N. Tunisian Sahara hosts giant Triassic, L. Paleozoic prospects

Fabrizio Rigo
Exploration consultant
Houston

Figure 5 -- Horizons B - C Isochron, Sud Nefta Area (48188 bytes)

Hoizons C - D Isochron, Sud Nefta Area (52091 bytes)

The area discussed in this article covers about 9 million acres in Central Tunisia, south of the Atlas mountains between the Algerian border to the west and the Jeffara fault to the east (Fig. 1 (57469 bytes)). Geologically this area represents the northernmost extension of the African plate, near its subduction under the European plate.

A regional geologic and seismic study has indicated that this area contains at least 12 large untested structural anomalies located below or at the edge of Silurian source sedirnents. Three smaller anomalies so far tested in a part of the Northern Tunisian Sahara (NTS), which geologically is not the most favorable, have tested oil and/or gas and condensate from Lower Ordovician reservoirs. From one of these anomalies, the Franig-1 well is now producing 1,000 b/d of light oil and condensate from an unstimulated reservoir.

Lower Triassic Kirchau sandstone, the reservoir of southern Tunisia's giant El Borma oil field, and basal Cambrian clastics, the reservoir of Algeria's super-giant Hassi Messaoud oil field, represent additional objectives that can be tested in the same wells.

The westernmost part of NTS has been seismically studied in detail and has shown the presence of several structural anomalies. Among them, Anomaly "A" (Fig. 4 (53561 bytes)) is an anticlinal horst 28 km long and 6 km wide, and Anomaly "E" is a seismic lead (which needs more seismic to be confirmed) measuring 20 by 4 km.

Potential cumulative reserves from Cambrian, Lower Ordovician, and Lower Triassic reservoirs in Anomaly A, using Lower Ordovician reservoir parameters of adjacent Sabria (SAB) and El Franig (FNG) producing wells, could be in the order of 10 tcf and 500-700 million bbl of oil and condensate. This is a conservative estimate since the Triassic Kirchau reservoir is normally good and Lower Ordovician and Cambrian quartzarenites must be highly fractured in this area, which is very close to the Atlas compressional belt.

Gas eventually produced in Tunisia is very competitive both locally and in the Southern European market.

Exploration in Tunisia

In the past three decades, oil and gas fields, some of respectable size, have been discovered in Mesozoic and Tertiary basins of Northeast Tunisia, both on and offshore. Most exploration and drilling in the country are still concentrated in these basins.

In southernmost Tunisia, the discovery of basal Triassic El Borma oil field in the early 1970s ignited exploration, resulting in the discovery of several satellite fields.

However, all of the NTS between El Borma and the front of the Atlas fold belt did not draw much attention in spite of discoveries in Ordovician clastics in Northwest Libya and huge Triassic and Cambrian reserves in nearby Algeria.

Why exploration was not very active in this area can be explained by looking at Fig. 1 (57469 bytes), where isopachs of Silurian shales are shown. On both sides of the Telemzane arch, Silurian source rocks are absent. In this bold headed area lower Paleozoic traps are mostly stratigraphic (pinchouts and truncations). Oil accumulations imply long range migration from distant Silurian source rocks. Ordovician Azzel shale source rocks are absent. Several dry holes have been drilled, and many others would have to be drilled to localize favorable spots, if any.

But the area farther north of the Telemzane arch appears more favorable also in view of the fact that oil and gas were found here. In the NTS exploration was aimed at stratigraphic traps in the El Borma productive Triassic Kirchau reservoir. Ordovician was considered a secondary objective and only by chance oil and gas were found at Sabria North-1 in Lower Ordovician sandstones below the Triassic unconformity.

Only 20 wildcats and nine confirmation wells near the Sabria, Franig, and Baguel discoveries have been drilled so far in this Lower Triassic-Paleozoic oil province, which covers an area in excess of 35,000 sq km or 9 million acres (one wildcat per 450,000 acres).

Five major oil companies explored the NTS during 1972-91. Arnoco was the most successful, with the discoveries of Lower Ordovician Franig-1, Sabria West-1, Sabria North-1, and Lower Triassic Baguel-1 wells. Confirmation wells drilled near these discoveries did not meet expectations. Structures were small and reservoir quality erratic. In spite of the fact that Franig-1 reserves were evaluated at 1.3 tcf, companies became pessimistic and abandoned the area. Huge structural anomalies east and west of the El Franig Sabria area were ignored.

Concerns were based on geological and marketing evaluations. The belief that Ordovician is a poor reservoir in North Africa is widespread in the petroleum industry. Triassic reservoir qualities are related to deltaic channelized or resedimented clastics, and difficult to locate. Tunisian gas, according to many companies, has no market.

A better geological approach and a more realistic evaluation of the market is instead:

1. Geology: The NTS area is squeezed between the southern front of the Atlas compressional belt and the Telemzane arch, an average width of 110 krn, along the foundering of the African plate below the European plate. A number of northwest-southeast to north-south transcurrent faults are present. The closer an anomaly is to the transcurrent faults and to the Atlas compressional belt, the better the reservoir quality of Ordovician quartzarenites should be.

This is not the case for the Franig and Sabria area, where transcurrent faults are small or absent and the distance from the Atlas is maximum (over 70 km). Triassic reservoirs, which are controlled by stratigraphy and paleogeography, should be considered only as secondary objectives in the exploration for Cambrian, Ordovician, and Permian plays, at least until the time that a reasonable amount of stratigraphic data is collected by wells drilled to these Paleozoic objectives.

2. Marketing: Tunisia is a quite substantial gas market, as demonstrated by the fact that British Gas is going to market its 2.5 tcf of reserves from Miskar offshore field through a 120 km pipeline at a cost of $600 million. Besides, the Trans Mediterranean pipeline still has 200 bcf/year of unused transportation capacity available for Tunisian gas, which is closer to European markets and does not have the royalty burden paid by Algerian gas producers to the Tunisian government for passage rights.

In summary, the few wells drilled in the NTS area cannot condemn it geologically. And it should be said that the belief that mainly gas can be found in Ordovician reservoirs is a prejudice, because in many anomalies these objectives are less than 3,500 m deep, and the area is part of a cold basin with a geo- thermal gradient of 2/100 m as shown by Sabria West-1, which tested 42-45 gravity oil at 4,000 m (Fig. 3 (46180 bytes)).

Stratigraphy

Cambrian is represented by a massive quartz sandstone, light gray, moderately sorted, fine to medium grained, occasionally coarse to very coarse grained, cemented with silica, and fractured (Fig. 2 (68602 bytes)).

The Ordovician sequence is incomplete due to local and regional unconformities. At El Franig and Sabria upper Ordovician is missing and middle Ordovician is represented by 150 m of Azzel shale formation locally eroded. The Azzel shale is a good, mature source rock. In the El Franig-Sabria area Azzel shale is transgressive over Lower Ordovician Atchane and Hamra quartzarenite, which are up to 100 m thick where they are not eroded. These are the producing reservoirs in the area.

The base of the Lower Ordovician is represented by a shale sequence with rare interbedded sandstone and siltstone stringers passing downward to intercalated sand and shale. Maximum thickness of the most complete Ordovician sequence in this area is 508 m (Sabria North-2).

The Silurian, found in the Sabria-El Franig wells, consists of 100-150 m of a uniform sequence of dark gray shales with traces of pyrite. A highly radioactive shale, called hot shale, found at the base of the Silurian, is considered the prime source rock in North Africa.

Along the southern flank of the Telemzane arch, Lower Triassic is represented by the Kirchau (Tagi) formation. This is the reservoir of El Borma, Debech, Larich, Makhrouga, and Sanhrar fields, which contain combined reserves in the order of 700-800 million bbl of oil.

The lower Triassic, in the El Franig-Sabria area, is composed of volcanics (dolerite-andesite) changing facies to the west and east of Sabria into the clastics of the Tagi formation. These consist of a light gray, greenish, friable, fine grained, occasionally coarse grained sandstone that is poorly sorted and has a high primary porosity (21%). Thickness of Lower Triassic clastics ranges from 0 m (area of volcanics) to 200 m.

