Chlorite Authigenesis and Porosity Preservation in the Upper Cretaceous Marine Sandstones of the Santos Basin, Offshore Eastern Brazil

The main reservoirs in Santos Basin, eastern Brazilian margin, are Upper Cretaceous, marine turbidite and shelf sandstones that show porosity greater than 20% below 4000 m depth. This anomalous porosity is ascribed mostly to the inhibitory effect of chlorite rims on quartz cementation, and pressure dissolution. The sandstones are mostly fine-grained lithic arkoses with up to 24% basic and acidic volcanic rock fragments (VRF) derived from the uplift and erosion of the Lower Cretaceous Serra Geral Formation of the Paraná Basin. The chlorite pore-linings were formed by the mesogenetic replacement of precursor smectitic coatings derived from the eogenetic alteration of VRF, and by the direct neoformation of chlorite rims. Ions required for these reactions were derived from detrital smectites (associated mudrocks, matrix, inherited coatings), VRF and derived eogenetic smectites, and Aptian evaporites. The best reservoirs are the Ilhabela Member turbidites, where moderate eogenetic smectite formation was followed by limited compaction, chloritization and chlorite neoformation. Sandstones with abundant precursor smectite were extensively obstructed by abundant chlorite. Sandstones with scarce chlorite often suffered intense quartz cementation and pressure dissolution during deep burial. Turbidites show better development of chlorite rims and porosity than the Juréia Formation shelf deposits, apparently owing to smectite generation from VRF alteration within a more closed eogenetic system and, potentially, to diffusion from interlayered mudrocks.