Proveniência sedimentar: métodos e técnicas analíticas aplicadas

Abstract

The analysis of sedimentary provenance is a tool increasingly used in basin analysis and particularly in the exploration for clastic hydrocarbon reservoirs. The original sand composition directly controls the types and intensity of the diagenetic processes promoting porosity reduction and generation in the sandstones. Therefore, provenance studies identifing the distribution of the original sands composition in time and space are fundamental for the prediction of potential sandstone reservoirs. Provenance analysis provides the mineralogic and lithologic composition of the source areas, infering their geographic location, climate and topography, determining the main routes of sand distribution, and the distance and time of transportation. The diversity of detrital heavy minerals occurring in sandstones and the occurrence in paragenesis diagnostic of specific source rocks make the analysis of heavy minerals the most sensitive and robust technique for provenance studies. The combination of detrital heavy minerals with quantitative whole-rock petrography, feldspars composition, quartz and rock fragments types allows to establish the complex history of the sediments from their source areas to the depositional sites. Furthermore, modern microanalytical techniques allow the precise determination of the chemical composition of individual grains through electron microprobe (e.g. garnets and tourmalines), the ages of zircon, monazite and titanite through SHRIMP (Sensitive High Resolution Ion Microprobe) or ICPMS - LA (Inductively-Coupled Plasma Mass Spectrometry by Laser Ablation). These parameters directly connect the grains of the sandstones with the composition and age of the igneous or metamorphic rocks of the source terrains. Additionally, apatite and zircon fission track studies may determine the main ages of uplift and erosion of the source terrains. Integrated provenance studies of sandstones from specific areas of several Brazilian margin basins (Santos, Campos, Espírito Santo, Jequitinhonha and Pelotas) allowed the identification of the main source-areas and routes of sediment dispersal. These works shall substantially contribute, together with diagenetic studies, for the reduction of risks envolved in the exploration for clastic reservoirs in the Brazilian margin basins.