High-temperature alteration and porosity generation in Upper Ordovician microbial reefs, Hudson Bay intracratonic Basin, Arctic Canada

Upper Ordovician biohermal mounds of the Red Head Rapids River Formation in the Hudson Bay Basin consist of microbial and sponge boundstone and cementstone. The cementstone is made up of isopachous layers and botryoids of former aragonite now calcite. Secondary dissolution porosity and small fractures are cutting through the bioherm. Secondary pore-fillings consist of drusy calcite cement and subsequent bitumen. The δ18OVPDB and δ13CVPDB values of late cements are invariably more negative than those of the marine cements. Fluid inclusion microthermometry documents the presence of two distinct populations of homogenization temperatures, one in secondary pore-filling burial calcite cement (Th average 93 ± 10 °C) and a second one in recrystallized marine calcite cements (Th average 118 ± 25 °C). The combined δ18OVPDB and Th data suggest that burial cements precipitated from a fluid having δ18OSMOW values between +0.5 and −2‰, whereas the marine cement data indicate resetting of the fluid inclusions in the presence of a high temperature, δ18OSMOW heavy brine (+2.5 to +12‰). The lower temperature fluid inclusions of the late pore-filling cement agree with recent Apatite Fission Track data suggesting early oil window temperature at maximum burial. The higher Th values recorded in the marine cement represent resetting of initial or entrapment of new fluid inclusions from fracture-controlled circulation of basement-derived fluids. The petrographic and geochemical data suggest that fracture-controlled high temperature brine circulation occurred after the inception of burial and recrystallization of original marine aragonite to calcite, which resulted in the generation of significant secondary porosity that was later filled by lower temperature burial cements and hydrocarbons.