W. Cameron Davies
Bass Resources Ltd.
London
Abderrazak Bel Haiza
Enterprise Tunisienne
d'Activites Petrolieres
Tunis
Tunisia's oil exploration and production industry is experiencing a boom of almost unprecedented proportions.
The catalysts are changes in the fiscal and legislative regime and a high success ratio in an exciting new exploration play, Upper Cretaceous Abiod chalk.
The foundation of this re-newed phase of activity was laid in 1985.
The Tunisian government decided to liberalize the terms and conditions that apply to the hydrocarbon exploration industry, in order to encourage the search for reserves to replace those being rapidly depleted through increased industrial, commercial, and transport consumption.
It was expected at the time that Tunisia would become a net importer of oil again by the early 1990s.
Production from giant El Borma field was declining at around 5%/year in spite of an extensive secondary recovery program.
TERMS SWEETENED
The initial amelioration to the fiscal and licensing terms was enacted in September 1985.
Before its effects could be perceived, the oil price had begun to spiral downwards and oil exploration budgets suffered large cutbacks.
It then became clear to the government that to compete for scarce exploration funds it would have to further improve Tunisia's position in the league table of countries in which oil companies would spend their increasingly precious exploration budgets.
By September 1987, a second revision to the legislation had been passed through the legislature easing the tax burden yet again and introducing the concept of the profit/investment ratio into the law.
The final piece in the jigsaw was enacted in 1990 giving further tax incentives and introducing more flexible licensing terms. In combination these changes began to attract a steady stream of applicants for open acreage but even so the net acreage under license was slowly decreasing.
TWO DISCOVERIES
The discovery by Marathon in 1986 of 20 million bbl Ezzaouia field near the holiday island of Djerba began to change the industry's perception of Tunisia as a highly prospective and increasingly attractive country in which to explore for hydrocarbons.
Ezzaouia field came on stream in late November into production facilities (OGJ, Dec. 3, p. 40).
This new perception was reinforced in 1988 by AGIP SpA's discovery of Maamoura oilfield in the Gulf of Hammamet. The field's reserves may be about 50 million bbl of oil.
The reservoir for this field is the Abiod chalk of Campanian/Maastrichtian age, which had not previously been considered as reservoir because of its low porosity and permeability.
However a flow rate of 3,300 b/d of oil was achieved from the second well on the structure, thus confirming that the Abiod could produce oil at potentially commercial rates.
The record of wells testing the Abiod reservoir in the light of this new perception has been remarkable.
The Maamoura discovery was followed up on land by Kufpec's 1 Sidi el Kilani well, reported to have tested at a stabilized rate of 2,600 b/d of oil, Shell's 1 Zinnia at 1,300 b/d of condensate with gas, 1 Tiref soon to be long term tested, and 6 Miskar, reported to have flowed in excess of 50 MMcfd of gas.
FISCAL, LEGISLATIVE FRAMEWORK
The objective of the Tunisian authorities' energy policy is to maintain a high level of exploration and production activity in the country and maintain energy self sufficiency as long as possible.
The new laws promulgated in 1985, 1987, and 1990 laid down a flexible framework. Oil companies can negotiate either a convention that has a back-in for ETAP or a production sharing agreement in which the incoming party acts as contractor for the state and controls all of the operations in the permit through a management committee.
Exploration can be carried out in three ways, namely, through:
1) An exploration license, which allows the holder to carry out all preliminary geological evaluations before making a work commitment.
2) An exploration permit, which gives the holder the exclusive right to carry out seismic surveys and to convert at his option to a full prospecting permit.
3) A prospecting permit, which gives the holder the exclusive right to carry out all exploration and drilling during as long as 5 years.
Most companies opt for prospecting permits. These are granted under a convention and memorandum of obligations, which are approved by law.
A joint venture agreement is simultaneously entered into with ETAP, which is given the option to participate once it has been decided to develop a discovery.
If ETAP decides to take up its option, it then pays its share of development and operating costs and also reimburses its share of past exploration costs attributable to the permit.
