A fluid inclusion study of cement and vein minerals from the Celtic Sea basins, offshore Ireland

Fluid inclusion petrography and microthermometry have been carried out on Jurassic sandstone cements (quartz, calcite and dolomite) and later fracture-filling calcite (±quartz) veins from the North Celtic Sea and the Fastnet Basins. Fluid inclusions in the sandstone cements are small (1-8 μm) monophase (liquid only) or liquid rich, two-phase (liquid+vapour) aqueous inclusions: all display irregular to elliptical morphologies. The two-phase aqueous inclusions homogenise to the liquid phase between ~60-122 C and have salinities ranging from ~7.0-16.0 wt% NaCl equivalent, indicating precipitation of the cements by moderate salinity waters. In contrast the veins host both aqueous and petroleum inclusions. The aqueous inclusions are liquid rich, two-phase (liquid+vapour) inclusions (~2-3 μm long) that are typically primary and show irregular and negative crystal shape morphologies. Their homogenisation temperatures (to liquid) range from 50-145 C and salinities from 6-20 wt% NaCl equivalent, indicating broadly similar salinities coupled with a wider range of minimum trapping temperatures to those recorded from the cements. Yellowbrown, two-phase (liquid+vapour) petroleum inclusions are hosted by vein calcite from several wells in the central North Celtic Sea Basin. Some of these inclusions appear to be primary and all are spatially associated with the aqueous inclusions. The two-phase (liquid+vapour) petroleum inclusions homogenise, mostly to the liquid, between ~50-130°C. Petroleum inclusions in the vein calcites indicate that petroleum was transported via fractures, at a large stage, through the Jurassic and Cretaceous strata of the Celtic Sea Basins, and apparently not through the sandstone matrix.