1 ) Overview
The Zag-Bas Draa Basin is located between the Anti Atlas to the North, the Ougarta chain to the East, the Reguibet massif to the South and Mauritanides and onshore segment of the Tarfaya basin to the West.
The basin is covering 95 050 km² and it’s one of the major under explored basins in North Africa, with a Paleozoic sediment column in excess of 6km in thickness including up to a kilometer of potential Silurian source rock.
2 ) Data base
No seismic data are available in Zag basin. Only 3 petroleum wells were drilled in the area between 1959 and 1964 (OD-1, FEH-1 and AZ-1).
3 ) Tectonic and Sedimentary Evolution
The basin fill of Zag basin consists of a Paleozoic sedimentary sequence which is more than 7000 m thick. In the early Paleozoic, the area has evolved as a relatively stable area. The sedimentation during this period was controlled by eustatic sea level change in passive margin setting.
During the late Devonian this passive margin was subject to the Hercynian orogeny which resulted in the uplift of the Anti Atlas and folding of the Paleozoic strata in Zag Basin.
4 ) Petroleum system
4-1 ) Source rocks
The Zag Basin comprises an extensive Silurien black shale deposits. In addition, more source rocks were identified in the lower Ordovician, late Devonian (Frasnian) and in the carboniferous (upper Visean).
Geochemicals analysis indicates that TOC values are ranging from 0.5 to 6.99% and the organic matter are type II to III. The geochemical data available shows that the Silurian source rock is still within the oil window in the southern flank, and may have reached gas window in the northern flank of the ZAG Basin.
4-2 ) Reservoir Rocks
The main reservoir rocks are late Cambrian sandstone, upper Ordovician (Bani formation), Lower Devonian (Rich formation) and Lower Carboniferous (Visean to Tournaisian). The porosity is ranging between 5 to 11% and can be increasing up to 15% by fracturation.
4-3 ) Seals
The main seals are: Lower Ordovician, Silurian, Late Devonian and thick interbeded carboniferous shale formation.
4-4 ) Traps
Both structural and stratigraphic traps are present in the ZAG basin. Lateral facies variation and syndepositional structure are the main potential stratigraphic traps mainly in the Cambrian, Ordovician and Devonian.
The structural traps are linked to the hercynian phase showing many types of folding inverted structure and faulted blocs.
5) Play Concepts
The Palaeozoic play is represented by early Devonian objectives charged by Lower Ordovician and Silurian source rocks and capped by a late Devonian shaly succession.
Additional play is represented by early carboniferous charged by late Devonian (Frasnian) and capped by middle to late Visean shales succession.
6 ) Prospects and leads
Due of the lack of seismic, the only outcrops structural analysis data available suggest the development of hercynian structures.
Many stratigraphic traps are expected in the basin, particularly associated with the Southward wedge-out of the Cambro-Ordovician succession, the southward increase in clastic deposits and the possible presence of Middle Devonian reefs.
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Basin
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ZAG
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Basin type
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Paleozoic basin
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Area
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95 050 km ²
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Exploratory wells
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27
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2D Seismic/3D Seismic
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0
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Source Rocks
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Lower Ordovician, Silurian, Frasnian
and upper visean: Type II & III
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Reservoir Rocks
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Late Cambrian, early Ordovician, early Devonian and lower Carboniferous
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Seal Rocks
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Silurian, Early Ordovician, late Devonian and Carboniferous
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