FLOODPLAIN DEPOSITS AND SOIL CLASSIFICATION: THE PREDICTION OF CHANNEL SAND DISTRIBUTION WITHIN THE TRIASSIC ARGILOGRESEUX INFERIEUR, BERKINE BASIN, ALGERIA

Abstract

Middle to Late Triassic fluvio‐lacustrine sandstones referred to as the Trias Argilo‐Grèseux Inférieur (TAG‐I) are an important hydrocarbon reservoir in the Algerian Berkine Basin. Three depositional sequences have been identified within the TAG‐I and a fourth sequence overlies it, forming part of the “Trias Argilo‐Carbonaté”. Subtle changes in the style of sedimentation through these sequences have been identified and are attributed to periodic rises in base‐level together with changes in subsidence rate and climate, leading to basinward, progradational shifts of the fluvial systems. The palaeoclimate changed from semi‐arid to sub‐humid with seasonal wetting and drying, as indicated by the evolved style of the deposition, the abundance of vegetation and the nature of associated palaeosols. Four main types of palaeosol profile have been identified and consist of green/grey, red, and red/brown to purple palaeosols together with green/grey to red pedogenically modified mudstones, depending on the level of maturity. The presence of sphaerosiderites in green palaeosols is a clear indication of the occurrence of wetland conditions. Palaeosol development and maturity are useful aids to stratigraphic correlation and as an indication of the proximity of reservoir channel sandstones. With improved prediction of the sandbodies’location, better models of reservoir distribution can be made leading to an enhanced field development plan