Regional Geology

The Aaiun basin remains one of the last frontier sedimentary basins in all of Africa. With limited exploration since the early 1970s advanced technologies are yet to be applied to this vast territory. The hydrocarbon potential of this region, both onshore and offshore cannot be underestimated as exploration levels continue to intensify both north and south of Saharawi Arab Democratic Republic.

The Aaiun Basin, is one of a series of mature passive margin basins that lie along the North Atlantic margin of Northwest Africa and the northeast margin of North America. The basin extends for almost 1100km along the margin from the Cap Blanc Fracture Zone in northern Mauritania, north through Western Sahara into southern Morocco to the intersection of the North Canary Island Fracture Zone and the South Atlas Fault. Rather than a single super-basin, the Aaiun Basin comprises two discrete sub-basins separated by the Dakhla Fracture Zone. The northern compartment, the Boujdour sub-basin, is largely characterised by an absence of Senonian aged sediments in the slope region, the result of major Early Tertiary canyon incision and the erosive effects of contour currents that developed across the slope and upper rise. Onshore the sub-basin is characterised by over 3km of Early Cretaceous deltaic sediments. Conversely, the southern compartment, the Dakhla sub-basin, is characterised by the presence of a thick (up to 1km) Senonian sand-rich succession in the deepwater portion and the absence of Cenomanian-Paleocene sediments onshore.


Northwest African margin basin compartments, associated fracture zones and regional onshore geology. (Onshore geology overview.pdf)

 

Petroleum Geology

Although the quantity and quality of geoscience information is limited it is probable that, all the key elements (reservoir, source, seal and trap) critical to exploration success are likely to be both present and effective within certain areas.

Early Cretaceous-Late Tertiary delta systems have been instrumental in establishing the presence and controlling the effectiveness of the various elements of the petroleum systems:

· Reservoirs - Reservoir quality sandstones have been transported across the shelf, down-slope and to the upper rise, accessed via multiple slope canyon conduits. Delta-front and deep water channelised turbidites and fans of Neocomian-Barremian, Aptian-Albian, Santonian-Maastrichtian, and Miocene age are considered to be primary reservoir targets, with the potential stacking of these units helping to mitigate exploration risk, and compound potential success.

· Source Rocks -The variable thickness of the Cretaceous and Tertiary sedimentary wedge has provided stratigraphic cover and loading within key areas of the Aaiun Basin to drive maturity of the Jurassic, Neocomian-Barremian, Aptian-Albian, and potentially Cenomanian-Turonian age source rocks into the oil window and beyond.

· Seal - The abundance of fine clastics carried basin-ward by the Cretaceous-Tertiary deltas has led to numerous regional sealing units throughout the succession

· Traps - Potential structural hydrocarbon traps are likely to be present along a seismically mapped large-scale prominent anticlinal trend, developed along the slope ('the slope anticline") and the associated tilted fault block complex. Additionally, outboard stratigraphic traps including channelised onlap wedges, perched canyon fill, and slope/basin floor fans are likely to be developed along the upper rise and slope.



Generalised stratigraphy of the Northern Aaiun basin

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Prospectivity

While the deepwater portions of offshore SADR remain virtually unexplored and are untested by the drill-bit, recent increases in activity and significant discoveries in the contiguous northern MSGBC Basin, including the deepwater oil discoveries at Chinguetti, Tiof and Tevet and gas discoveries at Banda and Pelican in Mauritania, highlight the potential for deepwater oil discoveries in the Aaiun Basin. Similarly, the onshore portion of the Aaiun basin has been largely neglected over the past 30. However analogous areas (onshore Mauritania and Morocco) are now receiving increasing levels of interest, which is testament to the potential which exists in these lightly explored regions.

Regional hydrocarbon occurrences on the North West African Margin

 

Petroleum Systems

Two distinct petroleum systems identified in the offshore area are considered to have excellent potential for the discovery and commercial development of hydrocarbons:

  • The continental slope region south of the Dakhla Fracture zone (i.e. the Dakhla sub-basin) contains two potentially oil prone source rocks, multiple reservoir intervals, and both structural and stratigraphic trap types. Within the slope region the primary Aptian-Albian source interval is interpreted to be present and mature, with additional potential at the Cenomanian-Turonian level. Maturity of these intervals is driven by the thick Senonian and Tertiary wedges, which along with the Neocomian-Barremian section provide multiple stacked reservoirs. In the northern portion of the Dakhla sub-basin, the southern extensions of the 'Slope Anticline' and arcuate fault zone have been mapped, and provide relatively low risk structural trapping mechanisms. In addition to the structural traps significant potential exists in stratigraphic traps in the form of perched canyon fill, onlap wedges and submarine fans.
  • The outer continental slope region north of the Dakhla Fracture Zone (i.e. the Boujdour sub-basin) is reliant on an appreciably different petroleum system to that in the Dakhla sub-basin, however significant potential is considered to exist, with a potential source rock, a thick reservoir section and large structural features all likely to be present. The Jurassic interval, a proven source interval in areas to the north including Morocco and Portugal, provides an excellent quality source facies, which is potentially oil-mature in the outer slope. The thick Neocomian-Barremian delta complexes provide stacked reservoir potential, and also drive the maturity of the underlying Jurassic source interval. The large 'Slope Anticline' and the extensive arcuate fault zone provide multiple relatively low-risk structural trapping mechanisms, while additional higher risk stratigraphic traps, including onlap wedges and submarine fans, are also expected to exist.

Interested parties are invited to purchase the Regional Report on the SADR, originally compiled by Fusion Oil and Gas. The table of contents of this report can be accessed at Regional Report Table of Content (1.1MB). Details of purchase of this report and its cost can be obtained from the SADR Petroleum Authority (kfadel@sadroilandgas.com).

 

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