Johann-Christian Pratsch
Petroleum geologist
Houston
A new major oil and gas ",exploration play has been defined in onshore central Tunisia by combining established data and new interpretations. Targets are Triassic and Paleozoic sandstone reservoirs, the stratigraphic equivalents of the main producing reservoirs in northeastern Algeria and southern Tunisia.
All major business parameters appear positive, from acreage availability and attractive petroleum law to market potentials. The following is a short summary of the underlying petroleum geology.
GEOLOGIC SUMMARY
Tunisia's geology reaches from the continental craton (North Africa) across Paleozoic shelf basins and highs into the Mediterranean Mesozoic/Tertiary realm with the Atlas mobile belt and its southern foreland area (Fig. 1)(61847 bytes). Onshore oil and gas fields occur
- in the Paleozoic Ghadames basin, part of the Paleozoic Saharan platform, and
- along the north and east end of the Central Tunisian Cretaceous carbonate platform, itself related to the general south-to-north increases of open-marine influences through time (Fig. 2)(35906 bytes).
The offshore fields occur
- in Tertiary rift features overlying the Mesozoic/Paleozoic shelf, and
- in Mesozoic features (faulted structures, reefs). Some of the Tertiary rifting extends into the eastern onshore where additional small fields exist. There are no major fields in the Atlas foreland area or in the large onshore region of central Tunisia.1
Present exploration concentrates on Triassic and Paleozoic targets in the southern Ghadames basin and on targets in the offshore and northeastern onshore.
It is today well established that all oil and gas accumulations in Tunisia occur inside or near generating depocenters, the deep portions of basins." Are there then additional basinal areas in the country that require active exploration and promise attractive results?
The regional structure of central Tunisia is that of a major sedimentary basin, as interpreted from regional Bouguer gravity data (Fig. 3)(40157 bytes). With dimensions of about 150 km west-to-east and about 300 km south-to-north, this basin is larger than any one of the known producing depocenters in the country.
No exploration directed at the main targets in the region is known at this time; its producing potential thus is unknown but considered to be large.
Two established petroleum-geological facts seem to be quite helpful as basis for our prediction that indeed large gas and oil reserve possibilities exist in this basinal region:
- Oil and gas accumulations occur in large quantities where a large and sufficiently deep generating depocenter exists containing source beds, reservoirs and seals. Further on, in all producing basins in the world basin structure or basin geometry control and determine the distribution of major oil and gas reserves by controlling and determining the direction of lateral oil and gas migration!
- All Tunisian oil and gas fields presently known are related to local generating depocenters,' ' an additional large depocenter thus will most likely contain additional large reserves.
Depocenters are best defined and described by regional thickness maps or regional structure maps. Where only potential-field data exist (gravity, magnetics) a close approximation to such regional maps is obtained if and where the geological sedimentary sequence in the area permits such interpretation;2-4 young, low-density sediments should overlay older, high-density sediments, and deep basement should be sufficiently more magnetic than overlying sediments. In many areas regional Bouguer gravity contours often can be used as approximate form-line contours of deep structural levels, even of top basement structure.4
CENTRAL TUNISIAN AREA
In central Tunisia a thick (over I 0,000 ft), relatively low-density sedimentary sequence (Recent to Triassic) overlies a denser Paleozoic sedimentary sequence, this in turn high-density basement rocks. In addition, central Tunisia is topographically relatively flat; this eliminates the need for often hazardous surface-related (Bouguer) gravity corrections.
Therefore, the published regional Bouguer gravity map (Fig. 3)(40157 bytes) is a close approximation to a regional thickness map of relatively low-density sediments Recent to Triassic) overlying higher-density rocks (?Paleozoics, basement); it is also a regional structure formline-map on top of high-density (?Paleozoic, basement) rocks. A complimentary magnetic base is not available to check our basic assumptions, nor are there any deep well data or deep seismic data. However, considering similar cases found elsewhere integrating Bouguer gravity geometry, basin geometry, geochemistry, and hydrocarbon migration assumptions, the case for a new major potential depocenter in onshore Tunisia can be made.
A published thickness map of Jurassic sediments in the region supports our interpretation (Fig. 2)(35906 bytes)]; it shows a west-to-east trending sediment thickness maximum in part of the region. There are insufficient wells and seismic data, though, to consider this map as final.2 Cretaceous regional thickness maps are influenced by the geometry of a long-living Cretaceous carbonate platform in central Tunisia; the seaward (northern) termination of the Turonian portion of this platform is shown (Fig. 2)(35906 bytes). Triassic distribution, sediment thickness, or facies in the region are not known at this time.
