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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. ([email protected])
2 Infoterra
Abstract
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
Summary
- The Cambro - Ordovician Play is an active E&P target in North Africa and the Middle East.
- Some of the most productive reservoirs were deposited in subglacial and proglacial environments.
- Subglacial reservoir systems are unpredictable and a challenge to develop. This is illustrated with an example from the In Amenas Gas Development (Algeria).
- Proglacial reservoir systems are organised into extensive sheets (10's of km) and less extensive channels; the sheet-like systems are much more predictable targets in the subsurface.
- Despite their apparent complexity, predictable vertical and lateral facies transitions are recognisable in glaciogenic sequences. This has been recognised in the outcrops and with appropriate datasets may also be recognised in the subsurface.
References
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.
Acknowledgments
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).