Stratigraphic Architecture Of The Triassic Reservoir in The Saharan Province, Algeria

Abstract

Abstrac tThe Triassic reservoir is one of the most prolific plays in Algeria since the beginring of the exploration . One of the exploration risk is linked to the distribution and heterogeneity of the reservoir: Triassic series veere mostly deposited in continental to lagoonal depositional environments, and reservoirs mostly correspond to channelised fluvial sandstones interfingering with floodplain and sabbkha sediments .A project was then designed to established the stratigraphic architecture of these series at a regional scale, using outcrop data in Algeria and Tunisia, 125 Wells with a very good core recovery, and palynological analysis . A new interpretation of the tectono -sedimentary evolution of the basins is then proposed . Eight depositional sequences veere determined and correlated all across the area, and detailed sedimentological interpretation of their internal architecture was made possible with the outcrop and core studies .Upper Triassic series veere onlapping Paleozoic sediments folded during the Hercynian orogen. During upper Ladinian times, the whole area was eroded and incised valley systems veere probably guided by the Hercynian unconformity morphology . Triassic sediments veere only deposited in the northern part of the Sahara and in southern Tunisia, along the Paleo -Thethys margin .During Carnian times, a rifting phase induced the reactivation of old Panafrican lineaments, and large North -South horsts and grabens veere formed . Two main basins, the Illizi -Berkine basin and the Oued Mya basin veere separated by the El Biod -Hassi Messaoud high, and bounded eastwards and westwards by uplifted areas . These highs veere associated to normai syn -sedimentary faults, inducing important thickness and facies variations ; and an active volcanism. In the Illizi basin, alluvial fans supplied sediments eroded from the highs to the graben axis, where extended floodplain and ephemeral lake sediments . Carbonated sediments, associated to transgressive events linked to the Paleo -Thethys opening, veere interfingered in these alluvial sediments . The Oued Mya basin was covered at the same time by a wide sandy braided -plain .During Norian times, fault activity decreased, and a post -rilt thermal subsidence affected the whole province . The whole fluvial system backstepped through time, and horsts veere progressively onlapped and covered by alluvial sediments . A wide sabbkha, connected to the Paleo -Thethys, extended in the North of the Illizi -Berkine basin, in which coarse -grained fan -deltas interfingered with the evaporites . Alluvial plain sediments covered the Oued Mya area. Triassic sedimentation ended with a regional transgressive event depositing a thin dolomite bed, overlaid by a thick series of Liassic evaporites .