TUNISIA'S UNEXPLORED BASINS SIMILAR TO PRODUCTIVE REGIONS ELSEWHERE

B. Duvernoy
Petroconsultants SA
Geneva

Tunisia offers infrastructure, stability, and promising geology contains relatively unexplored basins analogous to productive regions elsewhere in the world.

An important example is a foreland basin in the northeastern Maghreb, cornering northeastern Algeria, northern Tunisia, and extending on the offshore Pelagian Plateau. It is a basin type that some authorities either over-look with regard to Africa or claim to be missing from the continent.'

This basin has not been fully and rationally explored. Only a few old wells of moderate depth recognized the Tertiary formations-in and around the Medjerda Valley in northern Tunisia, for example.

This foreland area can be divided into three parts (Fig. 2):

• A northern one, the Tellian-Tunisian Trough, running southwest to northeast along the Atlasic Chain, where a thick series of Mesozoic sediments has been folded by the Atlasic orogeny before being overthrust by the post-orogenic Tortonian Numidian sandstones.

• A central one, which remained under shallow water during almost the entire Mesozoic and where neritic facies rapidly change into open marine or subcontinental deposits with large emersions.

• A southern one, the Chotts-Gabes Trough, similar to the northern one but not involved in the strong Atlasic tectonics.

The northern and southern troughs are surprisingly underexplored.

TELLIAN-TUNISIAN TROUGH

The lack of exploration in the Tellian-Tunisian Trough can be explained by the history of petroleum research and successes in North Africa.

Due to the well-known discoveries in Paleozoic and Triassic formations in the northern Sahara in the middle 1950s, Tunisia and neighboring countries have been considered as a possible northern extension of the Saharan Craton, deepening northwards into the Tethysian Sea and rapidly sinking down to inaccessible depths.

Moreover, the visible results of the Alpine tectonics (overthrust, diapirism) have for a long time puzzled petroleum geologists and geophysicists.

Abandoning this schematic "Saharan concept," one can consider northern Tunisia and adjacent areas as the external part of the Atlasic Chain, which has been pushed against the Saharan Craton, itself acting as a stable foreland with its own petroleum history. The transition zone between the northern part of the Saharan Craton covering central Tunisia and the southern part of the Tethysian Sea acts as a passive margin during the Mesozoic and as an active one during the Tertiary.

In this case one has to adopt and adapt the "exploration-work-in-foreland basins" to this specific country. The available stratigraphic data lead to a coherent paleogeoraphic and sedimentological history. On the edge of the Saharan Craton a thick neritic calcareous series forms during the jurassic - the Nara formation - while during the same time the northern trough (the Tellian-Tunisian Trough) receives a large amount of open marine shales with cephalopodae remains.

Along the slope of the neritic platform masses of horizontally and vertically graded debris flow into the basin, interfingering with the shales. This layout-a neritic platform bordered by a deep trough-is classic and very important from the petroleum point of view.

The trough acts as a kitchen, where marine shales rich with organic matter are deeply buried and mature, yielding their hydrocarbons into the interbedded calcarenitic reservoirs.

This petroleum framework becomes active and productive during the Tertiary tectonic and posttectonic phases.

At that time the folding of the Atlasic Chain generates anticlines and faulted traps (especially, overthrust anticlines and horsts; closures also could be secured by sharing out and facies changes)

The posttectonic phases, particularly the piling up of the Numidian overthrusts (Tortonian and younger), having thrown down the Jurassic sediments, achieved the maturation and expulsion of the hydrocarbons through compaction.

A PRODUCTIVE FRAMEWORK

The productivity of this paleogeo-graphic framework has been proven in eastern peninsular Italy, which is a foreland basin between the Apennines Chain and the stable Apulian Platform (considered as a detached part of the African Craton; Fig. 1).

The sedimentological framework described here in Tunisia occurred in the Adriatic zone during the Upper Cretaceous and Lower Eocene.

