--> Modeling the Genesis of Aptian Pre-Salt Deposits in Brazilian Basins

2018 AAPG International Conference and Exhibition

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Modeling the Genesis of Aptian Pre-Salt Deposits in Brazilian Basins

Abstract

The huge Aptian Pre-salt accumulations of Brazilian margin basins are constituted by alternation of crusts of fascicular calcite shrubs, which usually correspond to the best reservoirs, with layers of calcite spherulites within magnesian silicates such as stevensite and kerolite. Extensive petrographic characterization indicates that the calcite crusts formed along the sediment-water interface, while the spherulites formed within accumulating Mg-clays. The habit and distribution of calcite aggregates reflect changing geochemical conditions that periodically favored calcite precipitation over the precipitation and accumulation of syngenetic Mg-clays. To simulate the observed competitive accumulation and/or growth pattern between Mg-clays and calcite, a geochemical water-rock interaction simulator was developed to include: a) kinetic and equilibrium reactions; b) advective and diffusive mass-transfer; c) influx and evaporation of meteoric or hydrothermal waters; d) early diagenetic interaction of sediments with overlying waters; e) precipitation of Mg-clays in the water column and their accumulation at the lake bottom; and f) precipitation of fascicular calcite at the water-sediment interface and spherulites within the sediments. The simulator was used to carry out 1D and batch models, in a domain consisting of alkaline lacustrine water column overlying a shallow sedimentary column. The model demonstrates the occurrence of alternating periods of calcite supersaturation, when crusts were precipitating at the water-sediment interface and spherulites were growing within the Mg-clays, with periods of essentially Mg-clay accumulation, when only few spherulites were growing in the shallow sediments. Mg-clays dissolution, which petrographic studies indicate is a key porosity generating process in many intervals, has also been modeled. The results support our hypothesis that the major driving force that affected the lake water chemistry to produce the observed chemical precipitates was due probably to complex interaction between evaporation, CO2 loss, and variable influx rates of meteoric and hydrothermal waters. Simulation results can be correlated by comparing petrographic observations against rates of growth and/or accumulation of calcite and Mg-clays. The model provides important knowledge advancement toward assessing the distribution of calcite and Mg-clay deposits, in particular of the Pre-salt facies with the best reservoir quality.