Abstract: Source Potential of the Zairlan Onshore Pre-Salt Sub-basins of the West African Aptian Salt Basin
Krystyna Swirydczuk, Diemu Tshiband, Mbaku Nyimi, Iheta Musombo, Kalubi Mbalambala
Three pre-salt subbasins are located onshore in Zaire in the Congo-Cabinda Basin. Production exists to the west, and extensive outcrops of Mavuma tar sands are located immediately to the east of these subbasins.
Five pre-salt wells confirmed that thick Barremian lacustrine claystones of the Bucomazi Formation form the main source horizon in all the subbasins. Upper Bucomazi claystones average 4% and reach 12% TOC. Lower Bucomazi claystones average 2% (high of 6%). A mixed Type I/Type II algal oil-prone kerogen predominates. Up to 1% TOC is present in claystones in the underlying Lucula section. Dry pyrolysis shows significant differences in kerogen kinetics from subbasin to subbasin.
R0 and Tmax were used to model heat flow through time. Ages were from biostratigraphic analyses and radiometric dating of thin volcanics within the Lucula and Bucomazi formations. Apatite fission track analyses provided control on uplift history. Pseudowells were used in maturation modelling to predict source rock maturity in the subbasins. The upper Bucomazi is immature except in the deeper parts of two of the subbasins. The Lower - Bucomazi and Upper Lucula are mature in all subbasins and in the deepest subbasins are overmature. Oil generation occurred shortly after deposition of the Loeme Salt. Analyses of Lindu oil support this early migration.
Estimates of oil that may have been generated in the easternmost subbasin suggest that extensive Mavuma tar sands, which have been typed to lacustrine source, could have been sourced from this subbasin.
AAPG Search and Discovery Article #90951©1996 AAPG International Conference and Exhibition, Caracas, Venezuela