SECONDARY POROSITY GENERATION DURING DEEP BURIAL ASSOCIATED WITH OVERPRESSURE LEAK-OFF - FULMAR FORMATION, UNITED-KINGDOM CENTRAL GRABEN

The overpressure history of a sandstone can be estimated using a numer ical model if the burial curve and geological setting are known. From the resulting effective stress, the maximum potential porosity (MPP) c an be calculated. The MPP is the maximum porosity the rock could theor etically hold open at the modeled burial depth and pore pressure. Meas ured rock porosities should be at or below the MPP. We have determined the MPP history for the Fulmar Formation sandstones (Upper Jurassic) of the Central Graben, North Sea, and have compared the predictions to measured core data. We conclude that for the majority of the Fulmar F ormation sandstones, the porosity evolution is a simple pattern of red uction during burial caused by compaction and cementation. However, in wells sited close to regional overpressure leak-off points, the poros ity has been significantly increased from an end-of-Oligocene low (mea n 21%) to the present-day values (mean 31%). This porosity increase oc curred by feldspar dissolution, with the reaction products being remov ed from the sandstones. Secondary porosity generation and the export o f solute occurred while the sandstone was highly overpressured, althou gh still part of an open hydrogeological system. The generation of por osity within deeply buried sandstones is of commercial importance and potentially can be predicted.