--> The Paleogeography of the Lacustrine Rift System of the Pre-Salt in Santos Basin, Offshore Brazil

AAPG ACE 2018

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The Paleogeography of the Lacustrine Rift System of the Pre-Salt in Santos Basin, Offshore Brazil

Abstract

The Santos Basin, in Southeastern Brazil, comprises one of the most expressive lacustrine rift systems developed during the breakup of the Gondwana supercontinent in the Lower Cretaceous. The associated petroleum system is responsible for world-class discoveries during the last decades. In this work we discuss some aspects related to the evolution of this lacustrine system.

At the Hauterivian/Barremian transition, the stretching phase of the rift formed deep depocenters associated with the main faults oriented to NNE and characterized by extensive volcanism. The half grabens produced by mechanical subsidence controlled the paleogeography of the lakes; the syn-rift sedimentation was characterized mostly by fine-grained siliciclastic rocks, including Mg-silicates.

By the end of the Barremian, mudstone facies with variable siliciclastic and carbonate content, including high TOC intervals, were deposited in brackish-saline water lakes at wider depocenters generated at the onset of the thinning phase of the rift, with growing influence of thermal subsidence. Porous bivalve coquinas were deposited at the crests of tilted fault blocks. In the Aptian, fault activity migrated towards the East, with the development of low-angle detachments. The full development of the thinning phase during the Aptian, with thermal subsidence at high rates and active detachments, produced the expansion of the lake system, in which volcanism was less intense.

An important regional unconformity is recognized in the Lower Aptian and above it a carbonate sequence was deposited. Stromatolites and grainstones/rudstones are the most important reservoirs, deposited at structural highs controlled by the active normal faults and detachments, while fine-grained limestones with variable content of siliciclastics defined the background sedimentation in the sublittoral/profundal environments. Authigenic Mg-silicates, including kerolite, sepiolite and stevensite sometimes occur associated with carbonate facies and are indicative of alkaline conditions.

By the end of the Aptian at the exhumation phase, just before breakup, the carbonate sequence was covered by a thick salt layer. At that time the connection between the lacustrine system and the South Atlantic Ocean had already been established.