Fluid inclusion studies of well samples from the hydrocarbon prospective Porcupine Basin, offshore Ireland

Transmitted light and ultraviolet (UV) light microscopy combined with microthermometry of fluid inclusions in sandstones from 10 wells in the Irish Porcupine Basin reveal the presence of aqueous and oil-bearing fluids in cements and detrital quartz grains. Jurassic sandstone samples contain two-phase oil inclusions trapped during syn- and post-cementation and homogenise to the liquid state at ∼50 to 125°C. Furthermore, early trapping in cement of less mature relatively heavy oil (°API gravity∼25-35) followed by ingress of lighter more mature oil (°API gravity∼45-50) is indicated. Primary two-phase aqueous fluid inclusions trapped in cements are generally of low salinity (< 10 eq. wt.% NaCl) and homogenise to the liquid phase between 50 and 175°C, indeed similar to other Atlantic Margin basins, e.g. UK Rockall and West Shetland. Secondary aqueous inclusions in detrital quartz and calcite grains homogenise to the liquid state in the range ∼50 to 325°C, while salinities are generally < 10 eq. wt.% NaCl but can range up to ∼ 20 eq. wt.% NaCl.