Wave-dominated lacustrine margin of Aptian pre-salt: Mucuri Member, Espírito Santo Basin

Abstract

The Espírito Santo Basin, one of the Brazilian marginal basins formed during the Gondwana break-up, is composed by three main depositional units – rift, sag and drift phases. The Mucuri Member sandstones are the Aptian sag phase related, onshore reservoirs deposited at the margins of the lacustrine system were the pre-salt (pre-evaporitic succession) lacustrine carbonate reservoirs were generated. The main objective of this article focuses on the stratigraphic and sedimentological definition of depositional model from the top of the Mucuri Member, in the transition to Itaúnas Member evaporites of Mariricu Formation, since the depositional conditions of this interval until date remain under discussions and can contribute to the understanding of the correlated pre-salt reservoirs. Core description in detailed scale (1:50) associated with gamma-ray logs allowed the identification of nineteen facies, grouped into five facies associations: gravelly fluvial channel, poorly confined fluvial channels, upper shoreface, lower shoreface and offshore. Stacking patterns of facies associations allowed the definition of four cycles: T, T-R, normal R and forced R cycles. Although these cycles are easily recognizable in the cores, their correlation between wells was not possible. Geochemical analyses of 87Sr/86Sr ratio from anhydrites layers interbedded with the Mucuri sandstones confirmed a non-marine composition for the precipitating fluids. The integrated evidence suggests a wave dominated lacustrine coastal environment for the subaqueous Mucuri Member deposits.