OVERVIEW OF AFRICAN PETROLEUM SYSTEMS

Alain Perrodon
Petroconsultants SA
Geneva

Each continent has its own personality, none more so than Africa, which today is the true heir to the once great Gondwana supercontinent.

Bordered by two large, still spreading oceans and subjected to tensional forces for more than 200 million years, continental Africa has now reached its largest surface expression ever.

Africa's geologic history has determined that its intracratonic basins are of continental rift type, some having developed into sag basins, others into divergent margin type basins. Foreland basins, so rich on other continents, are here practically missing and from a petroleum point of view of very minor interest.

With a total sedimentary area of 15 million sq km distributed in about 80 basins, Africa possesses and produces circa 10% of the world's oil and 7% of the gas.

More than 95% of these hydrocarbons are concentrated in seven petroleum provinces, ruled by three great petroleum systems that in turn are tightly controlled and determined by the geodynamic characteristics of the basins.

For instance, the preponderance of narrow, deep rift type basins throughout the Mesozoic and Cenozoic gives rise to rich lacustrine source rocks, vertical migrations, traps of horst, salt dome or rollover type, and the necessity for very effective seals. On the other hand, Paleozoic sag basins of the Saharan area are characterized by marine source rocks, lateral migrations, and large arched domes or folded anticlines.

From a stratigraphic standpoint, these three petroleum systems belong to three geological time periods: Silurian, Cretaceous, and Oligo-Miocene. Generally the Silurian system occurs in Saharan sag basins, the Cretaceous-Eocene systems in a continental rift complex of western and equatorial Africa, and the Cenozoic in two deltaic basins.

The Upper Paleozoic and Jurassic, so rich in other continents, here play but a very minor part.

SILURIAN SYSTEM

This system, often reinforced by Devonian source rocks, is sourced by sapropelic, radioactive graptolitic black shales rich in kerogene II.

These sediments were deposited as black shale and carbonates of Middle-Upper Devonian age on the southern margin of the Proto-Tethys and today form a large part of the sedimentary sequences of Sahara, in Algeria, and for a lesser part in western Libya and southern Tunisia.

These source beds onset into the oil window during late Cretaceous and today over a large acreage are still inside it. In the western Saharan basins they are in the gas window.

Oil and gas are reservoired in sandstones of Cambro-Ordovician and Triassic age, and to a lesser degree in Devonian and early Carboniferous beds. Migrations are of vertical and lateral types with seals formed by shales and evaporites, and entrapment is mainly structural over paleoarches.

This petroleum system occurs in three Saharan provinces:

The "Triassic basin," delimited by the sealing salt layers of Upper Triassic-Liassic age, holds initial reserves of about 18 billion bbl of light oil and 4,000 billion cu m of gas rich in condensate.
The Ghadames-Illizi basins and to a lesser extent the Murzuk basin in Libya and Algeria, where about 4 billion bbl of light oil and 1,000 billion cu m of wet gas have been discovered.
The Ahnet-Timimoun and Reggane basins of western Sahara, which are mainly gas prone, with circa 200 billion cu m.

In this system, the habitat is typically concentrated and the Triassic basin has the two largest fields in Africa, Hassi Messaoud with 10 billion bbl of oil and 200 billion cu m of gas and Hassi R'Mel with 3,000 billion cu m of gas and 4.5 billion bbl of condensate, both in Algeria. In all, about 22 billion bbl of oil and 5,000 billion cu m of gas have been discovered in this Paleozoic system.

Despite a slowing down of the discovery success rate during the last few decades, there still remains reasonable hope for further significant discoveries, more particularly in the Triassic basin.

CRETACEOUS-EOCENE

This system exhibits a more varied landscape and may be divided into two subsystems: a minor lacustrine one and a major marine one.

The lacustrine Cretaceous subsystem occurs mainly in continental rifts of Central Africa and Atlantic margin basins, more particularly in:

The Lower Cretaceous coastal grabens of the Gulf of Guinea, from Angola to Gabon.
The Middle and Upper Cretaceous of Central Africa, Niger, Chad, Central African Republic, and Sudan, the source rocks of these grabens being more or less associated with Paleocene and Eocene lacustrine black shales.

All these lacustrine source beds, which are very rich in kerogene I-II, occur in elongated narrow troughs. They matured beneath the charge of thick Upper Cretaceous and Tertiary sequences in the more subsident basins particularly in the marginal ones where the sedimentary column reaches 3,000-3,000 m due mostly to deposition from several permanent river systems.

The oils are generally waxy, highly viscous, low in sulfur and gas-oil ratio. The hydrocarbons are reservoired mainly in detrital reservoirs of Cretaceous to Miocene age. The traps are sealed by interbedded shales and also by Aptian salt in the "Aptian basin" of Lower Congo and Gabon. Locally, as in the small pull apart Abidjan basin of Cote d'Ivoire, the source rocks pass from Albian lacustrine facies to Cenomanian marine facies.

This Cretaceous lacustrine subsystem has produced almost 10 billion bbl of oil, of which 8.1 billion bbl have come from coastal basins.

