Abstract:

Exceptionally high-porosity sandstones are reported from oil provinces worldwide, yet the mechanism of porosity preservation remains controversial. We present strong evidence that the exceptional porosities in the Jurassic Fulmar Formation within the North Sea are the result of early oil charging preventing cementation and chemical compaction in sands with relatively low detrital clay contents, although overpressure and grain coatings do have some effect. The most dramatic evidence that reservoir quality is related to early oil charge is the spatial distribution of the porosity: the maximum porosity of the Fulmar Formation is systematically highest immediately below the top seal, and decreases below this despite uniform sedimentology. Hence, porosity is preferentially preserved where oil first accumulated in these unusually homogeneous overpressured reservoir sandstones, at the top. Oil charge prevents or significantly retards chemical compaction and cementation of a reservoir, preserving porosity. There is independent evidence for early oil charging of reservoirs within the Central Graben, from fluid inclusion studies and K–Ar age dates of authigenic illite. Micro-quartz grain coatings, which have been proposed to preserve porosity during burial, are associated with only 5% extra porosity in sands that exceed the worldwide average sandstone porosity by almost 20%.

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