Upper Triassic is composed from bottom to top of an evaporitic sequence (the seal of the Kirchau reservoir), a shale-sandstone unit, followed by a carbonate sequence. Upper Triassic is over 1,500 m thick. It is mostly evaporitic in the adjacent Algerian area and dramatically thins at the Tunisian border and eastward (300 m at Sabria). The rest of the Mesozoic includes a second evaporitic cycle of Liassic age, a sequence of carbonate and clastics of middle upper Jurassic age, and a clastic continental Cretaceous facies. Thickness of post Triassic sediments increases from over 2,300 m at the BYS well in adjacent Algeria to 3,000 m in the Sabria area, indicating an inversion of the Triassic subsidence during Jurassic and Cretaceous times.

Reservoirs

Cambrian, the reservoir of giant Hassi Messaoud field in Algeria, displays sufficiently good lithologic characteristics to be considered in principle a good reservoir in the report area (Fig. 3 (46180 bytes)). However, no direct information on its reservoir characteristics is available. Fracture secondary porosity should be well developed in the highly tectonized area.

In the Sabria North-2 well, which drilled 79 m of Upper Cambrian units R and RO, oil shows are reported at 4,232-4,313 m. The Ordovician Hamra quartzarenite shows a fracture system with no evidence of secondary mineralization. Intergranular porosity in El Franig-1 cores averages 9.5% with a maximum of 12.6%. Permeability values range from 0.1 to 72 md with a mean value of 5 md. Within the poorly sorted zone, the coarse quartz formed a supporting framework that protected the finer grains during compaction and therefore preserved the primary porosity associated with these finer-grained sandstones.

Hamra in the Franig-1 well (gross pay 35 m) flowed 1,843 b/d of 44 gravity oil and 5.4-12.6 MMcfd of gas. Franig-1 has been put on a long production test at 1,000 b/d, which is delivered by truck to a pipeline terminal. The Lower Ordovician reservoir has not been stimulated at this time.

In the Sabria North-1 well Hamra flowed 930 b/d of 39 gravity oil.

Conformably beneath the Hamra lies the 50-70 m thick El Atchan sandstone, lithologically quite similar to Hamra. Net/gross reservoir ratio averages 0.6%. In Sabria-1, sandstone porosity ranges between 7.5-13.3%, and permeability is low with an average of 1 md and a maximum of 6.7 md. The Atchan reservoir in Sabria West-1 tested 530 b/d of 42-45 gravity oil and 1.5 MMcfd.

Reservoir quality, being dependent mostly on fractures, should improve in the anomalies located near the Atlas compressional belt and/or near transcurrent faults. Both areas west and east of the El Franig-Sabria wells are closer to the Atlas and display numerous systems of transcurrent faults. Seismic data show here a much denser grid of faults than in El Franig-Sabria. Near transcurrent faults seismic quality deteriorates at depth, suggesting highly fractured Lower Paleozoic sediments.

Source rocks

Source rocks in the area are found in Ordovician, Upper Silurian, and Middle Jurassic.

Ordovician. Geochemical analysis of cores at Sabria North-1 has demonstrated that Llanvirnian Azzel shale is a potential source with maximum TOC of 6% and an average TOC of 3.3%. Thickness of Azzel shale can reach up to 150 m.

Silurian. All over the North African Paleozoic basins the Tanezzuft Silurian shales are known as excellent source rocks. In the NTS area the Tanezzuft formation includes highly radioactive dark shales with black mudstone stringers, totaling over 50 m thick. In Sabria North-1 TOC values reached 11.92% with an average of 5.4%. The source potential is 27.6 kg/ metric ton and vitrinite reflectance 0.8%.

Middle Jurassic (Dogger). The Middle Jurassic is represented by interbedded mudstone and clay. A geochemical study from samples of Franig-2 has calculated a TOC of 9.79%. This rock may be considered as a potential source of hydrocarbons for the Triassic reservoir.

