Dynamic re-sedimentation of lacustrine carbonates in the Búzios Field, pre-salt section of Santos Basin, Brazil

Abstract

The huge oil accumulations in the South Atlantic Pre-Salt section have driven studies that seek a greater understanding of the origin and evolution of these important reservoirs. Lacustrine carbonate intraclastic deposits correspond to the main reservoirs in the Búzios Field, currently with the highest daily production wells in the world, and in other Santos Basin fields. However, few studies were dedicated to these deposits due to the great complexity of Pre-Salt deposits and the lack of analogs. These factors have generated intense controversies in the interpretation of their depositional models. A systematic petrographic characterization allowed detailed characterization of the textures and primary constituents in the rift and sag sections of a well in the Búzios Field. The bioclastic accumulations of the rift Itapema Formation, show features indicative of reworking, such as abrasion, fragmentation and disarticulation of the bivalve bioclasts. Moreover, the bioclasts occur mainly within intraclasts and as nuclei of ooids, characteristics of multicycle redeposition. The massive structure of these deposits, the lack of characteristic wave structures, their distribution in lows and conspicuous mixing with stevensitic grains indicate their gravitational redeposition, as result of the intense tectonic activity during the rift phase. The sag phase Barra Velha Formation deposits are composed of intraclasts eroded from the in situ spherulites and shrubs. These rocks are predominantly massive, lack subaerial exposure features, and display features that point to dynamic and multicyclic reworking. Furthermore, they occur intercalated with in situ deposits in high frequency, even at a thin section scale. These aspects, along with the consistency of isotope values, the widespread spatial distribution and preservation of stevensite across diverse Pre-Salt areas, suggest that gravitational, waves and currents redeposition models should be reevaluated. We propose that internal waves, generated by perturbations of the chemocline of a meromictic, stratified lacustrine system promoted recurrent reworking of the in situ deposits. This generated intraclastic levels with multicycle redeposition aspects, often interlayered with the characteristic in situ deposits. Understanding the processes that generated these deposits will contribute to reduce exploration risks and optimize hydrocarbon production from the intraclastic reservoirs.