Paleohydrogeological and thermal events recorded by fluid inclusions and stable isotopes of diagenetic minerals in Lower Cretaceous sandstones, offshore Nova Scotia, Canada

Abstract

Upper Jurassic–Lower Cretaceous sandstones of the Scotian Basin, offshore eastern Canada, are important gas reservoirs. Previous studies of fluid inclusions in Jurassic limestones and apatite thermochronology of Cretaceous sandstones have suggested a late Mesozoic thermal event. Fluid inclusions in different authigenic cements were analyzed to determine the temperature and composition of basinal fluid at the time of precipitation and the timing of hydrocarbon migration and entrapment. Fifty-one sandstone samples were analyzed for stable isotope composition (18O and 13C) of carbonate cements. Trapping temperatures for primary aqueous inclusions hosted in quartz overgrowths (89–175C) and in late carbonate cements (138–173C) are higher than predicted by two-dimensional modeling from burial alone based on postrift geothermal gradients. These inclusions contain high-salinity fluids (mostly 19–22 wt. % NaCl equivalent). Second, predominantly aqueous inclusions have much lower salinities (5.2–6.7 wt. % NaCl equivalent), and some contain liquid hydrocarbons. Late Fe-calcite cement in Hauterivian sandstone shows negative values of (13.17 to 9.2), whereas cements in deeper and shallower sandstones have higher 13C values. These data indicate that high-temperature, high-salinity fluids preceded hydrocarbon migration. The oldest and youngest rocks studied show less effect of high temperatures than do rocks that were buried to depths of at least 2 km (1.2 mi) after 135 Ma and before 100 Ma. Both fluid inclusions and 13C of carbonate cement suggest that highest temperatures were achieved during burial at 115 to 105 Ma. This Early Cretaceous thermal event had a geothermal gradient of at least 55C/km. It is earlier than the previously reported event based on apatite fission tracks but is consistent with the fission track data. The event is synchronous with regional evidence of volcanism, and its peak coincides with rapid salt-tectonic deformation in the deep basin. Hydrocarbon charge to the outer-shelf wells occurred after this thermal event and thus not before the Late Cretaceous.