The new laws have built in fiscal options to encourage development of marginal oil fields and find gas that can be sold to the domestic market at 85% of the price of heavy fuel oil.
EFFECTS OF THE CHANGES
The principal changes in the law were the lowering of tax and royalty rates, introduction of capital uplifts of 10-30% on exploration expenditures, the change to a sliding scale of royalty and taxation related to a profit to investment ratio.
The ratio is the accumulated net income divided by the total accumulated expenditure on the concession.
Finally, Tunisia simplified administrative procedures for the granting and management of exploration and production licenses.
These changes have encouraged the development of a number of previously marginal fields with reserves as small as 3 million bbl.
The reaction of the international oil industry to these terms has been favorable and led to the issue of prospecting permits to Shell, Amoco, Conquest, Coho, British Gas, Maxus, Sovereign Oil & Gas, and Conoco.
The specific provisions for natural gas are reported to have led directly to the reevaluation of large Miskar gas field (750-1,000 bcf), discovered by Serept/Elf in 1975 and under evaluation by British Gas.
The likely project cost will be about $600 million for the field and more than $1 billion for a gas fired power station to supply electricity to Europe via a link parallel to the existing Algeria-Tunisia-Italy gas pipeline across the Sicily channel.
REGIONAL GEOLOGY
Tunisia is divided conventionally into three principal structural provinces, namely,
The Sahara platform.
Western Tunisia.
Eastern Tunisia.
The Sahara platform consists of Precambrian basement overlain by a thick series of Paleozoic sediments only gently folded in the northern part.
Great thicknesses of Cambrian to Permo-Carboniferous rocks were eroded or truncated during the Hercynian orogenic phase, which juxtaposed the reservoir sands of the Triassic against the prolific Silurian Tannezuft shale source rock (mean total organic carbon 4.7%).
Maturation of these shales is thought to have started in post-Jurassic time, and all reservoir rocks in contact with them have been found to contain hydrocarbons.
The largest discovery and principal oil field of Tunisia, El Borma, is located here and produces about 55,000 b/d of oil from originally recoverable reserves of 700 million bbl.
WESTERN, EASTERN AREAS
Western Tunisia is characterized by broad scale atlasic folds separated by wide synclines grading further north into the overthrusts and nappes of the Tunisian atlas.
Oil production from this zone has been generally on a small scale with reserves in the 5-15 million bbl range and production of the order of 2,000-3,000 b/d (Douleb, Tamesmida, Semmama fields).
Eastern Tunisia consists of a large shallow continental shelf and a series of low plains. It can be divided into four zones, all of which are more or less important in the context of oil exploration and production.
The Gulf of Gabes, which was a zone of subsidence throughout Triassic, Jurassic, and much of Cretaceous, contains the important Ashtart oilfield, which produces about 27,000 b/d of oil from Ypresian nummulitic limestones.
The zone was protected from atlasic compression, and as with the Tripolitanian basin is characterized by distention faults forming horst and graben features and tilted basement blocks. These blocks of Jurassic and Cretaceous sediments are sealed by prograding late Cretaceous and Cenozoic formations.
Salt tectonics is also important here, and some Triassic diapirs have pierced later sediments to form long submarine salt walls.
CENTRAL ZONE, CAP BON AREA
The Central zone has been an area of irregular uplift and subsidence between the gulfs of Gabes and Hammamet. It is characterized by a series of northwest to southeast trending faults forming major grabens such as the Sidi el Hani trough onshore and the Jarrafa trough offshore.
These zones of subsidence have proved to be important oil kitchens not only for the Abiod but also for the Cenomanian/Turonian discoveries in the Sfax area.
Salt tectonics is less important here, but salt induced fracturing similar to that at Ekofisk in the North Sea enhances the reservoir properties of several of the recent discoveries.
The Gulf of Hammamet is a complex and unstable area with thick Cretaceous and Tertiary sediments accumulating in the depocentres and separated from them by partially eroded highs.
Finally, the Cap Bon peninsula is squeezed between the atlas front and the Gulf of Hammamet.
