Facies classification and patterns of lacustrine carbonate deposition of the Barra Velha Formation, Santos Basin, Brazilian Pre-salt

The lacustrine carbonate rocks of the giant Pre-salt oil-fields in the Santos and Campos Basins differ in a number of important aspects from classic lake deposits. Prior work has described distinctive and unusual features of deposition, early diagenesis and reservoir quality with in these rocks that defy classification using established schemes. In addition to their unusual textures, debate continues as to how to interpret the vertical stacking pattern of facies. A new facies classification scheme is proposed based on the relative abundance of three end-member components: mud (micrite and clay minerals), calcite spherulites and fascicular calcite shrubs. Although focused on the main textural and mineralogical features of the rocks as seen today, the proposed scheme takes into account the relative contribution of depositional and early diagenetic components, sediment mineralogy and the degree of dissolution. Application of this scheme offers insight into depositional conditions, whilst also linking to reservoir quality. Basic (elementary) cycles can be defined using this classification in combination with the variation in the abundance of detrital grains and the chemical composition. Three possible models are suggested to interpret the origin of the basic cycle within the Barra Velha Formation in the context of variation in climate, lake level and water chemistry. Integration of data from different sources, including petrographic, core description and chemical data for the basic cycle, offers improved process-based understanding of metre-scale facies patterns and provides a foundation for understanding larger scale vertical variations within and between unconformity-bound sequences.