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The Spectrum of Types of Passive Margins and their Differentiated Petroleum Potential

Author(s): Pedro Victor Zalán Zag Consulting in Petroleum Exploration, Brazil

Passive Margins can nowadays be classified as Magma-Poor or Sedimentary, Volcanic and Transitional, once crustal structure, phased evolution and compositional filling of the associated rifts are taken into account. The three comprise a spectrum of passive margins resulting from different modes of thermo-tectonic evolution during the rupture and breakup of mega-continents. The evolution of Magma-Poor Margins take longer and involve stretching and thinning mechanisms of the continental crust practically devoid of magma. This allows for the continental crust to breakup and for the lithospheric mantle to exhume. The final lithospheric breakup, however, is carried out by asthenospheric magma. Volcanic Margins are impinged directly by mantle plumes that carry a lot of heat, and, thus, magma. Magmatic activities are abundant before, during and slightly after rifting. This allows for extreme ductility and hyper-extension of the continental crust and a very quick evolution from the inception of rupture to the final concurrent crustal and lithospheric breakup. Grabens are filled mostly by volcanic material (SDRs). Transitional Passive Margins start as Magma-Poor Margins but, towards the end, evolve into Volcanic Passive Margins in the vicinities of the imminent breakup. No mantle exhumation occurs in these two types.

The different modes described also result in a differentiated petroleum potential. Magma-Poor Margins present wide and thick rift sequences developed during the stretching, thinning and exhumation modes. Large packages of lacustrine organic-rich shales are present in the grabens, as well as different types of reservoirs such as microbialites, coquinas, sandstones and fractured volcanics/basement. Eventually there may be so much oil generated in the rifts that a large amount may migrate upwards and be stored in post-rift reservoirs (turbidite sandstones, shallow marine carbonates). Petroleum systems associated to both the Rift and Drift Stages may be extremely rich, such as in the Santos, Campos, Espfrito Santo Basins in Brazil and in the Kwanza and Congo Basins in Angola and Congo.

On the other hand, Volcanic Margins barely contain any sedimentary deposits inside their grabens, if any. Volcanic and volcanic-derived rocks fill the underlying rift entirely.

The Spectrum of Types of Passive Margins and their Differentiated Petroleum Potential
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Lead author first name:
Pedro Victor
Zag Consulting in Petroleum Exploration
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