TUNISIA - The Geology.

As in neighbouring Algeria, the geology of Tunisia is dominated by a post-Paleozoic unconformity. The main source of its oil is a set of Silurian shales with most oilfields producing from Triassic reservoirs.

It is believed that the onset of generation was delayed until well after the Hercynian orogeny. Traps are found in faulted anticlines.

Tunisia is the smallest of the North African countries. Its 164,150 sq km area is wedged between Algeria and Libya. The northern region and a large part of the country's eastern frontiers are delineated by a Mediterranean coastline stretching about 1,300 km.

Geographically, the country is divided into a northern region lying above the eastward extension of the Algerian range of Atlas Mountains, a middle section of low plateaux and plains sloping towards Tunisia's eastern coast, and a southern region adjacent to the Saharan areas of Libya and Algeria.

Offshore prospects appear to be moderately promising, but more so for natural gas than for oil. The country has a broad continental shelf in the Gulfs of Gabes, Tunis and Hammamet. There is only a relatively narrow shelf to the north in the Mediterranean proper.

It is in the broad shelf that several E&P companies have been interested. A territorial settlement with Libya over the so-called "7th November" block offshore, in the Gulf of Gabes, has led to the establishment of Joint Oil Company (JOC) shared by Libya's NOC and Tunisia's state oil concern ETAP.

This area is rich in oil and gas, to be developed for JOC by a consortium of Nimir Petroleum of Saudi Arabia and Petronas of Malaysia. The geological features of one structure there, Omar, are similar to Libya's nearby oilfield of El Bouri which has a large gas cap (see July 1999 survey of Libya & who's who in Vol. 53).

The Pelagian Basin is the remaining major oil province of Tunisia. It is likely to prove to be more rich in natural gas than in oil. The basin straddles the coast and includes the Gulfs of Gabes and Hammamet, together with adjacent onshore areas. The hydrocarbon habitat is similar both onshore and offshore.

Traps were produced by anticlinal folding and the source is believed to be Late Cretaceous to Early Tertiary shales. Reservoirs are provided by Cretaceous carbonates, both limestones and dolomites with rare sandstones, and Tertiary sandstones and limestones.

The offshore Amilcar block of British Gas International (BGI), which contains the largest reserves of natural gas in Tunisia, lies in the Pelagian Basin. In this block are the Miskar field, the country's biggest producer of natural gas, and several other structures. Miskar produces gas from Cretaceous reservoirs called Abiod, R-1 and Bireno at an average depth of 10,500 feet. Miskar's recoverable gas reserves are estimated at about 20 BCM.

The biggest among the other fields now to be developed by BGI is Hasdrubal. This field's recoverable reserves are estimated at 28 BCM, lying in a nummulitic limestone reservoir called Ypresien El Gueria.

In the Gulf of Hammamet, an interesting oil discovery was made in January 1998 by Agip of Italy from a Miocene formation called Umm Douil and a Paleocene reservoir called El Haria. This is part of the Pelagian Basin. The wildcat, Baraka-1, was tested at the rate of 4,353 b/d of 43 deg. API oil and 4.48 MCF/day of natural gas at the depth of 2,670 metres. .

In 1982, Marathon of the US discovered an oil and gas reservoir in an Albrian-Cenomanian formation called Zebbag, in 8 metres of water. This lies in the Djeffara Basin about 17 km south-east of Zarzis. The company then estimated the field's recoverable reserves at 16 million barrels of oil and condensates and more than 100 BCF of natural gas.

The onshore section of the Pelagian Basin, first explored by BGI, is prov-ing to be gas-prone as well. Preussag Energie of Germany has discovered natural gas in several small structures in the Kerkennah Ouest Block B, which it acquired from BGI in 1997. The largest among these is the Chergui field, whose reservoir in a Mid-Eocene nummulitic limestone called "Reineche" is said to contain more than 70 BCF of recoverable reserves.

Another onshore Pelagian Basin discovery was made in early 1998 by Premier Oil. The company's wildcat tested at the rate of 3,600 b/d of 42 deg. API oil and 3.7 MCF of gas in carbonates of the Upper Cretaceous Bireno formation. The discovery was made at depths of 3,350-3,423 metres. Premier also made a discovery in its appraisal well, El Jem-2, which was tested at the rate of 245 b/d of condensate and 13 MCF/day of gas from an Upper Cretaceous limestone reservoir called Douleb.

Onshore, the geological elements which are best developed in neighbouring Libya and Algeria enter into Tunisia and cross the country. The Triassic Province, a prolific petroleum region in Algeria, enters into the southern tip of Tunisia. The Cretaceous trend for oil onshore is best developed in Tunisia. It is also found on the Algerian side of the border (see Feb/March 1999 survey of Algeria in Vol. 52). Several oil and gas fields have been developed there, with the main reservoirs in Triassic sandstones and subordinate reservoirs in Silurian clastics. Traps are found in faulted anticlines.

The Ghadames Basin straddling Algeria, Libya and Tunisia is an interesting prospect. The basin extends from Algeria and runs across Tunisia into Libya. On the Tunisian side, this has been relatively unexplored. Several companies are now drilling exploration wells on the Tunisian side, to the east of the prolific Berkine Basin of Algeria where large oil reserves have been discovered.

Agip, active on the Algerian and Libyan sides of this basin, has made some oil and gas discoveries on the Tunisian side in a Silurian formation called Acacus and a lower formation called Tannezuft. Agip is the biggest oil producer in Tunisia and Libya. On the Tunisian side of the Berkine Basin, Agip's Hammouda-1 wildcat was in 1998 tested at 3,508 b/d of 42 deg. API oil, 1,400 b/d of condensate and 14.2 MCF/ day of gas in the Tannezuft formation.

To the north of the country, a series of oilfields associated with the southern margin of the Atlas Mountains is found in a north-east and south-west trending basin. Reservoirs are in Cretaceous carbonates, mostly dolomites, sourced by Cretaceous shales, and traps are anticlinal closures.

In the Cap Bon region, in the north-east, an interesting oil discovery was made in 1998 by a company called Ecumed. The weld was tested at 3,007 b/d of 52 deg. API oil and 0.57 MCF/day of natural gas from a Ypresian formation called Bou Dabbous.

The Eocene has attracted the attention of foreign companies in recent years. This followed Marathon's discovery in June 1992 of gas and condensates in the Zarat block in the south. Located in 90 metres of water about 106 km north-east of Marathon's Ezzaouia oilfield, the first wildcat flowed at a combined rate of 17.5 MCF/day of gas and 1,498 b/d of condensates. The US company said the reservoir's depth was 8,629-8,842 feet and there was a column of over 97 metres in the Eocene El Gueria formation. The column contained a 25-metre oil zone and more than 67 metres of gas/condensate pay.

Most prospective areas in Tunisia are gas-prone and oil reserves are not likely to be of the size found in Libya, Egypt or Algeria. Nevertheless, Tunisia is interested in the gas prospects in view of its programme to shift to natural gas (see Downstream Trends).

Tunisian oil exploration, however, is verging on the phase of maturity and prospects for major discoveries in the established provinces must be considered low. As with Algeria and Morocco, there may be scope for a deep overthrust play associated with the Atlas Mountains, but the area for such a play in Tunisia is very much reduced.