The fluid regime during fracturing of the Embla field, Central Trough, North Sea

The deep Embla field, located in the Central Trough, North Sea, represents the first oil reservoir in Paleozoic rocks to be developed on the Norwegian shelf. The reservoir consists of braided fluvial and alluvial fan sandstones interlayered with floodplain/lacustrine mudstones and volcanics and intrusives. Extensive fracturing of the reservoir has led to the formation of numerous mineralised veins. Three main types of mineralised fractures can be identified: (1) early diagenetic veins and veins which occur in and around mafic igneous rocks; (2) carbonate-dominated veins; (3) barite, clay minerals and bitumen veins. This article presents fluid inclusion evidence from fracture-filling minerals, suggesting that the fractures were important fluid conduits of both petroleum and brine migration. The petroleum inclusion data are consistent with an undersaturated oil composition. A range in salinity between 23 and 12 wt % NaCl_eq is found for the aqueous inclusions. The variation in salinity indicates mixing between a high saline fluid, related to evaporites, and a fluid with lower salinity. Microthermometry data suggest that the fracturing events occurred at deep basinal levels, at temperatures in excess of 100°C.