M. Henniche¹, R. Eschard², A. Hamel¹, J. Proust³

(1) SONATRACH, Boumerdes, Algeria
(2) Institut Français du Pétrole, 92852 Rueil Malmaison Cedex, France
(3) CNRS , Rennes, Rennes

ABSTRACT: Sedimentary Architecture of the Fluvial Devonian Reservoirs in the Illizi Basin (Algeria)

One of the most prolific reservoir of the Illizi basin (Algeria) corresponds to the fluvial deposits of the Lower Devonian series. A regional study, combining outcrop observations and well to well correlations, was first performed in order to precise the sequential evolution of the Lower Devonian system in the Illizi basin. Then, this model was applied to the Tin - Fouye area, in order to precise the reservoir architecture of the field.

The Lower Devonian fluvial reservoirs resulted from the uplift and tilting of the Illizi basin, contemporaneous of the Caledonian unconformity at the Siluro - Devonian boundary. As a result, fluvial systems were reactivated, and a basin scale fluvial sand-sheet was deposited. This unit shows braided facies in the South of the Illizi basin passing to meandering then to estuarine systems northwards. This unit was then overlaid by transgressive aggrading mud-rich coastal to alluvial plain sediments, which had a regional extension and were used as marker for the correlation. Above, another massive braided sand-sheet was deposited, prior the Middle Devonian Emsian transgression. This sequential evolution was complicated by the uplift of structural highs during sedimentation.

The Devonian fluvial systems were characterized by 1) low-relief basin-scale erosional surfaces, corresponding to unconformities and by-pass surfaces 2) a very high sand/shale ratio, fluvial channels being amalgamated in basin-wide sand-sheets. This peculiar geometry was probably due to a low accommodation setting combined to a high sediment supply rate. The absence of vegetated floodplain made also possible continuous channel avulsions during sedimentation.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado