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° to 140°C, with a mean of 117°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

©Copyright 1997. The American Association of Petroleum Geologists. All rights reserved.

¹Manuscript received July 22, 1996; revised manuscript received January 16, 1997; final acceptance July 23, 1997.

²Scottish Universities Research and Reactor Centre, East Kilbride, Glasgow G75 0QU, Scotland.

³Department of Geology and Applied Geology, University of Glasgow, Glasgow G12 8QQ, Scotland.

We thank Shell UK Exploration and Production for funding Macaulay to undertake this research and for providing core samples. The authors appreciate the constructive reviews of Earl McBride and Glen Cayley. Thanks to Norman Oxtoby for his contributions to the fluid inclusion analyses, and to Douglas Maclean for photographing samples. The staff of the SURRC provided expert technical assistance with isotopic analyses. The SURRC is funded by NERC and a consortium of Scottish universities.