Geology and Major Oil Plays, Coastal Margin Basins, Brazil: ABSTRACT

Abstract

Six major tectonic-depositional sequences, reflecting rift and passive margin evolution, variously characterize the filling of Brazilian coastal margin basins: (1) Late Jurassic prerift, (2) Early Cretaceous tectonic rift, (3) Early Cretaceous quiescent stage (evaporitic or calcilutitic), (4) middle Cretaceous initial drift carbonate platform, (5) Late Cretaceous platform/deltaic progradational and deep marine retrogradation, and (6) Tertiary main passive margin progradation. Habitat of oil discovered to date meets two regional geologic conditions: (1) in tectonic rifts known to have basin core of starved, lacustrine shales, and (2) in basins which developed a quiescent phase during the transition from tectonic rift to passive margin. Two major plays characterize the central core rifts, including (I) underlying prerift sediments in fault contact with the central core, and (II) sublacustrine fans overlying the central core. These plays, typified in the Reconcavo basin, constitute about half the recoverable oil found to date. A structurally related variation of type II play and a third regional play exist where the quiescent condition occurred, including reservoirs of the rift below evaporitic or calcilutitic regional seals and carbonate platform and turbidite reservoirs in the passive margin above the quiescent episode. The subevaporitic-calcilutitic subplay is prominent where overlying regional seals are structurally unmodified, contains about 15% of the discovered oil, and has typical development in the Sergipe (evaporitic) and Potiguar (calcilutitic) basins. Where regional seals of the quiescent phase have been mobilized, structurally modified, or cut by subsequent submarine canyons, carbonate platform and turbidite reservoirs of the overlying passive-margin fills are the prominent play (type III). Thi play, with typical development in Campos basin, in Mosqueiro low in Sergipe basin, and onshore Espirito Santo basin accounts for 35% of the discovered oil. Exploration implications of the established plays are: (1) source is from tectonic or quiescent stage fill (Aptian or older); (2) structural integrity of the quiescent stage seals is critical to oil migration; and (3) tectonic rifts are productive when a core of deep lacustrine shales was developed. End_of_Article - Last_Page 510------------