Transition from "Ice - Contact" to Non "Ice - Contact" Depositional Systems : Implications for Reservoir Architecture - A Case Study from the Late Ordovician of the Tassili N'Ager Outcrop Belt & the Subsurface Illizi Basin, Algeria

Richard J. Dixon
Sunbury Business Park, BP North Africa, Sunbury Upon Thames, United Kingdom

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 of 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 lineation’s record periods of ice advance that punctuate the early fill sequence. Subsequent and significant ice retreat is marked by 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.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas