The palaeo environment and depositional facies of the drift section of Block 3A/4A in the Orange Basin, West Coast Offshore South Africa

Abstract

The Orange Basin is underexplored and has shown proven hydrocarbon reserves with the potential for further discoveries. This study is aimed at describing the depositional facies and palaeoenvironment of the drift section in Block 3A/4Aby doingdesktop studies, using seismic analysis, wireline logs and biostratigraphy data. The proximal side of the study area shows structural highs and stratigraphic features such as onlaps, downlaps and pinchoutsin sequences. These structures were mostly observed on the 13A sequence. The 13A sequence is suggested to have been from the Aptian age, which was deposited in transitional inner shelf to pro-deltaic environments. The biostratigraphy data also suggest that the 15A sequence was deposited during the Cenomanian on an outer shelf environment. The 15A sequence, together with the 14 and 16A sequences have faults towards the west, validating the shelf collapse event that took place during the Late Cretaceous. Towards the east of the block there is thinning of sequences which thicken towards the shelf. This could also be observed in the youngest succession, the 22A sequence.The thicker successions reveal similarlog patterns in some wells and sequences; this could be an indication of simultaneousdeposition. The overall picture of the drift sequences in the wells is that they contain sandstones with interlaminated silt which could serve as possible reservoirs and seals in hydrocarbon entrapments.