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Abstract: Factors Influencing Distribution and Quality of Rift Lacustrine Source Rocks in South Atlantic Rift Margin Basins of Brazil and West Africa

Apperson, K. D.; R. B. Allen, and K. Shaw - University of Utah

Recoverable oil reserves from West African rift margin basins are about four times the total recoverable reserves in Brazil (1996 estimates indicate 27,675 MMBO recoverable from West Africa, and 6,970 MMBO from Brazil). Organic-rich lacustrine shales deposited during the rift phase (typically Neocomian-Barremian) are important source rocks in the West African basins, and the principal source interval in southern Brazilian basins. Structural and stratigraphic frameworks of the comparably sized rift basins on both sides of the South Atlantic appear similar, as do the lacustrine-sourced petroleum systems. The reserves comparison is approximate because West African estimates include oil sourced from Tertiary marine shales, and Brazilian estimates include oil in Paleozoic and other basins. Nonetheless, a remarkable discrepancy in recoverable reserves still exists between the two sides of the former Gondwanan rift.

Factors contributing to differences in distribution and quality of pre-rift lacustrine source rocks in South Atlantic rift basins include: global tectonic events; asymmetry of rifting within the Proterozoic, deformation belt; orientation of basement structures with respect to rift margins; shelf/slope profile and shelf depth; source rock depositional environment and geographical distribution as highlighted by: geochemistry; timing of development of fault-bounded lake basins, versus regionally extensive lake basins; timing and role of the hinge line. Variations in these factors can adequately account for the great difference in recoverable reserves between south Brazilian rift margin basins and those of West Africa.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil