Albian Carbonates from Southeastern Brazil: Stratigraphy and Retreat of a Platform in Response to Halokinesis

Paulo R. Zarpelon, Adali R. Spadini, and Arnaldo S. Biassusi
Exploration & Production, PETROBRAS, Rio de Janeiro, Brazil

A regional study was done in albian (112 to 99,6 Ma) carbonates from Campos and Santos Basin (over 200,000 square Km), SE Brazil. This platform sequence developed after an evaporitic package related to a sag fase of a rift system (Aptian). Three major stratigraphic sequences (1,5 Ma - third order) were considered in these carbonates based on seismic, sedimentological and biostratigraphic data obtained from more than a hundred wells.

During the Early Albian, a structured ramp (Sequence I), with very shallow and dolomitized facies, was flooded and aggraded to widespread platform deposits (Sequence II). The sedimentary processes involved in this stage were affected by the paleo-structural relief, resulting in the preferential development of oolitic facies in highs as well as muddy and oncolitic-bearing sedimentation restricted to the inner portions. In central areas of both basins, thick packages (up to 1000m) of muddy facies with planctonic fauna indicate persistently deeper depositional environments, probably in connection to an open sea. Seismic features suggest that early salt tectonics enhanced the accommodation space for the formation of these depressions.

During the middle Albian, this platform was highly fragmented and rafted in response to salt tectonics and gravitational gliding. The resulting changeable tectonic framework strongly influenced the sedimentation (Sequence III) promoting a widespread back step of the higher energy sites and their development limited to structural highs, as well as the partial drowning of the deeper areas.

This internal architecture is in sharp contrast with the well developed prograding carbonate platforms observed in relative stable areas, particularly in the southern portion of the Santos Basin.

The ending of the platform sedimentation occurred at the beginning of the upper Albian due the complete drowning of the section.

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