Ridha Mrad, Ali M'Rabet, Nelia Chine
Entreprise Tunisienne d'Activites Petrolieres
Tunis
W. Cameron Davies
International Consultant
London
The oil and gas exploration industry in Tunisia is continuing to experience an almost unprecedented boom as the effects of the favorable fiscal and legislative regime work through and recent discoveries come on stream.
Perhaps the most significant of the new discoveries is 1 Belli on Cap Bon, which Marathon tested at a rate of 6,800 b/d of oil with reported potential of as much as 15,000 b/d.
Followup wells on this structure are planned, and a Marathon farm-in well on Springfield's Cap Bon permit tested an initial 5 MMcfd of gas from the Bou Dabbous (Ypresian) reservoir,
Marathon Ezzaouia field production has increased in stages to 34,000 b/d of oil. Kufpec's 1 Sidi el Kilani is on long term test at about 2,000 b/d, and AGIP/Samedan's 1 Zinnia discovery has been followed by 2 Zinnia, which flowed a rated 2,000 b/d of oil.
New permits or extensions have been awarded in recent months to Coho Resources Ltd. (Canada), Sovereign Oil & Gas plc (U.K.), OKGT (Hungary), Samedan Oil Corp. (U.S.), AGIP SpA (Italy), and British Gas (U.K.).
Previously underestimated reservoir targets are being reevaluated, including the Upper Cretaceous Abiod chalk, the Eocene Bou Dabbous limestone, and the lower to middle Turonian Bireno carbonates.
MID-CRETACEOUS DISCOVERIES
Within the Sfax onshore and Gulf of Gabes offshore regions, the Mid-Cretaceous Turonian carbonates of Cenomanian age (Zebbag formation) and the lower-middle Turonian Bireno member are both potential and currently productive reservoirs.
The Zebbag formation gave rise to three significant discoveries that culminated in the discovery and development of Ezzaouia field (Fig. 1).
Total's 1 Isis discovery in 1974 flowed 2,700 b/d of oil from the Middle Cenomanian rudist limestones.
Marathon discovered El Bibane and the onshore Ezzaouia fields in 1984 and 1987, respectively. El Bibane tested 4,800 b/d of oil and 11.5 MMcfd of gas, while Ezzaouia field produces 22,300 b/d of oil, both from Lower Zebbag dolomites.
In addition, a number of onshore discoveries have been made in the Sfax area in the Bireno carbonates:
- Miskar gas field (1974), which Elf Aquitaine delineated and which British Gas is studying.
- Total's Mahares discovery (1978).
- The former Houston Oil & Minerals Corp.'s Gremda and Guebiba fields (198081).
- British Gas's Rhemoura discovery (1991).
The Bireno reservoir's potential, in the same way as Abiod, has been underestimated and little attention focused on it.
As a new potential contributor to Tunisia's hydrocarbon production in the future, this target is being reassessed in terms of its paleogeographical distribution, key features, and reservoir characteristics.
LOWER-MIDDLE TURONIAN FACIES
The Lower-Middle Turonian facies in Tunisia exhibit important lateral variations due to pronounced paleogeographical differentiation (Fig. 2).
In northern and eastern Tunisia, this time interval is represented by the Aleg-Kef formations, which consist of marine shales rich in planktonic microfauna deposited in relatively deep marine basinal environments.
In central Tunisia from the east towards Kasserine, South Kairouan area, and northwest of Sfax, the basinal facies become platform carbonates and are represented by three lithostratigraphic units.
At the base, the Bahloul formation (late Cenomanian to early Turonian) consists of laminated argillaceous limestones containing large globigerinids (Whiteinella). This organic rich formation was deposited in an outer shelf to slope-basin environment under anoxic conditions.
The middle Annaba member (Lower Turonian) consists of marine marls rich in pelagic fauna (ammonites and forams) in its lower part and enriched in benthonic microfauna (forams, ostracods) and calcispheres towards the top. This widespread transgressive unit reflects open marine conditions deposited in a basin to outer shelf environment.
The uppermost Bireno member is a carbonate unit subdivided into two parts. The lower consists of fine wacke-packstones rich in rudist reef bodies that have been deposited on a shallow marine carbonate platform. They are progressively overlain by inter-supratidal to sabkha facies consisting of algal laminated dolomites (stromatolites) and evaporites (gypsum and anhydrite).
In many places in the Central Atlas, the upper part of the Bireno member starts with an erosional base locally overlain by carbonate breccias and conglomerates of tectonic origin due to a regional northeast-southwest distensive phase.
The Bireno facies are often affected by dolomitization that enhances the reservoir quality of these carbonates. It is also worth noting that in the transitional zone between outer and inner shelf, the Annaba shales are replaced by limestone equivalents. The latter together with the Bahloul and Bireno-carbonates form a carbonate unit above the Zebbag formation called the "Aleg carbonate."
In terms of sequence stratigraphy, the Bahloul, Annaba, and Bireno units form a third order sequence of deposition including a shelf margin wedge (Bahloul), a transgressive system tract (lower part of Annaba shales or the carbonate equivalents), and a highstand system tract (Upper Annaba shales and Lower Bireno carbonates).
The Upper Bireno-carbonates represent the lowstand system tract of another overlying sequence. Its basal erosional surface corresponds to a Type I unconformity occurring near-92 million years old and related at least in part to tectonic activity.
South central Tunisia and southern Tunisia are characterized by inner to littoral shelf facies of Beida and Aleg units (restricted thin carbonates and evaporates).
Frequently, the lowstand system tract deposits are lacking (i.e., Bahloul equivalent) due to subaerial erosion. Central Tunisia is thus characterized by the existence of emergent shoals and the emersion of Kasserine Island.
In addition, lateral variations in thickness of the Lower-Middle Turonian deposits exist and follow the paleogeograph. The maxima of thickness (400-600 m) characterize the platform border with thick carbonate wedges towards the north-northeast and east-southeast and basinal zones towards the north and east (Fig. 2).
BIRENO RESERVOIR CHARACTERISTICS
Most of the discoveries within the Bireno reservoir are located in the Sfax region, which is characterized by widely varying structural types.
These include tilted fault blocks, graben edge plays, related stratigraphic traps, depocenters, and anticlines not yet drilled or not drilled deep enough to reach the Turonian-Cenomanian facies.
In the wells of the Sfax area, the reservoir corresponds principally to the Upper Bireno carbonates.
The Bireno member includes medium to high energy limestones and low energy fine limestones relatively porous due to dissolution in addition to porous dolomitized rocks.
The Lower Bireno, as thick as 150 m, is composed at the base of lime-mudstones and wackestones being partly dolomitized and containing rudist debris and benthonic forams. They locally exhibit fractures cemented by calcite or containing oil shows. They are also affected by dissolution leading to the development of vuggy porosity.
The upper part consists of wackestones and pack-grainstones containing benthonic forams (miliolids), ostracods, locally abundant, rudist debris, and peloids. The top corresponds to algal laminated dolomicrite-dolomicrosparite associated to anhydrite-gypsum layers and micronodules.
These facies have poor to good intergranular and vuggy porosity (due to dissolution) and are impregnated with oil. Early fractures are oil filled, but late fractures are cemented by sparry calcite.
The Upper Bireno starts with laminated dolomicrite to fine dolosparite containing anhydrite nodules and having good intercrystalline, vuggy, and fracture porosity, often bleeding oil.
The dolomites are interbedded with mud-wackestones (partly dolomitized) and contain benthonic forams, ostracods, pelecypods, and gastropod and echinoderm debris, green algae in bioclastic and oolitic pack-grainstones. Towards the top the limestone textures consist of diagenetic packstones containing benthonic fossils associated with a few planktonic microfauna (forams, calcispheres) and glauconite.
Post compactional dissolution has led to the development of intergranular, moldic, vuggy and fracture porosity that contains the oil.
Evaluation of the porosity types from logs and by laboratory measurements confirms the good quality of the Bireno reservoirs. Porosity values range from 11-32% in the 1 Gremda well, 9-20% in the 1 Guebiba well, and average 15% in the 1 El Ain (formerly 1 West Gremda) well.
