Sedimentological evolution of the Givetian-Eifelian (F3) sand bar of the West Alrar field, Illizi Basin, Algeria

Commercial accumulations of hydrocarbons depend on several factors, among which sedimentological conditions are of prime importance. Such sedimentological factors are of particular importance in controlling both entrapment and reservoir quality in the western part of the Alrar field, within the F3 sandstone reservoir of Givetian-Eifelian age. Recent well results have shown facies and reservoir quality distribution to be more complex and generally more favourable than previously predicted. A series of tidally influenced littoral bars are now believed to have existed in the Alrar area at the time of deposition of the reservoir, these being elongated in a NW-SE direction in the West Alrar area, with an interbar area in the vicinity of well DZ-1. The highest reservoir quality is largely confined to this bar facies and understanding its distribution is therefore vital to locating development wells. Four possible sedimentological interpretations of the extension of the F3 reservoir to the south and west have been developed. In most areas, the pinchout seems to be gradual, but in other areas where the transition to lagoonal shale is sharp, fault control may apply. Significant uncertainties however remain in predicting this shale-out in undrilled areas.