Abstract

The overpressure history of a sandstone can be estimated using a numerical model if the burial curve and geological setting are known. From the resulting effective stress, the maximum potential porosity (MPP) can be calculated. The MPP is the maximum porosity the rock could theoretically hold open at the modeled burial depth and pore pressure. Measured 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 Formation sandstones, the porosity evolution is a simple pattern of reduction during burial caused by compaction and cementation. However, in wells sited close to regional overpressure leak-off points, the porosity has been significantly increased from an end-of-Oligocene low (mean 21%) to the present-day values (mean 31%). This porosity increase occurred by feldspar dissolution, with the reaction products being removed from the sandstones. Secondary porosity generation and the export of solute occurred while the sandstone was highly overpressured, although still part of an open hydrogeological system. The generation of porosity within deeply buried sandstones is of commercial importance and potentially can be predicted.

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