SEISMIC STRATIGRAPHIC ANALYSES OF EARLY CRETACEOUS (VALANGINIAN) SEDIMENTS OF GAMTOOS BASIN, OFFSHORE SOUTH AFRICA

The Gamtoos sub-basin located in the south-eastern margin of the Outeniqua Basin, offshore South Africa. It is a complex rift type basin that having both onshore and 12 km distal offshore components. It is a structurally complex, severely faulted basin due to comprises of the Agulhas-Falkland Fracture Zone (AFFZ) with relatively simple half grabens bounded by a major fault to the northeast and containing comparable thicknesses of sediments ranging in age from late Jurassic (Kimmeridgian) to- early Cretaceous (Hauterivian) wet-gas-to-oil-prone shales, with potential sandstones reservoirs in Valanginian. Sedimentary facies distribution within a sequence is controlled mainly by the depositional environment. In the present work the geophysical data e.g. seismic and well logs were analysed from five exploratory wells across the basin from the north to south. The present study aims to contribute for a better understanding of the depositional sequences within the Valanginian sediments for the hydrocarbon exploration. A comprehensive interpretation on the model proposed [1,2] was followed in the work. The results were integrated with GR-ray signatures of respective wells and 3D-stochastic lithofacies model was applied. The study reveal that the lithology comprises mainly of thick uniformly-graded coarse-to-medium-to-fine-grained sandstone-silt-shale and the deposition took place under various environments widely ranging between braided fluvial, delta, continental channel fill sediments to marine, probably a submarine canyon fill deposit and submarine fan lobe environment, fluvial flooding plain, basin floor shelf edge, storm dominated shelfslope fan and shallow marine to distal deep marine slope environment.