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Transition from Subglacial to Proglacial Depositional Systems: Implications for Reservoir Architecture, Illizi Basin, Algeria*


Richard J. Dixon1, T.L. Patton1, J.P.P. Hirst1, and J. Diggens2

Search and Discovery Article #50095 (2008)

Posted October 24, 2008

*Adapted from oral presentation at AAPG Annual Convention, San Antonio, Texas, April 20-23, 2008.

1 BP North Africa, Sunbury Upon Thames, UK. (

2 Infoterra


Late Ordovician glaciogenic sequences are important exploration targets and important producers of petroleum in North Africa and the Middle East. Glaciogenic sequences typically have complex architectures that reflect multiple phases of ice advance/retreat, a variety of depositional processes, and glacio-tectonism.

The Late Ordovician of the Tassili N’Ager outcrop belt illustrates the interaction of ice with the underlying substrate, leading to the development of extensive erosional features ranging in scale from mega-valleys, to palaeo-valleys and intra-valley channels. The palaeo-valleys mapped in the outcrop comprise slightly sinuous valley segments, up to 50 km in length, that alternately converge or diverge to generate an anastomosed array. The early fill is dominated by ice contact facies. Intra-valley surfaces marked by mega-scale glacial lineations record periods of ice advance that punctuate the early fill sequence. Subsequent and significant ice retreat is marked by the deposition of an extensive suite of high and low-density turbidites interbedded with debrites.

Turbiditic facies initially have a sheet-like architecture overstepping the palaeo-valley margins, but pass upward into sinuous and meandering channel/levee complexes. Sedimentary structures are suggestive of sustained, high concentration flows. The sequences are interpreted as the equivalents of a large, deltaic system located some distance to the south of the outcrop. The delta system was fed by glacial outwash, but the ice was not in direct contact with the sea. Sediment was transported into deepwater via a glaciofluvial outwash plain and a delta front / slope system. Large volume flows probably passed directly from the subaerial environment to the subaqueous environment without any period of “storage” on the shelf.

Selected Figures

Figure 1 Gondwana glaciation in the Late Ordovician (plate reconstruction from Stampfli and Borel, 2002).
Figure 2 The Cambro - Ordovician play in North Africa (Algeria and Libya).
Figure 3 Location map showing BP outcrops and recent Libyan licence areas.
Figure 4 Glacial palaeo - valley model and subglacial/proglacial facies definitions.
Figure 5 Typical Ordovician glacial valley; Iherir Oasis, Algeria
Figure 6 Facies models: Proglacial facies associations and boundary conditions.



Beuf, S., B. Biju-Duval, O. De Charpal, P. Rognon, et al., 1971, Les grés du Paléozoïque inférieur au Saraha; sedimentation et discontinuités, evolution structurale d'un craton: Collection Science et Technique du Pétrole, v. 18, p. 464.

Hirst, J.P.P., A. Benbakir, D.F. Payne, and I.R. Westlake, 2002, Tunnel valleys and density flow processes in the Upper Ordovician glacial succession, Illizi Basin, Algeria; influence on reservoir quality: Journal of Petroleum Geology, v. 25/3, p. 297-324.

Stampfli, G.M., and G.D. Borel, 2002, A plate tectonic model for the Paleozoic and Mesozoic constrained by dynamic plate boundaries and restored synthetic oceanic isochrones: Earth and Planetary Science Letters, v. 196/1-2, p. 17-33.


Many thanks to John Diggens (Infoterra) for numerous entertaining and thought - provoking discussions of process-based, glacial sedimentology!.

Many thanks to Alastair Hutchison and our colleagues at In Amenas Gas (BP/Statoil/Sonatrach), for access to the In Amenas core data and for some good technical data exchange.

Many thanks to Remi Eschard and Guy Desaubliaux (IFP) for a strong technical relationship, excellent guidance in the Tassili N'Ager (both technically & logistically) and for excellent discussions in the field.

Also thanks to a wider group of external consultants / technical experts who have provided a great “sounding board” for the development of the ideas presented in this talk. We would like to thank Prof. D.G. Roberts, Dr. Dan Le Heron (Royal Holloway), Prof. Doug Benn (Svalbard), Dr. Lidia Lonergan (Imperial College), Dr. Owen Sutcliffe (Neftex), Dr. Andy Whitam (CASP), Prof. A.H. Hawat (Garyounis University, Benghazi), Dr. Mads Huuse (Aberdeen), and Dr. A. Moscariello (TU Delft).

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