rated as a frontier zone of major interest in a recent IHS Energy Group study.

This two-part article highlights the key concepts published in two companion reports, "Deepwater Exploration Opportunities-South Atlantic African Basins" and "Deepwater Exploration Opportunities-Brazilian Basins."

The deepwater offshore zones of the Atlantic Ocean from the Amazon River mouth to Walvis ridge are at first glance homogeneous in structure on both sides of the Atlantic.

Generally speaking the margin is of the passive, starved type where water depth quickly reaches 3,000 m. It is essentially constructed of limestone sediments. Mostly these conditions are not favorable for oil and gas accumulation; however, in places there are very interesting variations to this pattern, and these offer the likeliest opportunities for hydrocarbon prospectivity.

In particular, they are responsible for the Congo fan and Campos basin petroleum provinces and the discoveries in the Niger Delta deep offshore.

The discovery of Ceiba in an apparently very different geological setting surprised many because no successful drilling had been recorded within thousands of kilometres of the three provinces mentioned above. We can predict the discovery of other new provinces from the detailed study of petroleum geology made in the two deepwater exploration opportunity reports.

The reports are based on a series of regional seismic sections, and it is possible to explain the many cases of exploration failure and show that the potential petroleum provinces discussed constitute up to one-fifth of the sedimentary basin located in up to 3,000 m of water.

The purpose of this article is to display some of the key points examined over hundreds of pages and in 70 large regional seismic sections and through many source rock maturation simulations in the two deepwater reports.

Economic ingredients

Simply stated, the following factors are the generally accepted requirements for successful exploration in deepwater environments:

  1. Thick, continuous reservoirs with high flow rates and a large drainage radius;
  2. Recoverable reserves of at least several hundred million barrels;
  3. An accumulation that is sufficiently geologically simple and well defined by seismic at the producing horizon to allow for highly accurate modeling.

If major discoveries had not already been made in the deep offshore these restrictions would be discouraging since very few fields discovered on the shelf and onshore conform to these requirements. The Atlantic passive margin, however, has developed in such a way that the geology of the shelf has very little in common with the geology of the upper slope, which itself hardly resembles the lower slope and even less the vast and serene tranquility of the abyssal plain.

In general, the discoveries made onshore and on the shelf are grouped in isolated petroleum provinces (occupying less than one tenth of the sedimentary area) resulting from a combination of favorable local conditions. In the deep offshore it is possible to find this same situation, with prolific oases in a vast desert so to speak but not necessarily in the same regions as found on the shelf.

Broadly speaking, the greater the water depth the greater the likelihood of finding oil, especially in 3,000-3,500 m of water. Thus, nature offsets the costs associated with water depth, and one can foresee economically viable exploration in the ultradeep offshore.

The rift

The thick layer of salt deposited at the end of the rifting phase in the South Atlantic plays a major structural part in the geology of the deep offshore.

Broadly speaking, the petroleum geology of the deepwater offshore South Atlantic area evolved from the Aptian rift transition zone marking the opening of the Atlantic (Lower Albian in the north). However, because the economics of deepwater fields requires a very large drainage area and high flow rates, rift sediments will not yield the right type of reservoirs. That is why, as a rule, all drilling to presalt targets has been and will be fruitless.

Yet in many areas-in particular the Campos basin in more than 1,000 m of water-the sequences lying just below the salt contain major oil-prone source rocks. Obviously their oil maturation could only take place in optimal burial conditions.

Break-up of Africa, Brazil

The break up of Africa and Brazil during the Atlantic opening is complex:

  • In time because near the Walvis ridge in the south we know of deepwater facies from the Middle Aptian, whereas in the Central Atlantic the series are not marine until the Upper Albian.
  • In manner because the opening leads to a simple passive margin only over the 500 km between southern Gabon and central-southern Brazil. The farther one gets from this central region, the more the geology is constrained by shearing, as for example the dominant shearing between the Barreirinhas basin and Ivory Coast.
  • In location because continental separation is not parallel to the present-day shorelines but zigzags in the Central Atlantic, obliquely crossing the South Atlantic, leaving practically all the rift and its salt in the north on the African side and in the south on the Brazilian side. The special feature of this asymmetry is that the rift and its discontinuities had a marked structural influence on subsequent events, and it is possible to draw a soundly reasoned parallel between, for instance, the Senonian basins of Port Gentil in Gabon and Santos in Brazil.
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From the opening to the present time, the deep offshore Atlantic has hardly been more than a starved margin with a thin shaly cover, very monotonous, and subjected to permanent thermal subsidence. Such an environment develops fine, essentially limestone-marl deposits in euxinic conditions. Over time it is favorable to the local and repetitive formation of oil prone source rocks that would have had no chance of reaching maturation and would have had very few reservoirs to charge if it were not for events independent of the thermal subsidence (Fig. 2).

These events are associated with phases of plate readjustment caused by the opening of the Central-Southern Atlantic and the influence of Andean subduction. Together, these events led to local modifications that brought with them the final ingredients for creating prolific petroleum systems.

These structural events relate to phases of basement uplift during episodes of shearing. These episodes are associated either with slight changes in the direction of sea-floor spreading or, as in the Senonian and the beginning of the Tertiary, with an increase in the intensity of sea-floor spreading combined with the birth of the Andes and an oceanic lowstand.

This gives rise to strong clastic influxes which result in the petroleum wealth of the deep offshore, as strong clastic influxes mean definable conical sedimentary bodies and turbiditic sandstones. A cone of sediment automatically infers a thickening of the sedimentary column and increased source rock burial. Therefore, the existence of prolific zones is directly linked to strong clastic influxes reaching the sea and in turn to ancient river systems.

Source rock maturation

The two deepwater reports mentioned previously use simulated pseudo wells from 70 regional seismic sections distributed over the entire central South Atlantic to model potential source rock maturation.

The results of this modeling fit well with the real data obtained in the Campos and Santos basins (both oil and gas) and with the distribution of hydrocarbons observed on the Congo fan (mainly gas in the center, with full oil maturation on the crown). They also explain the failure of the many unsuccessful wells drilled.

Consequently, due to the large number of simulations carried out, all drawn from seismic sections calibrated to nearshore wells, we are able to suggest that if seismic is available it is possible to master the issue of turbiditic sandstone charging.

One of the sites used to model geological development and hydrocarbon charging is situated just north of Ceiba field, on the Equatorial Guinea border with Cameroon, and in the study the presence of oil in Senonian turbidites is unambiguously shown.

As the model was published more than a year before Ceiba was found those who read the report will not have been surprised by the discovery. However, while it is probable that Ceiba will not remain an isolated discovery, it is obvious that anyone tackling the area without due care may find themselves seriously disillusioned as was shown by the dry holes Evouga Marine 1 and Padouk 1 off Gabon. Although source rocks are broadly present in this region, prediction of trapping and reservoir distribution are not simple matters.

Trapping

None of the major discoveries in the South Atlantic has been made in a purely structural trap. The whole range of combination traps can be found, from the predominantly structural such as Roncador in the Campos basin to the almost completely stratigraphic traps of fields such as Marlim, also in the Campos basin.

Next: The second and concluding part of this article contains analysis of the distribution of turbidite sandstone bodies and basic structural factors.

The author

Raymond Joyes spent 35 years with Elf, latterly as worldwide director of exploration, strategic studies, and asset evaluation. He has served as head of global geology, head of Eastern European strategy, and exploration manager in Congo, Iran, Brazil, and Gabon. Since 1994 he has served as a consultant to IHS Energy Group, specializing in deep water oil and gas exploration reviews. He is an engineering graduate of the Institut Francais du Pétrole and received specialty training at Ecole Nationale du Pétrole et des Moteurs. He completed his doctorate degree at the Museum National d'Historie Naturelle.