It is formed by the large Sidi Abderahman anticline and consists principally of a thick sequence of shales with only two carbonate reservoir sequences of Upper Senonian (Abiod) and Ypresian (Metlaoui).
The Ypresian (Metlaoui) trend has been extensively explored following the Ashtart discovery, but the Abiod has until recent discoveries been considered a secondary objective.
The latest finds have shown that this judgment was wrong, and several companies are reentering to test Abiod in old wells in which shows were observed while drilling and electric logs had perhaps been wrongly interpreted.
ABIOD FORMATION
The Abiod formation of Campanian/Maastrichtian age is essentially composed of carbonates, generally, chalky limestones and it derives its name from Oued el Abiod, where the type section was described by P.F. Burollet in 1956.
It is divided into three members:
A) A lower carbonate member comprising chalky mudstone, rich in foraminifera (Globigerinidae, Globotruncanidae, and Orbitoides) also containing frequent fragments of pelecypods and echinoids. This member rarely exceeds 70 ft in thickness.
B) A middle member is an argillaceous limestone rich in benthic foraminiferous such as Pseudotextularia and Neoilabellina. It is normally of the order of 100 ft thick.
C) The upper carbonate member is the most important from the economic point of view because it often has porosities in excess of 20% and fracture permeabilities of 1-3 Darcies.
It comprises three submembers starting with a chalky limestone having rare thin argillaceous bands and a basal calcareous glauconitic sandstone. It grades into a more interbedded chalk and light gray marl which when drilled becomes a sticky amorphous mass. These two sub-units reach a combined thickness of about 230 ft.
RESERVOIR FACIES
The principal reservoir facies of the Abiod is a massive chalk with very rare green marl horizons.
It is irregularly stratified with slumps and turbidite fan deposits that enhance the porosity to good reservoir quality. The turbidities are composed of calcarenite and nodular fragments of Inoceramus, Echinoides, and rudistids frequently dolomitized and brown in color.
This member can exceed 600 ft in thickness. The upper member is overlain by the El Haria shale formation, an extensively distributed excellent seal for this carbonate reservoir.
The Abiod formation extends over most of northern and eastern Tunisia, is known to outcrop in the north and west, and is present in wells throughout the pelagian shelf from Cap Bon to the Gulf of Gabes.
It gives a clear response on the sonic velocity log but in several recent wells has not exhibited any oil or gas shows during drilling, although it has produced more than 3,000 b/d of oil under subsequent test.
Nondeposition occurred in the area of Tunisia southwest of the city of Kairouan known to paleogeographers as Kasserine Island.
SOURCE ROCKS
There are two principal source rocks for the recent Abiod discoveries, namely the late Cenomanian or early Turonian Bahloul formation and the Albian to Cenomanian Fahdene formation of open marine facies.
The Bahloul is a thin, dark colored, commonly laminated, marly limestone characterized by a euxinic fauna of globular plankton such as Whiteinella gigantea, Whiteinella parabudia, and Whiteinella aprica with various forms of Heterohelix.
There is abundant organic rich material in the darker horizons probably derived from the decomposition of phytoplankton.
Total organic carbon levels in the Bahloul can be as high as 8.5% and average 3% over a thickness of several metres. The oil is a light 41 gravity.
These sediments formed in slope facies or restricted euxinic basins within the shelf.
The Fahdene, in contrast, consists of a mixed facies of dark shales, bioclastic limestones, and marls. Reefal buildups may develop in the lower or middle members of the formation, but the principal source sequence occurs in the middle.
The Mouelha formation of Albo-Vraconian age is a dark gray to black claystone with thin beds of black limestone up to 300 m thick and containing microfauna characteristic of the Upper Albian, e.g., Rotalipora breggiensis and Ticinella primula.
Geochemical analysis shows that the Mouelha formation has TOC values ranging from 0.65% to 3.6%, although individual values of up to 14% have been recorded with yields of 14 kg/t.
This formation, although less organic rich than the Bahloul, tends to be an order of magnitude thicker and rather more widespread geographically.
SEAL
The Maastrichtian El Haria formation is a fairly uniform dark gray shale with some minor interbeds of limestone towards the base.
The El Haria is rich in benthic and planktonic forams and ostracods and was deposited over an area broadly similar to the Abiod.
Its isopach distribution is also similar to that of the Abiod, and it thus provides a blanket seal except over one or two eroded highs (e.g., Kerkennah/Isis).
However in these few locations the overlying Paleocene shales complete the sealing process.
RECENT ABIOD DISCOVERIES
Explorationists and production engineers began to take a fresh look at the Abiod as a potential reservoir and to upgrade it from a secondary to primary target following the discovery and testing of the 1 Maamoura Est well by AGIP in 1988.
The well flowed 3,300 b/d of oil from fractured Abiod close to the top of the reservoir section. The oil was of excellent quality at 41 gravity, but the reservoir was poor, having a high clay content and a primary porosity of about 8%.
Potential reserves are reportedly about 50 million bbl.
The difficulties of producing from fractured reservoirs are well known, and the application of horizontal drilling technology for the first time in Tunisia is being investigated as a means to prevent early water breakthrough and improve economic potential.
At the end of 1988, Kufpec spudded a well at Sidi el Kilani about 70 km south of Maamoura to test the potential of a common primary objective, the Turonian carbonates at about 2,800 m.
No hydrocarbon shows were observed while drilling through Abiod, but sidewall cores were taken and found to contain oil. Later examination of electric logs confirmed the presence of hydrocarbons.
Good oil and gas shows were found in the primary target, but subsequent tests were not productive. However, on a final test of Abiod after acidizing, the well flowed at a stabilized rate of 3,300 b/d of 400 gravity oil.
A long term test was conducted for 30 days at 1,500 b/d and a surface pressure of about 600 psi. Petrophysical measurements on the sidewall core samples showed the chalk to be cleaner than that at Maamoura with no clay minerals present. Calculated porosity was substantially higher at 24% on trend with the expected porosity of chalk at a present depth of burial of 1,600 m.
OTHER DISCOVERIES
In July 1989, Shell completed and tested the 1 Zinnia well on the north shore of the Gulf of Hammamet about 25 km north of the Maamoura discovery.
The primary Turonian target proved unproductive, but again the Abiod flowed at a rate of more than 50 MMcfd of gas and 1,300 b/d of condensate.
The long term test of the reservoir confirmed the discovery as economic, and the condensate will be trucked to the refinery at Bizerte. The gas will substitute for imports in the gas fired electricity generating station at Korba.
Springfield drilled and tested 1 Tiref close to Zinnia and was rewarded with positive results in the Abiod at 2,800 m, a depth similar to that of the Zinnia reservoir. The well is being prepared for a long term pump test at a reported 500 b/d of oil.
Finally, British Gas tested Abiod gas in 6 Miskar at more than 40 MMcfd, a flow rate greater than that of the original primary reservoir target in Miskar gas field.
By adding Miskar field Abiod reserves to those of the Turonian/Coniacian Miskar reservoir it seems probable that the field will now prove to be a candidate for a major contribution to Tunisia's indigenous energy economy.
CONCLUSION
Tunisia's revised and favorable fiscal and legislative framework is encouraging international oil companies to take a fresh look at the economics of existing unexploited oil and gas discoveries.
Fields that were marginal under the old hydrocarbons law have been put on stream, e.g., Gremda and Mahares.
New discoveries in previously recognized reservoirs such as the Zeebag being developed at Ezzaouia by Marathon and ETAP are confirming Tunisia's self sufficiency in oil production in spite of the decline in the productivity of the oldest fields.
Recent discoveries have upgraded the Abiod chalk reservoir to that of primary exploration target, and the discoveries of the past 2 years have suggested that this reservoir may add substantially to Tunisia's reserves and help maintain the country's self-sufficiency for years.
Copyright 1990 Oil & Gas Journal. All Rights Reserved.