Calcarenitic slumps gliding from the neritic Apulian Platform into the Scaglia chalky basin have been proven productive in several medium size gas and oil fields: San Giorgio Mare, Maria a Mare, Sarago, Mormora, David, etc. The intercalations of the white calcarenitic layers in the red-layered chalk can be observed on outcrops along the coast at the Monte Conero (south of Ancona) and in the innerland.2

"The oil accumulations, which constitute about 10% of the original Italian reserves, were mainly discovered in the transitional facies between platform sediments and pelagic deposits," according to L. Matavellie et al.3

The calcarenitic Cretaceous reservoirs of the Adriatic basin represent hydrocarbon volumes of 50 million tons of oil and 10 billion cu m of gas with very low percentages of sulfur, CO2, and N2.4

Since Gautier, authors agree to consider the North African Atlas as the prolongation of the Italian Apennines, both chains being part of the orogenic belt born from the Tethys. 5 In Tunisia the corresponding petroleum system has till now not been proven, but several factors corroborate its probable existence.

• The edge of the neritic Central Tunisian Platform is lined by several little fields, which have no common characteristics but their geographic and paleogeographic location. They occur in the Coniacian (Ras Toumb, Djebel Onk, Gherguit el Kilal in Algeria; Douleb, Semmama in Tunisia), in the Campanian-Maastrichtian (Zinnia, Tiref in Tunisia), and in the Eocene (Belli in Tunisia). Other wells tested small flows of oil (Magroun, Rouissate, Oued Balhoul). These structures could have trapped hydrocarbons coming from the Tellian-Tunisian Trough, which accumulated in the first reservoir they found on the way of their migration. They could appear as oil seeps testifying to the presence of mature oil in the basin.

• Studies on the outcrops of the Jurassic basinal shaly M'Cherga formation show a content of up to 14% organic matter of "type II" and a hydrocarbon potential of 10-74 kg/ton. Moreover, the region we are considering contains the majority of the living oil seeps in Tunisia, most of them seeping out on the edge of Triassic diapirs. The Triassic itself does not seem able to have generated hydrocarbons, as one can see from its outcropping facies.6

• The presence of slumps on the southern edge of the trough is proven by the outcrops which show intercalations of intraformational conglomerates in the neritic Jurassic formation along the Zagouan-Bou Kornine Ridge.7 8 Moreover, the Tellian-Tunisian Trough is divided by a central narrow neritic discontinuous ridge, the Setif-Bizerte Axis, where reefal facies have been described and which also provided the deepest part of the trough with their debris.

CHOTTS-GABES TROUGH

South of the Central Tunisian Platform, the Gulf of Gabes and its western prolongation onshore, and the chotts area crossing Tunisia and extending into eastern Algeria lies the main North Saharan fault zone.

These faults have been active since at least the Permian and have been reactivated several times with more or less pronounced horizontal (compressive or distensive) component.

Thus the area is nowadays marked by a topographic depression like the Algerian and Tunisian chotts themselves. The trough has been less tectonized than the northern one, being protected by the Central Platform and practically out of the Atlasic folding. Nevertheless it suffers a continuous subsidence and received several thousands meters of sediments.

The trough itself receives , a 6,000 to 8,000 m thick succession of Middle Jurassic to Lower Tertiary marine to paralic carbonates and clastics," according to Schamel et al.9

This zone has been but slightly explored because the huge sebkhas constitute a harsh working environment. A few ancient seismic lines have been shot, and the structural framework is not well known.

Consequently, few wells have been drilled on the edge of the trough. They found oil and gas in the Paleozoic series, especially on the southern edge, in the Sabria and Franig areas. To the north the Mesozoic series has not been sufficiently recognized by seismic as well as by wells to depict a good paleogeographic and sedimentological framework.

Eastwards, south of Djerba Island, El Biban-1 and Ezzaouia-1 found oil and gas for the first time in Tunisia, in Jurassic sands and carbonates. These successes provide encouragement to continue and intensify the exploration of the Jurassic series all along the edges of the trough.

Three factors plead for an active exploration of this zone:

• The smoothness of the tectonic affords hope for large and regular structures.

• The Silurian black shales, which are the main source rocks of the Saharan fields, pinch out southwards on the flank of the Nefusa Arch, and deepen northwards where they could have fed the Mesozoic reservoirs through migration along the faults.