Burial history and time of maturation has been calculated in the Franig-2 well with a geothermal gradient of 2.5 C./100 m. The results were:

• Top of oil window at 2,200 m in Malm age.

• Base of oil window at 3,300 m in Silurian age.

• Base of Silurian reached the onset of oil generation in Albian tirnes.

• Top of Silurian reached the onset of oil generation in Campanian times.

Dogger reached the onset of oil generation in Paleocene to Oligocene times.

Silurian source in the Franig area is at present in the onset of gas-condensate generation at the Ordovician level. These results tie with the hydrocarbons produced in the Franig-Sabria area, except for the area to the west, where Sabria West-1 tested 42-45 gravity oil at 4,000 m. In this western area the geothermal gradient is lower, due probably to distance from the area of lower Triassic volcanism.

Migration of hydrocarbons generated in the Silurian occurred in late Eocene-Oligocene times, when the first alpine compressional forces had started to affect the area, the faults were rejuvenated, and fracturing of the reservoirs was dramatically increasing.

Seismic

In the past 3 years the author of this article has examined over 3,000 km of seismic lines in the NTS area from the Algerian border to the Jeffara fault (Figs. 4-7) with the exception of the area from El Franig-Sabria to the Kebili transcurrent fault (Fig. 1 (57469 bytes)), which is covered by exploration permits and seismic data are confidential.

The study remained at a regional level except for the area of Sud Nefta, located between the Algerian border and the Triassic volcanics of Franig-Sabria. Here a detailed study was performed by the author together with a colleague geophysicist.

The easternmost portion of the NTS area shows the presence of five major transcurrent faults, two of them forming a prominent horst known as Matmata high. Along or near this high and in the graben to the west, at least six large northwest-southeast trending anomalies have been identified. Depth of Cambro-Ordovician reservoirs in these anomalies is in the order of 3,000-3,500 m, which is the shallowest depth for these objectives in NTS. The four northernmost anomalies are covered by 50-150 m of Silurian source rock sediments; the other two are near the Silurian truncation boundary.

The area west of the Matmata high is part of a deep Permian basin containing numerous Upper Permian reef anomalies (OGJ, Jan. 2, 1995, p. 56).

The Sud Nefta area covers about 1.5 million acres. Four seismic contours were drawn:

• Horizon A-base Upper Cretaceous. Shows a monocline dipping to the north with an average gradient of 8 msec/km and an increase in thickness of Upper Cretaceous and younger sediments of approximately 400-450 m from the southern to the northern portion of the area. Apart from small gentle noses the only closed anomaly (Anomaly "E") is found near the Algerian border. It has a closed surface of about 30 sq km (7,500 acres) at this level.

• Horizon B-intra-Triassic. Same monocline as above with a nosing on Anomaly "E" and local very small structures with vertical closure less than 25 msec two-way time. Lower Triassic-Middle Cretaceous interval has a uniform thickness in the area.

• Horizon C-Hercynian unconformity or base Triassic. The northward dipping monocline is hereby affected by two structural alignments trending southwest-northeast. The one in the northern part of the area is more prominent and shows also a northwest trending component. The average north-south gradient across the area is 0.65 rnsec/km, with a minimum of 0.56 msec/km between the two structural alignments, indicating the presence of a paleohigh below the Hercynian unconformity and a thinning of Lower Mesozoic sediments along this axis. This is well expressed in the B-C isochrons, which show a 50 msec two-way time (or 100+ m) relief below the Hercynian unconformity.

• Horizon D (Fig. 4 (53561 bytes))-At this horizon, which can be located in the Ordovician-Cambrian transition zone, the regional north-south gradient is zero. A dominant northwest-southeast structural alignment with a maxmum vertical relief of 200 msec two way time shows the presence of an anticlinal horst feature in the Lower Paleozoic (Anomaly A). Anomaly A is bounded to the southwest by a 60 km long transcurrent fault, down to the southwest, and its closed surface is defined by the 2,650 msec contour. Its length is 28 km and average width 6 km (Fig. 7 (101502 bytes)), giving a total of 168 sq km of closure (16,800 hectares or 41,500 acres). Maximum vertical closure is 175 msec and average closure 100 msec two way time.

