ABSTRACT: South Atlantic Rift Tectonics: A Key to Understanding Exploration Fairways

FOLEY DONALD C., Conoco, Inc., Houston, TX

A key to 80% of the exploration success in the South Atlantic is the presence of pre-salt organic shales, which are often referred to as "Bucomazi" oil source rock. The very rich Bucomazi shales are composed of lake deposits that accumulated in early rift basins on what is now the continental shelves, slopes, and rises of the South Atlantic.

As a consequence of nonuniform rifting, the Bucomazi source rock is absent from parts of the South Atlantic margin. A simplified interpretation of the South Atlantic's structural patterning demonstrates that the South Atlantic rift is simply the opening of an ancient (Precambrian) suture between the African and Brazilian cratons. There are two general types of basement: (1) The "suture" belt is comprised of semiparallel thrust and fold structures. The structures are very probably the remnant of a 500-700 m.y. old plate tectonic suture between the African and the South American cratons. (2) The African and Brazilian cratons are generally composed of Archeozoic, older than 2500 m.y., igneous/highly metamorphic rock.

Cretaceous-aged sea floor spreading often occurred on the contact between the Precambian "suture" belt and the "craton." On those continental margins that contain no "suture" belt, the sedimentary section is steeply seaward dipping and narrow. Where the "suture" belt is present, there is a broad continental shelf with thick Mesozoic sequences that often include "Bucomazi" source rocks.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)