Abstract

The Fulmar oil field lies at the faulted western margin of the Central Graben in the central North Sea. Chalcedony and quartz, and later dolomite, occur as diagenetic cements in veins within the Upper Jurassic Fulmar Formation. Quartz in one particular vein contains primary aqueous fluid inclusions with homogenization temperatures ranging from 85 degrees to 140 degrees C, with a mean of 117 degrees C. The measured d18O of the authigenic quartz ranges from +25.1 to +27.1o/oo SMOW. The d18O of the water from which most of the quartz precipitated is calculated to have been around +8o/oo, which is significantly more positive than the measured present-day formation water d18O of +4o/oo. Quartz and chalcedony cements in the vein are postdated texturally by dolomite that has a d18O of +23.5o/oo SMOW. High salinities are recorded in the fluid inclusions in the quartz, the majority ranging from 15 to 19 wt. % NaCl equivalent. Also, the quartz contains inclusions of anhydrite. High-salinity aqueous inclusions, combined with anhydrite inclusions in the quartz and the 18O-enriched calculated water composition, suggest that the quartz precipitated from an evaporite-influenced fluid. The same fluid contributed Mg for dolomite precipitation. More deeply buried sections, including the Permian Zechstein evaporites underlying the Fulmar field, are the most obvious source of an 18O-enriched saline fluid. Mineralization of the vein resulted from cross-formational flow of warm fluids from depth through the Zechstein and into the Jurassic succession over vertical distances of 600-2000 m. Fluids most probably migrated up faults and fractures during episodes of salt movement or overpressure release.

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