CARBONATE CEMENTATION PATTERNS AND DIAGENETIC RESERVOIR FACIES IN THECAMPOS BASIN CRETACEOUS TURBIDITES, OFFSHORE EASTERN BRAZIL

Massive turbidite arkoses contain more than 80% of the oil reserves of the Campos Basin, the main petroleum province of Brazil. The porosity and permeability distribution in the Namorado (Albian-Cenomanian) and Carapebus (Turonian-Santonian) sandstones is controlled by carbonate cementation at shallow depths below the seafloor and by compaction and silicification of mud intraclasts. The carbonate cementation followed two patterns: (1) in the Albian sandstones, concretionary sulphate re duction and fermentation calcite cementation occurred around bioclasti c levels with marine microcrystalline cement; (2) in the Upper Cretace ous sandstones, coarse sulphate reduction and fermentation carbonates precipitated along the intercalated shales. The sources of the carbona te cements included sea water, bioclasts, bacterial alteration of orga nic matter and Albian carbonate rocks. Four diagenetic reservoir facie s were characterized: (1) massively cemented facies along bioclastic l ayers with marine cements and intercalated shares; (2) partially cemen ted facies bordering massively cemented facies with cements derived fr om the burial dissolution of early carbonates; (3) porous facies with dominantly primary porosity preserved due to the late subsidence and e arly oil saturation of the reservoirs; and (4) intraclastic facies in channel/levee deposits with mud intraclasts compacted to pseudo-matrix and silicified.