Controls on sandstone diagenesis, Scotian Basin, eastern Canada: basinal fluids and salt tectonics.

Sedimentation and diagenesis in the Scotian Basin were strongly influenced by salt tectonics. Thick Upper Jurassic-Lower Cretaceous deltaic sandstones are important reservoir rocks. Published fluid inclusion data provide a record of changing salinity and temperature of migrating formation water, with the most saline fluids at times of active salt deformation. Recognised high temperature events correspond to high regional mantle heat flow in the middle Cretaceous and the Paleogene release of over-pressured fluids known from apatite fission track studies. Geographic variations in timing of salt deformation resulted in variable diagenetic conditions along the strike of the basin. In reservoir sandstones, albitization of detrital K-feldspar and plagioclase resulted from the high Na+ content of formation waters that migrated through permeable pathways at the time of silica and carbonate cementation, with strong albitization coinciding with transport-controlled dissolution of K-feldspar. Such saline fluids recorded in fluid inclusions created widespread dissolution of silica and carbonate grains and cements and this secondary porosity was host to a variety of late diagenetic minerals including minerals transported by halogen-rich fluids such as sphalerite and titania (TiO2) minerals. Among the titania polymorphs, anatase is mostly early diagenetic whereas brookite is usually late diagenetic and is abundant in reservoir sandstones above the free-water level. Detrital Ti-bearing minerals are unusually abundant in the Scotian Basin and provide a source of Ti for chelation by organic acids. Early diagenesis of titania minerals is related to humic acids in meteoric water at marine lowstands. Late diagenetic mobility of Ti and precipitation of brookite is enhanced by Ti complexing with organic acids in saline, hydrocarbon-rich fluids. [ABSTRACT FROM AUTHOR]