Petroleum Systems along the Western South Atlantic Margin Assessed from Oil Geochemistry and Basin Redefinitions
The authors have assessed petroleum systems of the western South Atlantic margin basins analyzing a large (>800) sample set of oils for source, maturation and migration history. We rely on statistical evaluation of crude oil geochemistry for the marginal basins of Brazil (Foz do Amazonas south to Santos) plus Argentina and Chile onshore (Cuyo, Neuquen, San Jorge) and offshore (Colorado, Austral, Magallanes). This approach works because the basins share a common origin as Jurassic - Cretaceous separation of South America from Africa formed a series of failed rift-drift basins with genetically related syn-rift lacustrine and transitional-marine environments. Study objectives were to 1) group oils into families and sub-families by source type and depositional environment; 2) identify areas with lacustrine-sourced oils; and 3) with gravity & magnetic data, ascertain tectonic relationships among the oil-bearing lacustrine and marine-source compartments. A fourth objective followed, to infer likely sources where we lacked oil samples.
A distinct difference between basins north of the Rio Grande Rise - Walvis Ridge (RGWR) and those to the south is the respective presence versus absence of salt. During the Aptian, a closed evaporite basin developed north of the RGWR in which thick salt beds were deposited. The evaporites require a marked change to marine conditions, opening the door for earlier transitional (sag?) source intervals north of the RGWR. Initial results allow confident separation into four broad groups or families:
B) Lacustrine Fresh/Brackish
C) Hypersaline/Transitional Marine, and
D) Lacustrine Brackish/Saline.
Groups C and D are limited to oils from Ceara/Potiguar/Sergipe and Espírito Santo/Campos basins respectively. B represents the broadest geographic group of oils, spanning basins in Brazil (Reconcavo, Camamu, Potiguar, Ceara and Para Maranhao), Argentina (Cuyo, San Jorge, Neuquen, Austral) and Chile (Magallanes) and including oils from lacustrine sources and additional settings previously interpreted as sag or mixed. A has the most internal scatter as the sources represent a greater time span with corresponding paleoclimate variability.
Further subdivision of oil families based on maturity, lithofacies, and biodegradation is consistent with our updated tectonic mapping. The interplay between data sets has supported some redefinition of basin and sub-basin outlines as these new family extents are drawn.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009