The petroleum potential of the passive continental margin of South-Western Africa : a basin modelling study

摘要

The Petroleum Potential of the Continental Margin of South-Western Africa - A Basin Modelling Study The hydrocarbon potential of the continental margin of south-western Africa was assessed with means of a 2D basin modelling study of the hydrocarbon generation, migration and accumulation of the Kudu gas field. The basin model is based on well and seismic data from offshore Namibia and constrained by geochemical data on source rocks, natural gas samples and hydrocarbons desorbed from near-surface sediments. The modelling was conducted with the basin modelling software PetroMod (IES, Germany). The source rock samples investigated in this study originate from different phases concerning the opening of the South Atlantic Ocean. Lacustrine source rocks deposited during the Permian prerift phase are represented by the Whitehill and Irati shale samples from onshore Namibia and Brazil, respectively, and by samples from the Cruz del Sur well, offshore Argentina. Marine and terrestrial source rocks of Barremian to Aptian age deposited during the drift phase of the Atlantic opening were drilled in the wells Kudu 9A-2, Kudu 9A-3 (offshore Namibia), DSDP 361 (offshore South Africa), and Cruz del Sur (offshore Argentina). High petroleum generation potential was recognised for marine Aptian rocks from the DSDP 361 well, Neocominan and Paleozoic rocks from the Cruz del Sur well and for Permian lacustrine Irati shale samples. The dry Kudu gas with only minor condensate quantities is reservoired in the feather edge of a seaward dipping reflector sequence in predominantly aeolian sandstones. Remarkable about the gas field is that its reservoir is overlain by Aptian shales which are supposed to act as seal and source rock simultaneously. The 2D basin model confirms the possibility of downward hydrocarbon expulsion from the Aptian source shales in the underlying Aptian to Barremian reservoir. The downward expulsion is driven by the pressure gradient between the due to hydrocarbon generation high pressured shale and the lower pressured permeable sandstone. Secondary migration takes place buoyancy-driven in up-dip i.e. landward direction. Thus, the basin model indicates that hydrocarbons from basinward parts of the source rock migrate towards the trap. The terrestrial influence on those distal parts of the source rock is inferred to be less than in the drilled proximal part. Thus, the filling of the reservoir with hydrocarbons from the terrestrial influenced source rocks encountered in the Kudu wells as well as with hydrocarbons from more basinward marine source rocks is inferred. This interpretation is corroberated by the 13C values of methane, ethane, and propane from the Kudu reservoir which argue for a marine source rock. The maturity is estimated from the gas' isopotic signature to approximately 1.4% Rr which is not in concordance with today's maturity measured at the Barremian and Aptian shales in the Kudu area but with the modelled maturity at the time of petroleum expulsion. The gas was found to be dry in spite of its moderate maturity which might hint at oil to gas cracking induced by the high reservoir temperatures due to high heat flow and deep burial. The Kudu condensate shows a high content in aromatic compound thus indicating terrestrial input to the source rock. Therefore, the condensate is considered to possibly stem from more proximal parts of the source rock. The analyses of source rocks from the southern South Atlantic reveal a certain petroleum potential of Lower Cretaceous as well as Paleozoic source rocks. The potential of the Aptian to Barremian source rocks is evidenced by the Kudu and the Ibhubesi field offshore Namiba and South Africa, respectively. This finding is corroborated by the presence of thermogenic hydrocarbons in near-surface sediments taken at the continental margins of south-western African and Argentina. From the basin model downward expulsion of petroleum from the Aptian and Barremian source rocks seems reasonable. Thus, further reservoirs of the Kudu type might be present in the southern South Atlantic.