The Bouguer gravity-indicated basinal area (Fig. 3)(40157 bytes) shows a major basinal low of about 150 km west-east and 300 km north-south, as mentioned above. It lies north of the intra-Paleozoic Telemzane arch or high, itself a result of pre-Permian orogeny, uplift, and erosion (Fig. 4)(33984 bytes) and a major paleo-tectonic feature of the North African platform. Previous interpretations of these gravity data were limited to a structure map of the Moho crustal discontinuity;1 we extend the interpretation into shallower sedimentary depth.
FORMATIONS EXPECTED
This basin is expected to contain a thick (?over 5,000 ft) Paleozoic sedimentary section ranging in age from Permian to Cambrian. Shales
and sandstones will be major lithologies; the sediments will be similar to those present in northern Algeria and southern Tunisia, but with a more pronounced marine influence (more shales) from south to north.1
Potential source beds, sandstone reservoirs, and sealing shales are expected in Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian, and Triassic units. Triassic and Permian units will lie above a regional ("Hercynian") unconformity. Triassic evaporates will act as regional seal for Triassic and Paleozoic sandstones just like in the producing area of Algeria and southern Tunisia.
On the southern basin flank several wells have been drilled across the intra-Paleozoic Telemzane high 5 (Fig. 5)(26096 bytes); these wells clearly show the type of Paleozoic section that can be expected in the basin. Especially interesting here are reported oil source beds in Ordovician, Silurian, and Devonian shales in wells on the north flank of the Telemzane high, that is, on the southern flank of the onshore Tunisian Central basin. Small oil reserves exist here in Ordovician sandstones.
The regional basin structure ("basin geometry")2 4
will be one of a west dipping basin with major north, east, and south basin flanks. This basin geometry is favorable to major lateral hydrocarbon migration from the basin center to the basin flanks. Hence, large oil and gas reserves will be found on the basin flanks as well as in intra-basinal structural (and stratigraphic) anomalies. This is one major reason to initiate exploration north of the Telemzane high, along the north-south fault zone, and along the northern flank of the basin. The other reason is the reduced depth to Paleozoic targets along the basin flanks.
North of the Central Tunisian Paleozoic basin, Tertiary to recent tectonism of the Atlas belt has influenced final deformation and deformation style. Tectonic deformation increases in intensity from the Atlas belt across its immediate foreland from south to north. Atlas derived compressional stress also has led to associated regional shear faulting, this in turn to local shear-related transpressional and transtensional deformation; in other words, not all surface compression reported from north and central Tunisia is directly related to Atlas belt compression. In addition, Triassic evaporates are expected to form major tectonic detachments. Hence, surface structure in Central Tunisia may be found to be quite different from (?mainly tensional) deformation underneath Triassic evaporates in Paleozoic targets.
RESERVES
It is clearly somewhat early to define future oil and gas reserves in this basin. They are seen as large when we consider
- the size of the basin,
- its long and positive history of regional sealing and hydrocarbon preservation by Paleozoic shales and Triassic evaporates, and
- when comparing its potential with that of the Paleozoic platform fields to the south.
Should the future reserves of this basin consist of more gas than oil, there will be an increasing possibility for major gas export to energy-hungry Europe via existing pipeline to southern Italy or via the planned pipeline system to Spain and France. Increasing normal future gas demand and any interruption of gas imports into central Europe will enhance North Africa's reserve potential, and hence that of the Central Tunisia basin.
REFERENCES
- Ferjani, A.B., Burollet, P.F., and Mejri, F., Petroleum geology of Tunisia, ETAP, Tunis, 1990,194 p.
- Pratsch, j.C., The location of major oil- and gasfields: examples from the Andean foreland, journal Petroleum Geology, Vol. 17, No. 3, 1994, pp. 327-338.
- Yukler, k, Moumen, A,, Daadouche, I., Bouhel, H., Meskini, A., Saidi, M., and H. Jarraya, Quantitative evaluation of the geologic evolution and hydrocarbon potential of the Gulf of Gabes, proceedings of 4th Tunisian Petroleum Exploration Conference, Tunis, 1994, pp. 169-212.
- Pratsch, J.C., Tunisia's oil and gas potential and future plays, proceedings of 4th Tunisian Petroleum Exploration Conference, Tunis, 1994, pp. 1-24.
- ETAP and Schlumberger, Tunisian Exploration Review, 1990,107 P.
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