Sequence stratigraphy and sedimentology of the late Triassic TAG-I (Blocks 401/402, Berkine Basin, Algeria)

Abstract

The Berkine Basin is an intra- or pericratonic basin that developed during the Middle to Late Triassic on the margin of the Saharan platform. The basin lies to the east of the north–south trending Hassi Messaoud Ridge which separates it from the Oued Mya Basin to the west. These Algerian basins lie to the south and east of the network of rift basins that developed in the Iberian peninsular and along the margins of the North Atlantic seaboard. The principal hydrocarbon reservoir is the Upper Triassic Argilo-Gréseux Inférieur (TAG-I) which, in Blocks 401a and 402a, is Carnian to Norian in age. Elsewhere in North Africa, especially to the south and east, there are older Triassic formations. For example, in Zarzaitine in the extreme south east of the Triassic Algerian outcrop, Anisian vertebrates are documented. The TAG-I sits unconformably on Palaeozoic basement rocks and with the basal Lower Carbonate comprises a laterally and vertically variable sequence which has been sub-divided into four depositional sequences: Sequence 1, an unconformity-bounded, ephemeral fluvial interval that fills palaeorelief on the Hercynian unconformity; Sequence 2, an initially upward-fining, and subsequently upward-coarsening package of perennial fluvial sandstones and floodbasin shales with thin crevasse splay elements and interfluve palaeosols; Sequence 3, an erosively based, fluvio-lacustrine section characterised by fluvial sandstones with associated crevasse sandstones and floodbasin/lacustrine shales. This sequence is the main hydrocarbon reservoir section and is divided into two main packages 3A and 3B; the base of 3B is distinguished by basin-wide fluvial incision and the widespread channel sand deposition; Sequence 4 (Lower Carbonate) is a coastal plain and shallow marine system comprising shales, sabkha-type evaporites and bay-fill sandstones. The four depositional sequences reflect differences in depositional style resulting from base level shifts, tectonics and climate throughout TAG-I times. The overall increase in relative sea level was interrupted by periods of incision, which may relate to periods of rifting and erosion of the rift shoulders of the Berkine Basin. The initial valley fill (Sequence 1) was deposited under relatively arid or semi-arid conditions. During Sequences 2 and 3A, perennial fluvial systems with anastomosed channels and floodplain lakes became dominant and the climate increasingly humid. At this time a major longitudinal drainage, divide developed due to intrabasinal rifting. The basin was ultimately flooded by the transgressive systems tract of Sequence 4.