EL OUATAOUI,A., ONAREP
Abstract: Petroleum Potential of the Atlas Fold Belt
Moroccan High Atlas chain is an important feature of moroccan geology. It delineates two main structural domains: The Meseta at the North and the Anti Atlas at the South. It owes its birth to Triassic faults, which seem to be a reactivated of ancient faults.
Actually, High Atlas belt stretches from the Atlantic Ocean at the west to Tunisia via Algeria country at the East.The moroccan segment among this long chain is roughly 500 km long and 100-km wide It is an intracratonic chain, which underwent the Triassic, and Jurassic aborted rift system, followed by the alpine compression and inversion. Its implication for hydrocarbon exploration in Agadir, Ouarzazate, Boudnib, Essaouira,Tadla, Missour and High plateaux basins is due to its adjacent position to these troughs.
Wide Paleozoic outcrops form the central block in the occidental High Atlas (Fig. 1). They are composed of carbonaceous and detrital sediments of Cambrian and Ordovician ages. According to some authors, this block has separated the occidental High Atlas from the central and eastern High Atlas since Triassic times.
Triassic series also outcrop in this chain. They outcrop, either spreaded at large scale at the centre of this belt or injected through faults at the eastern part of the chain.Triassic period is characterized by the rifting phase of Eastwest trend and the individualisation of tilted block basins filled by fining upward clastic deposits overlain by volcanic rocks and evaporite sequences.
During Jurassic times extension occurred during the Liassic The High Atlas basin is compartmentalized into low and high areas. Liassic period evolved carbonate platforms with reefs on the border of the troughs and marls in the center of troughs. Since Dogger the subsidence rate has decreased and the sedimentation has become continental.
Restricted intramountain basins received post Jurassic sediments during minor Tertiary transgressions.
-The first play is related to the Triassic rifting contemporaneous to the opening of the Atlantic Ocean.This event gave birth to a number of horsts and grabens, as encountered in the Argana Valley (Occidental High Atlas) The synrift sediments encompass either reservoir rocks (sandstones) as well as the seal (salt). Hydrocarbon supply could be either from silurian deposits or from distal Triassic facies in the graben. Geochemical analysis of silurian source rock in different areas revealed fair to good TOC values.Tadla basin, located at the north flank of the chain showed a Silurian source rock of 12 % TOC.The degree of maturation is, in some places within oil window.This architecture (low and high) will be preserved afterwards. It is also benefit for the Jurassic petroleum potential (presence of reefs and marls)
-The second play is related to fore deep basins, such as the Agadir basin (Fig. 2) , which is.adjacent to south front of the occidental High Atlas.This basin is filled with continental facies issued from the eroded High Atlas chain, and transported offshoreward along the south flank of this chain. Reservoir rocks located in proximal zone, and source rocks in distal zone, are present in this bassin.The high rate of sedimentation and the compaction phenomenon were in favour of an early maturation of the cretaceous and the tertiary source rocks of this basin. Geochemical Analysis of source rock samples taken from the vicinity of this depocenter showed an immature source rock , but with a TOC value of 7%.The traps are mainly of stratigraphic types.
-The third play is related to the tectonic style of this chain (Fig. 3) . Recent studies showed that low-angle thrusting model is a more accurate style for the margins of the High Atlas than high-angle faults (old interpretation). Overthrusting and detachment within the Triassic sequence caused a displacement of the Triassic and Jurassic series over the cretaceous ones. Triassic and Jurassic series could be folded as a result of this displacement. These folds provided good traps that could be sourced from mature cretaceous source rock beneath the overthrust sheets. Other source rocks could be brought to the maturation level by thrusting effect. This tectonic style has also the advantage of preserving the ante-Triassic structures. The third play could be found, for example, in the Tadla basin at the north flank of the High Atlas.
Other concepts may be identified for example at the vertical contact zone of Paleozoic sequence with the Atlasic Basins.
AAPG Search and Discovery Article #[email protected] International Conference and Exhibition, Birmingham, England