  A few satellite anomalies are shown around it (anomalies B, C, D). But Anomaly E, located west of Anomaly A, is intriguing. It is an anomalous feature because it is reflected as high as horizon A (Cretaceous), contrary to all other anomalies.

Anomaly E was a paleohigh during Lower Paleozoic times, as indicated by isopachs (Figs. 5 and 6), and it has strongly been rejuvenated by Alpine tectonism.

Seismic contours of horizon D show several prevalent northwest-southeast regional transcurrent faults associated with numerous minor tensional faults. Faults are more numerous here than in the El Franig-Sabria area, probably an effect of proximity of the Sud Nefta area to the Atlas compressional belt. The northern nose of Anomaly A has the highest density of faults, indicating the foundering of the African plate under the Atlas folds.

Anomaly A is going to be drilled during 1996. If Cambrian, Lower Ordovician, and Lower Triassic clastics are found productive, total reserves could be in the order of 10 tcf of gas and 500-700 million bbl of light oil and condensate. Satellite anomalies B, C, and D may contribute additional reserves, making this area the most attractive play in Tunisia.

The area of the Matmata high, which is strongly tectonized, could run second with potential reserves quite close to those of the western area, due to the considerable size of five Lower Ordovician seismic leads already identified. Lesser depth of the Lower Paleozoic objectives would make oil and liquids prevalent over gas in these areas.

Isochrons of Figs. 5 and 6 indicate that anomalies A and E were paleohighs during Ordovician and Triassic times and Anomaly C definitely a paleohigh at least in Lower Triassic times.

Conclusions

The closest production to the Sud Nefta Anomaly A is found 60-80 km east-southeast, in El Franig and Sabria fields. The occurrence of light oil at Sabria (32-44 gravity) and at El Franig (44-48 gravity), with high gas/oil ratio indicate that the area is near the gas/condensate window, located to the north.

An evaluation of the potential reserves in anomaly A is somewhat difficult. Normally the reservoir parameters of the Ordovician and Cambrian are mediocre but are improved by fracturing in the formation. Anomaly A due to its proximity to the area of collision of the African and European plates and to several transcurrent faults, should display an optimal fracture system. These assumptions are confirmed by the numerous faults shown in the seismic lines that have been interpreted.

A simplification of potential reserves in only two of the anomalies identified in the Sud Nefta area, using reservoir parameters of Franig and Sabria, is as follows:

Anomaly A-Ordovician reservoir: 6.44 tcf of gas and 214.6 million bbl of liquids (assume GOR 30,000 scf/ bbl). Lower Triassic and Cambrian may add similar sized reserves. Triassic estimated depth is 3,900 m, Ordovician 4,300 m, and Cambrian 4,500 m.

Anomaly E-Triassic reservoir: assuming 10,000 ft closed acres by 100 ft reservoir thickness and 300 bbl/ acre-ft, oil reserves could be in the order of 300 million bbl. Western closure for Ordovician and Cambrian reservoirs in lacking control with available seismic data.

Petrel Robertson exploration consultants in Calgary recently made a very detailed interpretation of this area tying reprocessed seismic lines to wells in the area. Its estimate, only for Ordovician reserves of Anomaly A (a little different by shape than our anomaly) is 3 tcf and 280 million bbl of liquids.

Reservoir parameters used for this evaluation are the same as in Franig, which is a prudent approach for an area more tectonized like Sud Nefta.

The author's conclusions are that the NTS is the area in Tunisia that offers the best chance for the discovery of giant oil and gas fields. All favorable parameters-source rocks, reservoirs, migration, and entrapment-are present in this area to justify the risk of drilling. In many of the anomalies the three objectives (Lower Triassic, Lower Ordovician, and Cambrian) can be tested in the same well.

Acknowledgment

I would like to remember the late George Castellucci, a brilliant geophysicist and an old friend, who worked with me on seismic interpretation of the Sud Nefta area.

The Author