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Palynofacies characteristics and palynological source rock assessment of the cretaceous sediments of the northern Orange Basin (Kudu 9A-2 and 9A-3 boreholes)

A sequence of five palynofacies types characterised by diagnostic palynomaceral assemblages defines five depositional environments within the Cretaceous sediments of the northern portion of the Orange Basin. Near the base of the sequence below seismic horizon P2, presence of a “p-wafer” palynofacies implies buoyant inertinite shards (palynowafers) were transported basinwards by bottom currents and settled from suspension in a low-energy, offshore environment. Occurrence of a mixed dysaerobic palynofacies between horizons P2 and PI indicates intermittent formation of semi-amorphous organic matter (with some wet gas-prone hydrocarbon source potential) during quiescent, mildly anaerobic periods, with accumulation of structured terrestrial organic material under more oxygenated bottomwater conditions. Within the overlying sequence between horizons PI and P, a xenomorphic palynofacies provides evidence of anaerobic conditions necessary for generation and preservation of large amounts of amorphous organic matter, resulting in the formation of a sapropelic, oil- to wet gas-prone source rock sequence during the Aptian. Subsequently, a low rate of sedimentation within an oxygenated water column during the Albian and Cenomanian produced an assemblage of sorted, blade-shaped humic debris described as a p-wafer/ tracheid palynofacies between horizons P and N. Well-preserved, poorly-sorted phytoclasts form a tracheal/exinitic palynofacies within the Upper Cretaceous interval above horizon N, indicative of rapid sedimentation. Optical assessment of spore colouration indicates that the source rocks below horizon P are thermally post-mature within the dry gas phase. Above horizon K4, an abundance of dinoflagellate genera indicative of marine regressive conditions (Dinogymnium, Andalusiella and Palaeocystodinium) together with abundant Azolla megaspores and Pediastrum coenobia, suggest a relatively nearshore, deltaic environment of deposition with freshwater influence. The presence of reworked striate bisaccates of Permo-Triassic age above horizon P indicates post-Aptian erosion of Upper Ecca or Beaufort Group sediments formerly existing to the west of their present outcrop positions. Recycled Lower and Upper Cretaceous miospores were derived from an uplifted basin margin during the Senonian. The occurrence of Phelodinium boloniense, Andalusiella polymorpha and Andalusiella mauthei within the Campanian to Maastrichtian interval above horizon N is the first record of a Malloy peridiniacean suite south of the Walvis ridge.