GEOSCIENCES Exploring the Atlas Trend offshore Northern Tunisia

Geoffrey Grant Geco-Prakla Exploration Services The Central Mediterranean and the Geco-Prakla / ETAP seismic survey. Illustrative section from the Gulf of Tunis, extensional faults related to major wrench system. Summary stratigraphy of northern Tunisia. Illustrative section across the Tellian Basin, northwestern Tunisia.

Geoffrey Grant
Geco-Prakla Exploration Services

The Atlas - Appenine mountain chain is an area of complex geology dominated by thrust tectonics. The trend has been extensively explored both on- and offshore Italy and, to a lesser extent onshore Algeria where, in 1953, the Oued Guetterini Field produced 2 million barrels of light oil from an Eocene limestone at the base of a stack of thrust sheets. In Tunisia, however, only the southern margins have yet been explored. Onshore the mountainous, wooded terrain, presents great logistical difficulties which makes seismic acquisition very expensive. However, where the Atlas trend continues offshore, i.e. off Northern Tunisia and Sicily, good quality seismic can be obtained much more cost effectively. The shelf area to the north and northeast of Tunisia is broad and water depths are generally less than 250 meters. Exploration just across the frontier in Italian waters has already yielded the Nilde discovery (10,000 b/d), but offshore Tunisian exploration has concentrated in the Pelagian Basin along the eastern seaboard. This is perhaps understandable as the geology is relatively less complicated in this dominantly extensional basin than it is in the thrust and fold belt to the northwest. However, numerous oil seeps in the Tellian Atlas and gas shows in wells offshore northeastern Tunisia prove that a viable hydrocarbon system does indeed extend into the northern waters. In order to encourage exploration of the Atlas trend in Tunisian waters, Geco-Prakla and ETAP, the Tunisian State Oil Company, have cooperated in acquiring some 1600 km of new non-exclusive 2D seismic data, soon to be offered to the industry.

Little is known of the pre-Mesozoic stratigraphy of northern Tunisia. Regionally it would seem that the ancient Saharan platform, uplifted during the Hercynian event, was onlapped by Mesozoic strata as the Tethys Ocean opened through left lateral motion between Europe and Africa.

The oldest outcrops in Northern Tunisia are Triassic (perhaps Permo-Triassic) red sandstones exposed at Jebel Hairech, near the Algerian border. These weakly metamorphosed sandstones are overlain by dated Middle to Upper Triassic dolomites, which may in part, be a condensed equivalent of the thick evaporitic sequences known over the rest of northern Tunisia. The majority of Triassic outcrops in northern Tunisia consist of reddened evaporitic shales. Traditionally, the scattered and stratigraphically isolated nature of the Triassic outcrops has been interpreted to have resulted from diapirism. There is, however, little field evidence to support this hypothesis, particularly in the light of the absence of halite or indeed of pseudomorphs after halite. The evaporite minerals are in fact dominated by gypsum and anhydrite. More recent interpretations suggest that the distribution of the Triassic outcrops has resulted from low-angle thrusts clipping through a previously deformed stratigraphy. Middle to Late Triassic carbonates exposed at Jebel Ichkeul, near Bizerte on the northeastern coast, seem to represent a more marine equivalent of the evaporite facies and suggest a connection to Tethys in the northwest, a precursor to the Jurassic transgression which saw marine conditions over much of Tunisia.

The Triassic-Jurassic boundary lies within a 400-meter thick limestone, which ranges from Rhaetian to Lower Pleinsbachian in age. This is overlain by a condensed sequence of dolomites, limestones, marls, and radiolarian cherts recording the interval up to the Oxfordian. At Jebel Hairech, the whole of the Middle and Upper Jurassic is represented by less than a hundred meters of deepwater radiolarian cherts and carbonates. However, farther east, relatively thick Kimeridgian to Tithonian shallow water limestones occur. These facies and thickness variations are taken as evidence for significant transtensional tectonism during the Middle to Late Jurassic.

Differential subsidence is even more evident in the Lower Cretaceous sequences. Thick deepwater turbiditic sandstones and shales characterize two northeast to southwest trending troughs (the Tellian Trough of northeastern Algeria and the Tunisian Trough, which is more or less coincident with the modern Medjerda Valley). The Constantine High, which separated the troughs, received shallow-water carbonate deposits. This differential subsidence continued into the Cenomanian-Turonian, when there is evidence that the Constantine High was frequently exposed while over a thousand meters of shales with minor limestones accumulated in the Tunisian Trough. Similarly, Coniacian to Campanian marls and limestones are significantly thinner over the high than to the south. The tectonic origin of these parallel highs and lows is unclear, especially as the presumed extensions into Tunisia of both the Tellian Trough and Constantine High are hidden beneath the "Numidian Nappes". Paleomagnetic data indicate a major change in Tethyan tectonics during the Turonian as the opening of the North Atlantic lead to dextral transpression between Europe and Africa. This reversal was manifest as subduction and collision along southern Europe and the Alpine Arc.

Campanian to Mastrichtian chalks blanket the area, but still show some significant thickness variations, as do the El Haria shales which mark the Cretaceous to Paleocene transition. The Lower Eocene nummulitic limestones, which are an excellent reservoir in central and southeastern Tunisia, are unlikely to be encountered off the northern coast, where they are replaced by the deepwater, organic rich "globogerina marls" of the Bou Dabous Formation. These are overlain by thick Middle and Upper Eocene marls. These marls thin markedly in the region of Ain Draham, northwestern Tunisia, providing evidence for the continuing influence of the Constantine High. The dextral transpression, which had characterized the Late Cretaceous and Paleocene gradually changed to a more orthogonal north-south compression during the Eocene and Oligocene. Continental collision between the Corsica-Sardinia-Calabria microplate and Tunisia-Sicily uplifted the Tellian Atlas and lead to the deposition of thick, Oligo-Miocene "molasse" and "flysch" sandstones unconformably on folded and thrust units. The bioclastic Ain Grab Limestone is an excellent seismic marker near the base of the Miocene and is overlain by shallow marine Beglia and Birsa sandstones. Relatively low energy shallow marine deposits dominate the rest of the stratigraphic column. These are interrupted by Late Miocene, black, evaporitic shales which record the Messinian drawdown event recognizable throughout the Mediterranean. Late Miocene (Burdigalian) intrusive igneous rocks in northwest Tunisia and adjacent parts of Algeria confirm continuing collision and subduction.

The principal proven reservoir in the thrust belt is the Miocene sandstone. This is the reservoir for the Nilde oil field off Sicily and for Tazerka and others in the Gulf of Hammamet, Tunisia. In addition, gas shows in KRB-1 and KRK-1 offshore northeastern Tunisia were encountered in this unit. These sandstones are not the only potential target, however, as proven reserves are found nearby in Lower Cretaceous sandstones (Cap Bon gas field) and Upper Cretaceous chalks (Zinnia & Tazerka) as well as in Eocene chalks (Belli). In addition, regional geology suggests that viable reservoir rocks are likely to be found at numerous horizons within the older thrust sheets; e.g. Triassic sandstones, Jurassic carbonates, and lower Cretaceous sandstones as well as Oligocene sandstones. Cretaceous, Eocene, and possibly Miocene source rocks are known to exist nearby while adequate sealing lithologies occur throughout the sequence.

Study of existing seismic data allows the area to be divided into three distinct zones, based on structural style. The southeastern part of the survey area is dominated by transtensional tectonics and is effectively transitional with the northernmost part of the Pelagian Basin of eastern Tunisia. Many of Tunisia's recent oil and gas discoveries have occurred in this basin, notably in the Gulf of Hammamet. This region is characterized by wrench and normal faults with a thick Tertiary to recent fill. As one moves to the north and northwest the effects of the collision between Africa and Europe become more apparent and the structural style here is dominated by thrust faults and compressional folds, indicating generally northwest to southeast transport. These provide large attractive reservoir targets at moderate depths. Careful geological analysis will be necessary to calculate the relative timing between oil generation and trap formation, hence the need for high quality seismic data to penetrate the complex strata.

The major decollement for these thrusts appears to be near the base of the Triassic evaporite sequence. Near the "thrust front" this shows up as large detached anticlines in which significant thickening of the Triassic sequence, due to imbricate stacking, has jacked-up the overlying beds. In the northeastern part of the area along the Italian frontier, a more classical staircase geometry is developed, which gives rise to large scale thrust culmination anticlines within the Cretaceous to Paleogene sequence. In general, the transport direction of these thrusts is from northwest to southeast, but local confrontation between forethrusts and backthrusts gives rise to triangle zone traps.

In the southwestern part of the area, between the mainland and the Gallite Islands, a separate subbasin can be identified. This is known as the Tellian Basin and is a broad sag basin with little evidence for significant extensional faulting. It is thought to have developed as a result of tectonic loading during the Tellian Atlas collision event, but may be much older, sharing a common origin with the Tellian Trough of Algeria. Little is known about Cretaceous to Paleogene deposition in this basin, as these units are not very well imaged on older seismic. It is possible to see major compressional structures lying beneath the Middle Miocene unconformity. Thick Oligo-Miocene submarine fan sandstones of the so-called Numidian Flysch are a potential target within these thrust sheets as are older units. Potential Miocene carbonate build-ups lying above the unconformity are also an attractive reservoir target at relatively shallow depths.

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