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 sandston es interlayered with floodplain/lacustrine mudstones and volcanics and intr usives. Extensive fracturing of the reservoir has led to the formation of n umerous 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 miner als and bitumen veins. This article presents fluid inclusion evidence from fracture-filling minerals, suggesting that the fractures were important flu id conduits of both petroleum and brine migration. The petroleum inclusion data are consistent with an undersaturated oil composition. A range in sali nity between 23 and 12 wt % NaCleq is found for the aqueous inclusions. The variation in salinity indicates mixing between a high saline fluid, relate d to evaporites, and a fluid with lower salinity. Microthermometry data sug gest that the fracturing events occurred at deep basinal levels, at tempera tures in excess of 100 degrees C. (C) 1998 Elsevier Science Ltd. All rights reserved.