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Veins Record Vertical Flow at a Graben Edge: Fulmar Oil Field, Central
Calum I. Macaulay, Adrian J.
Boyce, Anthony E. Fallick, and R. Stuart Haszeldine3
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
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
1997. The American Association of Petroleum Geologists. All rights reserved.
received July 22, 1996; revised manuscript received January 16, 1997; final
acceptance July 23, 1997.
Universities Research and Reactor Centre, East Kilbride, Glasgow G75 0QU,
of Geology and Applied Geology, University of Glasgow, Glasgow G12 8QQ,
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.
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