SOURCE ROCKS, SEALS
As with the Abiod formation (OGJ, Dec, 10, 1990, p. 50), there are two principal source rocks for the Bireno discoveries, the late Cenomanian-early Turonian Bahloul formation and the Albian Fahdene formation.
The Bahloul formation is one of the main source rocks in Tunisia. It consists of dark colored laminated limestones and marls containing planktonic forams deposited in basinal to outer shelf environments during an ocean anoxic event (Event E2).
It is widespread, particularly in north central and eastern Tunisia, where its thickness is typically 3-50 m and the total organic carbon (TOC) has values ranging from 2.17.9% (average 3.8%). The shales are generally mature and have hydrocarbon potential ranging from 4-35 kg/tonne of rock.
The Fahdene formation (Albian to Cenomanian) is another potential source rock consisting of a thick sequence of shales and marls with thin limestone intercalations, rich in pelagic fauna, and deposited in deep marine environments during the ocean anoxic event E1.
The middle Mouelha member (Middle Albian-Vraconian) consists of as much as 300 m of dark shales and thinly bedded black limestones with TOC values from 0.65-14% in north central Tunisia and averaging nearly 1% in the Gulf of Gabes. They have good hydrocarbon potential with values ranging from 9-17.1 kg/tonne of rock.
The Coniacian-Santonian Aleg shales constitute a good seal rich in planktonic and benthonic fauna. The isopach of the Aleg shales is comparable to that of the Lower-Middle Turonian Bireno facies. However, due to uplift and erosion occurring during the Middle Turonian and Coniacian-early Santonian, they may be locally missing where the Bireno carbonates are unconformably overlain by the chalk facies of the Abiod formation.
On this case the Maastrichtian-Laoeocene deep marine El Haria shales complete the sealing process.
BIRENO FINDS, DEVELOPMENTS
After two discoveries of the 1970s at Miskar (1974) and Mahares (1978), the beginning of the 1980s was encouraging with three more discoveries on stream.
The 1 Gremda, drilled in 1980 by Houston Oil, found oil and gas in Bireno carbonates at 2,718-42 m. The well flowed 1,360 b/d of 38 gravity oil on a short test, but the test was suspended when hydrogen sulfide was detected.
The 1 Guebiba, drilled in 1981 by Houston Oil on a faulted structure, reached the Bireno reservoir at a depth of 2,804 m. The net pay was 4.5 m thick, and the well flowed 1,700 b/d of 34 gravity oil and 86.7 MMcfd of gas.
The 1 El Ain, drilled in 1982 about 2 1/2 km southwest of 1 Gremda, encountered 29.5 m of net Bireno pay at 2,570 m. A long term test flowed 2,550 b/d of 40 gravity oil and 1.2 MMcfd of gas.
These discoveries were left undeveloped due to low oil and gas prices in the late 1980s, and consequently the Bireno carbonates remained a secondary target until 1 El Ain was brought on production.
Encouraged by the good performance of the reservoir, British Gas upgraded again the Bireno carbonates as a primary target and drilled 1 Rhemoura. The well, situated 5 km southeast of 1 El Ain, reached the top of Bireno at 2,873 m and encountered a net pay of 53 m that tested 7,600 b/d of 35 gravity oil.
This promising discovery has rekindled oil and gas activity in the Sfax area, and British Gas is considering the progressive drilling of a number of new Bireno prospects.
CONCLUSION
The stable political regime and the favorable fiscal and legislative conditions have encouraged the development of the formerly marginal fields at Gremda, Guebiba, and Mahares.
In addition, the British Gas 1 Rhemoura discovery has upgraded the Bireno limestone to the status of a primary reservoir objective.
This encouraging development plus the other recent discoveries in the Abiod chalk and in the Bou Dabbous limestone by Marathon at 1 Belli have enhanced prospects for Tunisia's oil production, which reached an all time high of 131,918 b/d in August 1991.
Copyright 1991 Oil & Gas Journal. All Rights Reserved.
