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ABSTRACT: Cementation and Diagenetic Fluid Mixing in South Brae Oil Field, North Sea, U.K.

O. M. McLaughlin, R. S. Haszeldine, A. E. Fallick, G. Rogers

The reservoir rocks in the Brae oil field consist of small overlapping submarine fans that form a sediment apron along the western margin of the Viking graben. It is located in the United Kingdom sector of the North Sea and lies 240 km east of the Orkney Islands.

Carbonate cementation was the first major diagenetic event, followed closely by compaction of the sediment and a minor development of authigenic chlorite and pyrite. This was ensued by a widespread long-term secondary quartz overgrowth phase. A dissolution event followed, subjecting feldspar, the micaceous minerals, and the early calcite cement to varying amounts of alteration. Clay minerals, principally illite and kaolinite, and some late calcite and dolomite infilled these dissolution pores at a later stage.

Progressive burial diagenesis has resulted in extensive reaction between the sediment and pore fluids, and the history of cementation is preserved in the isotopic composition of the authigenic calcite. Cemented horizons, veins, and concretions were analyzed. Concretions display the most depleted isotopic carbon values, with the carbon becoming progressively lighter towards the margin (-9.53 to -10.51^pmil PDB). The veins have a similar depleted signature of ^dgr13C (-8.4^pmil PDB) but show the most positive ^dgr18O PDB and ^dgr18O SMOW, perhaps indicating their formation by a later pore fluid source enriched in ^dgr18O. These values are not indicative of an end-member CO2 source but may suggest a mixture of bacterial fermentation nd sulfate reaction.

These calcites have formed in a marine environment, but the mean 87Sr/86Sr calcite ratio (0.7119) is considerably more radiogenic than 87Sr/86Sr of Upper Jurassic marine waters (0.707). The radiogenic strontium may have been contributed by the dissolution of silicates, such as feldspar supplying 87Sr/86Sr (0.720). These ions could have been supplied within the sandstone, indicating a closed diagenetic system.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990