Integrated analysis of the quality heterogeneity in Albian carbonate reservoirs from the Campos Basin, Brazil

Albian limestones of the Quissamã Formation are important oil reservoirs in the Campos Basin, Brazil. However, these reservoirs present wide heterogeneity in the distribution of porosity and permeability, promoted by variations in their depositional texture and composition, and dominantly by the differential intensity and distribution of diagenesis. The main diagenetic processes were micritization, eodiagenetic and mesodiagenetic calcite cementation, and mechanical and chemical compaction. Generation of secondary porosity by dissolution occurred both during eodiagenesis, by meteoric waters, and during mesodiagenesis, by acidic fluids percolating through fractures caused by halokinesis and through stylolites. Preservation of porosity in some reservoirs was promoted by early oil charge, inhibiting cementation and compaction. Most deposits from the South of the basin show inferior reservoir quality compared to those in the central area, due to stronger cementation and compaction. The heterogeneous distribution of permeability and porosity was systematically demonstrated by flow zone analysis, and related to the preservation or reduction of interparticle porosity, and locally to the presence of fine-grained or muddy deposits. Unravelling the depositional and diagenetic controls on porosity and permeability of analyzed reservoirs shall contribute to reduce exploration risks, and to optimize recovery from producing oilfields.