• The area lacks a continuous thick evaporitic series of the Triassic. Till now no diapiric structure has been reported in southern Tunisia, except offshore in the deepest part of the eastern Gulf of Gabes.

If a new giant field is to be found in Tunisia, it should be discovered in relation to the southern Chotts-Gabes Trough.

ESTIMATES

The major Tunisian offshore oil discovery (Ashtart) represents 300 million bbl. Assuming a $20/bbl stable oil price, it has been calculated that in similar conditions a 50 million bbl field of 30 gravity oil will generate positive cash flow at a 15% discount rate, with a stand-alone development plan and offshore loading.

In Libya, due to the greater water depth (more than 200 m) a 50 million bbl field with 34 gravity oil could generate a 20% rate of return (in spite of a $145 million development program on a stand-alone basis). A 25 million bbl oil field could be profitable if connected with other nearby discoveries.

Onshore, in northern and central Tunisia, even one 5 million bbl field with 40 gravity oil (reservoir depth around 2,500 m) will be profitable as the rate of return of the project reaches 16%.

Moreover, Tunisia is willing to develop its gas production, and the fiscal system is particularly favorable for gas projects in terms of lower royalties and tax rates. A gas field of 3 billion cu m appears as the threshold of profitability.

In northern Algeria the lack of transport facilities diminishes the profit of a 25 million bbl field with 30 gravity oil, the oil being trucked to an oil pipeline station. A 100 million bbl oil field, in spite of construction of a 50 km oil and gas pipepline, will generate a 42% rate of return.

CONCLUSION

Tunisia is a relatively easy country in which to work in terms of climate, road network, oil facilities and transport network, political stability, liberality of the oil law and regulations, flexibility of the fiscal system, and exploration costs.

It is unfortunate, therefore, that entire paleogeographical provinces remain almost unexplored despite the results obtained in similar basins around the world.

In spite of the difficulties inherent in a poorly explored basin, but thanks to the progress of technology it is probable that in coming years petroleum research will extend in northern Maghreb to explore the southern margin of the Tellian-Tunisian Trough and the margins of the Chotts-Gabes Trough, where it is likely that hydrocarbons have been generated and trapped.

REFERENCES

1. Perrodon, A., "Overview, of African petroleum systems," OGJ, Vol. 91, No. 28, July 12, 1993, pp. 115-118.

2. Duvernoy, B., Reulet, J., "Reservoir de turbidite calcaire: la formation 'Scaglia Calcaire' (Bassin des Marches, Italie)," in "Essai de caracterisation sedimentologique des depots carbonates," Centre Rech. Elf Aquitaine, Vol. 2. 1977, pp. 1-26-231.

3. Mattavelli, L., Novelli, L., Anelli, L., "Occurrence of hydrocarbons in the Adriatic basin, Italy, special publication, 1991.

4. Andre, P., Dubois, P., "Determining the petroleum potential of the offshore areas of the Mediterranean Sea," Bull. Centres Rech. Elf-Aquitaine, Vol. 16/1, 1992, 8 p.

5. Gautier, E.F., "Structure de l'Algerie," Ste. d'Editions Georaphiques et Scientifiques, Paris, 1922.

6. Belayouni, H., Chandoul, H., M'Rad, R., "Oil seeps in Northern Tunisia-Field Trip Guidebook," introduction, 3emes Journees Geol. et Explo. Petrol Tunis, May 1992.

7. Turki, M.M., "Polycinematique et controle sedimeiitaire associe sur la cicatrice Zagouan-Nebhana," These University. Turns. INRST, Vol. 7, 1988, pp. 1-252.

8. Schamel, S., Mellgard, A., "Structure and stratigraphy of Djebel Zagouan and adjacent areas," workshop notes and guidebook, Tunis, South Carolina University Schamel ed., 1982, R, P.

9. Schamel, S., Reed, J.K., Traut, M., Ben Hassine, K., "Geologic evolution and hydrocarbon prospects of the Gafsa trough, Central Tunisia," Abst. 3emes journees Geol. et Explo. Petrol. Turns E.T.A.P., May 1992.

Copyright 1993 Oil & Gas Journal. All Rights Reserved.