Distribution of silicification intervals throughout the Barra Velha and Itapema formations: Host rock controls and chronology of silica precipitation (Pre-Salt, Santos Basin, Brazil)

Abstract

The Barra Velha and Itapema formations, Aptian, Santos Basin, constitute large hydrocarbon reservoirs, mainly in carbonate rocks. These rocks host up to centimeter-thick silica bodies with a variety of textures and geometries along different carbonate facies, suggesting syngenetic, diagenetic, and hydrothermal processes in their formation. The mineralogical and textural changes related to these processes can directly influence the porosity and permeability of the reservoir rocks, turning the study of the silicified intervals important in the exploration and production of these deposits. We present here a systematic macroscopic and microscopic description of the silica bodies and its distribution through the carbonate facies the of Barra Velha and Itapema formations from the Sapinhoá and Búzios fields. Data allow us to identify four geometric types of silica bodies: tabular, irregular, oval and disseminated, which are composed of chalcedony, microcrystalline and macrocrystalline quartz. Tabular, oval, and irregular silica bodies are cross-cutted by mechanical compaction structures such as vertical fractures, boudins and faults. In addition, these bodies include relicts of the host rock lamination, which in turn deflects around them. Thus, silica precipitation must have been penecontemporaneous or in the early stages of diagenesis, before the main mechanical compaction stage. These features were not observed in the disseminated silica type, in the contrary, this type involves and overlaps oval and irregular bodies, indicating that this silica precipitation probably occurred in latter stages in relation to other body types. The genesis of disseminated bodies can then be related to diagenetic or hydrothermal processes during diagenesis. Therefore, the geometry of silica bodies and their spatial distribution are influenced by the host carbonatic petrofabrics and the chronology of silica precipitation within the sedimentary to diagenetic evolution of the sedimentary package. Finally, we present a systematics that can be easily applied to recognize these silica types in other exploration fields of pre-salt interval. The results presented here have great potential to be applied in the research on pre-salt intervals from other Brazilian and African basins since the host carbonate rocks are very similar and favor the development of similar silicification processes and results.