|
Fault Analysis and Diagenesis |
1 Geochem Group Limited, Chester Street, Chester CH4 8RD, UK
2 NAM BV, Postbus 28000, 9400HH, Assen, The Netherlands
3 Advanced Geochemical Systems Ltd, 40 Springfield Drive, Crewe, CW2 6RA, UK
Diagenesis in reservoir sandstones from the southern East Irish Sea Basin has resulted in the precipitation of multiple generations of quartz and carbonate cements. Other cements are volumetrically minor and do not affect sandstone reservoir quality, which overall retains high porosities and permeabilities. Stable isotope data suggest that precipitation of caliche calcite cement during early diagenesis occurred at low temperature (c. 30°C) from meteoric porewaters (–3
SMOW). By contrast 
34S data for evaporitic gypsum/anhydrite from the Ormskirk Sandstone Formation and Mercia Mudstone Group suggest that locally, an early marine sulphate source was available following deposition. Late burial anhydrite and barytes cements were derived through dissolution and reprecipitation of the early sulphates, according to stable isotope data. Fluid inclusion data for burial diagenetic cements record elevated temperatures (>100°C) for authigenic quartz, carbonate and sulphate minerals, which precipitated from concentrated brines. Stable isotopes indicate that porewaters enriched in 18O evolved during burial and late carbonates incorporated increasing quantities of carbon derived from organic maturation at depth. An episode of hydrocarbon migration during burial, predating the influx of hydrocarbons currently found, is preserved as bitumen in the sandstones. Further cementation postdated this episode, including late calcite cements postdating the migration of the current hydrocarbon charge. Petrographic data indicate that the elevated temperatures recorded in burial cements were not achieved entirely through depth of burial but probably reflect periods of elevated heatflow, as a result of the circulation of hot fluids through the reservoir sandstones.
