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Abstract: Tectonic Evolution of South Atlantic Salt Basins

Mohriak, Webster U.; Palagi, P. R. - Petrobras/E&P; Mello, M. R. - Petrobras/Cenpes

An integrated study of the tectonic evolution and petroleum geology of the salt basins along the South Atlantic rifted margin was carried out along the segment from Campos to Sergipe-Alagoas (Brazil) and from Kwanza to Gabon (West Africa). Based on a regional integration of geological and geophysical data, particularly seismic reflection profiling, results of exploratory drilling, and geochemical data of oils and source rocks, we address the following themes: a) the tectono-sedimentary evolution of the basins; b) the deep structure of the rifts; and c) similarities and differences between petroleum systems in conjugate basins.

The sedimentary basins at both sides of the South Atlantic are traditionally considered as independent basins because of geographical and technological limitations. However, with the evolution of the geological knowledge in the last few years and the necessity to rationalize basin analysis and petroleum exploration, we have attempted to provide a large-scale view of these basins, considering them a single geological entity bearing equivalent structural and stratigraphic elements leading to analog petroleum systems.

The stratigraphic column of the South Atlantic salt basins may be divided in 5 megasequences: the pre-rift (PRMS), rift (or continental, RMS), transitional (or evaporitic, TMS), transgressive marine (TMMS), and regressive marine (RMMS). These megasequences are represented by time-equivalent successions at conjugate margins, deposited at similar depositional environments. These geological units are divided by major regional unconformities, namely: a) the regional unconformity separating Paleozoic from Mesozoic (mainly Upper Jurassic) pre-rift sediments; b) the Early Cretaceous boundary between the pre-rift and the rift sequences (locally named Rio da Serra unconformity in Brazil); c) the pre-Aptian unconformity, locally corresponding to the breakup unconformity; and d) the Early Tertiary unconformity.

The South Atlantic salt basins are marked by a similar geodynamic mechanisms of formation and similar patterns of stratigraphic evolution within a regional framework, but in some aspects, the conjugate margins are characterized by peculiarities in their geological evolution. Comparative geochemical analyses of oils and source rocks from several boreholes have revealed significant similarities in the organic facies of potential source rocks, but marked differences in thermal history and petroleum potential. The exploration in both margins aimed at different objectives through time. This resulted in analog petroleum systems with a present-day unequal distribution of petroleum accumulation in each petroleum system.

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