The marine Cretaceous-Eocene subsystem, largely Upper Cretaceous in age with some important contributions from Paleocene and Eocene sediments, occurs mainly in the rifted basins of the Mediterranean front: Sirte, Suez, and Pelagian, and locally in the offshore part of two coastal equatorial basins, in Gabon and Angola.

In the Mediterranean basins source rocks are composed of Upper Cretaceous and Paleocene black shales rich in kerogene II and largely of Eocene age in the Pelagian basin.

These source beds matured mainly in Tertiary time during phases of rapid subsidence and high thermal flux.

They still are largely within the oil window.

Migrations are mainly vertical. Hydrocarbons are reservoired in detrital type Nubian and Miocene sandstones and in carbonate rocks, including reefs, of late Cretaceous-early Tertiary age.

Traps, mainly block faulted type, reefs, and pinchouts, are sealed by interbedded shales. The Sirte basin, the most important, holds many giant pools.

These Cretaceous petroleum systems have furnished some 62 billion bbl of oil, of Which more than 80% is located in the Mediterranean (Sirte basin 65%) and 1,400 billion cu m of gas. Contrary to the Silurian system, the Cretaceous appears richer in oil than gas.

Several of these continental grabens are still very much underexplored, and new hidden grabens or part grabens probably still remain to be discovered.

TERTIARY SYSTEM

This system is concentrated in the two deltaic basins of the Niger (Nigeria and Cameroon) and Nile (Egypt) rivers.

Source rocks are largely composed of mixed material components of allochthonous humic organic matter subjected, through bacterial degradation, to considerable enrichment in amorphous hydrogen rich matter. They were deposited in lacustrine and distal parts of the deltas, grading mainly in age from Oligocene to Miocene. Pliocene shales are gas prone. Maturation and migration have recently taken place and are still occurring, the latest following the growth faults.

Oil and gas are reservoired in sandstones inter-bedded by shales. The sands usually have been deposited in regressive off-lap sequences corresponding to a variety of environments varying from coastal to deep slope. Frequently porosities reach as high as 40%, and permeabilities lie in the 1-2 darcy range. Traps are structural and as a result of syn-sedimentary deformation are generally small to medium in size, complex but numerous. Habitat is typically dispersed.

Thirty billion bbl of oil and more than 3,500 billion cu m of gas have been discovered in this system, mainly in the Niger delta basin.

As has already been noted, the other petroleum systems, more particularly the Jurassic, have so far contributed very little petroleum. Liassic source rocks are known from Morocco (Rharb and Essaouira basins), and Callovian-Oxfordian black shales occur in the Western Desert (Egypt).

Upper Cretaceous source beds could contribute to oil genesis in this basin, where so far little more than 1 billion bbl of oil have been discovered. A Jurassic petroleum system could be present in marginal basins of the Indian coast.

CONCLUSION

The African continent appears to be irregularly and only moderated explored, with a mean density of five new field wildcats per 10,000 sq km of sedimentary basin and one of the highest technical finding rates.

Several large basins still have much underexplored acreage, particularly with advanced technology. Recently some attractive grabens have been discovered and therefore new interest areas and petroleum systems probably still remain to be discovered.

The marginal basins of the East Africa coast have surely not had their last word. On the opposite margin of the Red Sea, Yemen was for a long time a very unattractive exploration play.

Very little is known about deep offshore plays. The results so far obtained from the Campos basin on the South American side of the Atlantic Ocean show their attractiveness.

In salt basins, great strides have been made with new technology in deciphering the architecture of deep formations. A better knowledge of the petroleum systems and the characteristics of African geology, should enhance the chances of success.

All in all, new discoveries in the order of 30-45 million bbl of oil and 7,000-10,000 billion cu m of gas do not seem unreasonable, though the likelihood of discovering further giant fields is increasingly small.

Large basins may always hide at least some prospective oil bearing acreage, and if many large areas are barren today, productive acreages are often far richer than originally hoped.

Many basins such as the Saharan and Gulf of Guinea, two rich petroleum provinces today, have shown numerous dry wells before first discovery.

The lack of success encountered to date serves only to foster hope.

Africa, as always, remains an exciting challenge.

BIBLIOGRAPHY

Chatelier, S.Y., and Slevin, A., Review of African petroleum and gas deposits, Journal of African Earth Sciences, Vol. 7, No. 3, 1988, pp. 561-578.

Clifford, A.C., African Oil-past, present, and future, AAPG Memoir 40, 1985, pp. 339-372.

Demaison, G., and Huizinga, B.J., Genetic classification of petroleum systems, AAPG Bull., Vol. 75, No. 10, 1991, pp. 1,626-43.

Grunau, M.R., Rift systems can point the way to hydrocarbon richness, World Oil, Vol. 211, Nov. 5, 1990, pp. 96-98.

Klemme, H.D., and Ulmishek, G.F., Effective petroleum source rocks of the world: stratigraphic distribution and control deposition factors, AAPG Bull., Vol. 75, No. 12, 1991, pp. 1,809-51.

Perrodon, A., Petroleum systems: models and applications, Journal of Petroleum Geology, Vol. 15, No. 3, 1992, pp. 319-326.

Petroconsultants SA, Overview of African Sedimentary Basins: Hydrocarbon Systems, Reserves, and Potential 1993.