ABSTRACT: The Geochemistry and Diagenesis of Stratabound Calcite Cement Layers within the Rannoch Formation of the Brent Group, Murchison Field, North Viking Graben (Northern North Sea)

DAWS, JULIE A., and D. J. PROSSER, University of Aberdeen, Aberdeen, Scotland, and A. E. FALLICK, Scottish Universities Research and Reactor Centre, East Kilbride, Scotland

Up to four calcite cemented horizons (Doggers) form impermeable barriers to fluid flow within the Rannoch Formation and are correlatable across the Murchison field. Calcite precipitated during early diagenesis, within high porosity-permeability sandstones at the top of coarsening-(shoaling-) upward shoreface cycles. Calcite Carbon 13 and Oxygen 18 compositions range from -4.07 to -13.43% PDB, and -7.127 to -16.66% PDB, respectively. High Sr concentrations are consistent with marine carbonate sources (possibly shell fragments), but no viable intraformational carbonate source has been identified. 87Sr/86Sr compositions (0.711089-0.712656) are higher than middle Jurassic sea water (0.7073) and perhaps consistent with precipitation from modified porewaters containing significant proportio s of continentally derived "meteoric" fluids enriched in 87Sr as a result of basement weathering. Carbon 13 compositions are more depleted than those typical of carbon derived from shell debris, perhaps indicating mixing of porewaters and incorporation of carbon from basinal derived fluids. Correlation between decreasing Oxygen 18 and decreasing Carbon 13 suggests a modified porewater system, produced by mixing of meteoric porewaters (flushing sandstones during early diagenesis/burial) with modified marine porewaters.

Geochemical and isotope data reveal trends that can be interpreted differently in terms of either "static" or "evolving" porewater composition models. Static composition porewater models using middle Jurassic "meteoric" water (-6% SMOW) could be used to predict cement precipitation temperatures. However, Oxygen 18/Carbon 13 trends favor an evolving porewater system, subject to differing degrees of fluid mixing/modification during early burial, and estimates of precipitation temperature based on isotope data alone should be treated with extreme caution.

Geostatistical analysis of closely spaced minipermeameter data indicates that third-order permeability heterogeneities generated by the presence of bedding and associated bounding surfaces are important in controlling the development of "vertical" permeability within the Etive Formation.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)