Sedimentology and alluvial architecture of the Bajo Barreal Formation (Upper Cretaceous) in the Golfo San Jorge Basin: Outcrop analogues of the richest oil-bearing fluvial succession in Argentina

Abstract

This study addresses the sedimentology, architecture and dimensions of fluvial deposits in the Bajo Barreal Formation (Upper Cretaceous) in the Codo del Senguerr anticline, Golfo San Jorge Basin, Argentina. The stratigraphic framework of the 450–650 m thick alluvial succession was carried out through description and interpretation using 18 detailed sedimentary logs along a 4.5 km wide exposure, where 314 fluvial channels were analysed. The Bajo Barreal Formation is a high-accommodation fluvial succession, dominated by fine-grained floodplain deposits, with isolated-to-vertically stacked, sheet-like, low-sinuosity or braided fluvial channels of limited lateral mobility clustered in several coeval channel belts, in which fluvial channels were relocated by avulsion. The Lower Member consists of reworked ash-fall materials preserved in floodplain areas (78%), floodplain sandstones (4%) and fluvial channels (18%). The mean thickness of sandbodies is 2.96 m (n = 118) and mean true width is 112 m (mean W:T = 43), with thicker and wider sandbodies in upper levels of the Member. The Upper Member consists of grey siltstones and mudstones preserved in extensive lowland areas (78%) and fluvial channels (19%), with scarce preservation of sandstones in the proximal floodplain (3%). Fluvial channels are narrow low-sinuosity sheets with comparable thickness (mean = 3.21 m, n = 196), but greater width than those of the Lower Member (mean width = 147 m, mean W:T = 50). Paleoflow data from 298 fluvial sandbodies indicates a paleoflow direction toward the SE (Az. 112°). Rivers flowed parallel to inherited early Cretaceous normal faults and are oriented to high-angle (>70°) to the current axis of the Codo del Senguerr anticline, suggesting that the uplift of the anticline occurred after the deposition of the formation. The variation in geometry of fluvial channels in both Members of the Bajo Barreal Formation could help in planning and developing primary and/or enhanced oil recovery projects in nearby oilfields, and provide data necessary for modelling the subsurface connectivity of hydrocarbon reservoirs.