--> The Neoproterozoic São Francisco Basin, Eastern Brazil, Geologic Evolution and Petroleum System, Martins-Neto, Marcelo, #90100 (2009)

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The Neoproterozoic São Francisco Basin, Eastern Brazil, Geologic Evolution and Petroleum System

Martins-Neto, Marcelo1

1Vale E&P, Rio de Janeiro, Brazil.

The existence of a petroleum system in the Precambrian deposits of São Francisco basin can be attested by several occurrences of gas seeps and the presence of gas in water wells along the basin. Seismic, well and outcrop integrated studies indicate that the record of the late Mesoproterozoic to early Cambrian succession started with the development of a full rift to passive-margin basin setting and ended with a foreland cycle. The passive-margin sequence has an overall wedge shape, with maximum thickness reaching ca. 6000 m in the westernmost part of the preserved basin. Seismic data indicates for the foreland sequence a classic wedge shape with preserved thicknesses of ca. 4000 m to the west, at the depocenter, and of ca. 800 m to the east, close to the flexural border. Deformation and trap formation are mostly related to the east-verging external zone of the Brasília fold-belt, where three tectonic styles can be identified: thin-skinned imbricated fan, basement-involved thick-skinned thrusts and strike-slip flower structures. Compressional structures include fault-propagation folds, pop-up anticlines and snake-head anticlines. Black shales/marls have been found in passive-margin deposits outcropping in the Brasília belt, with TOC ranging from 1 to 15 %, averaging 3.0 to 4.0 %. Geochemical data from gases show compositions from methane to more dense components, indicating a thermogenic origin. Most potential reservoir rocks are shallow-marine sandstones of the upper passive-margin succession, as well as vuggy or fractured carbonates of the foreland sequence. Marine clay-silt and clay-carbonate rocks occur in all stratigraphic units, constituting local and regional seals. The basal detachment of the thin-skinned belt is also considered a regional seal. The geologic location of basement-involved thrusts, connecting potential source and reservoir rocks, indicate them as the main migration routes in the São Francisco basin petroleum system. Strike-slip faults can also have played an important role as migration pathways. A very favorable situation for migration and focalizing through carrier beds can be observed in the contact between the shallow-marine sandstones of the passive-margin upper sequence and the basal detachment of the thin-skinned belt. However, the present available data allow only speculations about source-rock maturation and hydrocarbon migration in the São Francisco basin.

AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil