Diagenetic porosity creation in an overpressured graben

R. S. Haszeldine*, M. Wilkinson, D. Darby, C. I. Macaulay, G. D. Couples, A. E. Fallick, C. G. Fleming, R. N.T. Stewart, G. Mcaulay

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract / Description of output

The Central Graben of the North Sea is an actively subsiding, highly overpressured, basin. At the present day, hydrocarbons are migrating both vertically and laterally through the stratigraphy to fill reservoirs from Permian through to Eocene. The late Jurassic Fulmar Sandstone approaches rock fracture pressure in many Central Graben areas. Petrographic studies show that sandstones originally contained as much as 35-40% potassium feldspar. This has dissolved rapidly below 3 km, so that by 4.5 km sandstones may contain only 5% feldspar. This loss is not matched by growth of authigenic clays or feldspars; aluminium is interpreted to have been exported from the sandstones by an unknown mechanism. In some cases, excess kaolin may form in overlying Palaeogene sandstones. Improved stratabound secondary porosity (an additional 5-15%) is created along the top of reservoir sandstones and at structurally high leak-off points. Leak-off points maintain high porosities, partly because overpressure reduces the effective compactional stress. This secondary porosity forms zones of good reservoir quality in predictable positions. Oils migrating vertically and laterally into Palaeogene sandstones have been bacterially oxidized, to form calcites with distinctive carbon isotope signatures. The distribution of such cements can be a guide to deeper leak-off positions. Modelling of modern temperatures across the graben shows that spikes of elevated temperature occur above leak-off positions, demonstrating that vertical fluid-flow is still active. The Central Graben, and any actively subsiding deep basin, can be viewed as one interconnected system, where localized deep porosity is created, and shallow porosity is destroyed. The organic, inorganic, temperature and overpressure fluid systems are all interactive.

Original languageEnglish
Pages (from-to)1339-1350
Number of pages12
JournalPetroleum Geology Conference Proceedings
Volume5
Issue number0
DOIs
Publication statusPublished - 1999

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