The diagenesis of Middle Jurassic reservoir sandstones of Bruce Field, U.K. North Sea
The earliest volumetrically significant diagenetic phase in the Middle Jurassic reservoir sandstones of Bruce Field (U.K. North Sea) is poikilotopic calcite. Integration of petrographic and isotopic data with burial history modelling indicates that it precipitated from Jurassic sea water at temperatures of about 40oC during the Early Cretaceous at a burial depth of about 600m. Its development was controlled by a favourable combination of many factors: a 'background' of organic-rich, argillaceous and shelly sediments to generate and supply the reactive species by essentially 'mudrock' reactions; relatively open pores into which reactive fluids were focussed; the presence of detrital shell as 'seeding points'; and the presence within the sediment pile of high permeability storm beds along which decompacting fluids could escape. Similar studies show that neoformed kaolinites precipitated from essentially meteoric pore waters at temperatures in the region of 50oC during the Cretaceous. I suggest that the emergent East Shetland Platform dominated the hydrology of these sandstones at this time, providing sufficient hydrostatic head to introduce the meteoric waters into the reservoir section. Quartz overgrowths, late stage ferroan calcites, dolomites and ferroan dolomites/ankerites, illitization of kaolinites and minor 'fibrous' illite neoformation are all interpreted to have occurred during a final stage in the burial history of these sandstones based on their relative paragenesis, stable isotopic compositions and the thermometric properties of fluid inclusions contained within authigenic quartz and carbonates. Such data indicate that this latest stage of mineralization occurred at temperatures of about 90-110oC. The similarity between these temperatures and present day formation temperatures (average 99-105oC) suggests that the development of these minerals may still be continuing at the present day.