Aspects of the Neoproterozoic Petroleum System in the São Francisco Basin, Brazil

Globally, Precambrian hydrocarbon systems are a new frontier, both from an academic and commercial perspective. In the São Francisco Basin of Brazil, a functioning Proterozoic hydrocarbon system occurs, yet the origins and the timing of its generation remain unknown. A basin-wide reconnaissance level study was approached to evaluate its hydrocarbon source potential and to determine its evolution from the Proterozoic inception, through to the present day. Three units were targeted in this study: the Meso/Neoproterozoic Canastra Group (Paracatú Formation), the Meso/Neoproterozoic Vazante Group (Serra do Garrote Formation) and the Cryogenian/Ediacaran Bambuí Group (Sete Lagoas Formation). The results indicate fair-good organic enrichment for the Paracatú and Serra do Garrote formations and almost null potential for the Sete Lagoas Formation in the location of study. Even with uncertainties, it is inferred that the three units are presently overmature, although it is not discounted that they were active generators in the past. New
Re-Os dates for the Paracatú Slate Formation yields a depositional age of 1002±45 Ma, which is in agreement with published U-Pb detrital zircon age. Seismic interpretation, on the other hand, support an intracratonic setting for the deposition of the Bambuí Group and specifically for the Sete Lagoas Formation provides insights into restricted circulation associated to palaeotopography developed during the Jequitaí glaciation. Thermochronology dovetailed with analysis of porosity in borehole, reveal that significant post-Bambuí Group erosion has taken place. At least 3-6 km of strata is estimated to have been removed following the amalgamation of Gondwana. Maximum palaeotemperatures beneath this now missing cover were reached during the early Palaeozoic. Through basin modelling, it is shown that the Palaeozoic overburden
allowed the Bambuí Group to enter the oil window. Hence, assuming adequate organic content in the strata, any hydrocarbons are predicted to have been generated during late Ediacaran to Cambrian times. This requires that the traps were already in place and hence long retention. For the older units, it can be argued that the São Francisco Basin has had the potential for hydrocarbon generation at least since Tonian times (i.e. ca. 1 Ga).