Brittle deformation and hydrothermal alteration in the Barra Velha Formation, Santos Basin, offshore Brazil

The Barra Velha Formation is a prolific Aptian (Lower Cretaceous) oil and gas producer located in the Santos Basin, southeastern Brazilian coast. We used structural core description, petrography, geochemistry, and geochronology to show evidence from drill cores that the Barra Velha Formation was affected by several events of brittle deformation and interacting hydrothermal alteration. The U-Pb geochronology provides an age of ca. 116 Ma for depositional host rock calcites, although recrystallization is possible. An early brecciation event occurred at ca. 108 Ma. Recracked fractures (>10 cm wide) are interpreted as spring mound vents. A dolomite fault mineralization is dated at ca. 95 Ma. Events affecting fractures include widespread dissolution, with secondary porosity filled by barite, quartz, and solid bitumen. Fluid inclusion geochemistry suggests that diagenesis occurred with the participation of seawater. We show a timeline of diagenetic events and infer that fracture porosity is more likely to be preserved where deep faults enabled fluids from greater depths to ascend into the reservoir, where they created large vuggy cavities by dissolution of host rock.