Source Rock Prediction in the Brazilian Proterozoic São Francisco Basin

Globally, late Precambrian petroleum systems are a new frontier, both from an academic and commercial perspective. In the São Francisco Basin of Brazil, a functioning Proterozoic petroleum system occurs in a succession of probable Tonian-Ediacaran age. Gas seeps at outcrop, and significant gas shows have been encountered in several recent drilled wells, yet the origins of these hydrocarbons, the timing of their migration, and mechanism of entrapment, remain unknown.

Organic enrichment is reported in Meso-Neoproterozoic strata on the São Francisco Craton. Whilst the precise stratigraphic levels in which this enrichment occurs is undocumented in the public domain, three tectonistratigraphic units have untested source rock potential. These are 1) the Meso/Neoproterozoic Canastra and Paranoá groups, 2) the Tonian/Cryogeninan(?) Vazante Group and 3) the Cryogenian/Ediacaran Bambuí Group. These strata occur in the Brasília belt that fringes the São Francisco craton to the west and in the intracratonic São Francisco Basin and record a complete cycle of supercontinent breakup and assembly to form the megacontinent of Gondwana.Within these strata are potential source rocks and detailed study will investigate likely kitchen areas, migration pathways and trap mechanisms.

Regional field studies in 2011 targeted the study of some of these potential source rock intervals within the western margin of the craton. Based on literature review, their recognition in the field focused in their colour and portable gamma-ray spectrometer response. The latter contributed to 1) a proxy for authigenic uranium enrichment in oxygen deficient depositional environments and 2) to assist surface to well correlation. Subsequent geochemical analyses assessed organic content and maturation to provide information regarding the nature of these organic-rich rocks, their thermal history and likelihood for oil/gas generation. In addition, regional 2D seismic was interpreted to recognize the sequence of tectono-sedimentary events recorded in the area and the continuity of the units under evaluation. Re-Os dating was also incorporated to obtain depositional ages for source rocks and the timing of petroleum generation. Further gas sampling from seeps is planned to complement studies by determining the chemical composition, hydrocarbon gas types and maturity. The data presented in this paper, it is hoped, will therefore provide information in the nature and timing of hydrocarbon generation.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California