Oil exploration and its relationship to the world of trapped micron scale fluids: A review of the applications of fluid inclusion microscopy to the study of aqueous and hydrocarbon fluid dynamics in sedimentary basins

Fluid inclusions are micron scale samples of aqueous and hydrocarbon fluids trapped in annealed microfractures developed during burial, or earlier in authigenic minerals e.g. quartz and/or calcite during cementation. Microscopic studies are carried out on specially prepared doubly polished fluid inclusion wafers (~ 150 microns thick) of well core, sidewall core and cuttings. Using a combination of transmitted light and UV light microscopy, laser Raman microscopy and microthermometry, facilitates the collation and comparison of fluid inclusion data. Textural and compositional data relating to the trapping history of fluids can be further constrained using P-T modelling software. The results of fluid inclusion studies of North Atlantic offshore basins i.e. Irish, and Newfoundland and Labrador offshore sectors highlight the use of these analytical and fluid modelling techniques. For example, Porcupine Basin aqueous basinal fluids trapped in cements are consistently of low to moderate salinity (<10 eq. wt.% NaCl), comparable to those found elsewhere on the Atlantic margins e.g. UK Rockall, West of Shetland region, and in the Jeanne d'Arc Basin offshore Newfoundland and may reflect the paucity of evaporites at depth in these regions (Parnell et al., 1999, Parnell et al., 2001 and Feely and Parnell 2003). Migration of at least two chemically distinct hydrocarbon fluids occurred post cementation, as lateral flow along Jurassic sandstones with limited vertical flow along faults (Conliffe et al., 2009). In the Saglek Basin offshore Labrador both monophase (liquid) and two-phase (liquid + vapour) hydrocarbon fluid inclusions occur in the Cretaceous Markland Formation. The two-phase hydrocarbon inclusions yield homogenisation temperatures of ~80°C. The aqueous fluid inclusions represent low temperature (~100°C) and low salinity (~5 eq.wt% NaCl) fluids, and are similar to those recorded in the Porcupine Basin offshore Ireland. P-T modelling of these fluids indicate trapping pressures and temperatures of ~300 bars and ~110° C.