Some intervals of the deeply buried Upper Jurassic sandstones in the Danish Central Graben of the North Sea have higher than expected porosities of up to and above 20% even at depths of more than 4 km. These relatively high porosities correlate with the presence of microquartz coatings in the sandstones. The presence of microquartz coatings is generally delimited to sandstones deposited in the shoreface environment (Ravn Member and ‘Outer Rough Sand’) and offshore gravity flows, while microquartz is absent in sandstones deposited in fluvial, back-barrier and estuarine environments (Gert Member). Occasionally, impressions after sponge spicules occur in the microquartz-bearing sandstones, implying that the presence of microquartz is determined by the preferred habitats of silica sponges. Microquartz has been observed at burial depths of 2.8–4.8 km. Below ca. 3 km the individual crystals slowly begin to increase in size, eventually growing into incipient and interlocking quartz overgrowths. No microquartz is present below 4.8 km. This may be due to thermodynamic or chemical instability of microquartz at increased depth and temperature. Instead, unusual quartz-calcite interactions are present in calcite-cemented sandstones, perhaps indicating recrystallization of an initial grain-coating microquartz or opal phase.


Article metrics loading...

Loading full text...

Full text loading...


  1. Aase, N.E., Bjørkum, P.A. and Nadeau, P.H.
    [1996] The effect of grain-coating microquartz on preservation of reservoir porosity. AAPG Bulletin, 80, 1654–1673.
    [Google Scholar]
  2. Andsbjerg, J. and Dybkjær, K.
    [2003] Sequence stratigraphy of the Jurassic of the Danish Central Graben. Geological Survey of Denmark Greenland Bulletin, 1, 265–300.
    [Google Scholar]
  3. Bloch, S., Lander, R. and Bonnell, L.
    [2002] Anomalously high porosity and permeability in deeply buried sandstone reservoirs: Origin and predictability. AAPG Bulletin, 82, 301–328.
    [Google Scholar]
  4. Japsen, P., Britze, P. and Andersen, C.
    [2003] Upper Jurassic-Lower Cretaceous of the Danish Central Graben: structural framework and nomenclature. Geological Survey of Denmark Greenland Bulletin, 1, 233–246.
    [Google Scholar]
  5. Johannessen, P.N.
    [2003] Sedimentology and sequence stratigraphy of paralic and shallow marine Upper Jurassic sandstones in the northern Danish Central Graben. Geological Survey of Denmark Greenland Bulletin, 1, 367–402.
    [Google Scholar]
  6. Lander, R.H., Larese, R.E. and Bonnell, L.M.
    [2008] Toward more accurate quartz cement models: The importance of euhedral versus noneuhedral growth rates. AAPG Bulletin, 92, 1537–1563.
    [Google Scholar]
  7. Leinfelder, R.R., Werner, W. and Nose, M.
    [1996] Paleoecology, Growth Parameters and Dynamics of Coral, Sponge and Microbolite Reefs from the Late Jurassic. Glob. Reg. Control. Biog. Sedimentation., 1, Reef Evol. 227–248.
    [Google Scholar]
  8. Maast, T.E., Jahren, J. and Bjørlykke, K.
    , [2011] Diagenetic controls on reservoir quality in Middle to Upper Jurassic sandstones in the South Viking Graben, North Sea. AAPG Bulletin, 95, 1937–1958.
    [Google Scholar]

Data & Media loading...

This is a required field
Please enter a valid email address
Approval was a Success
Invalid data
An Error Occurred
Approval was partially successful, following selected items could not be processed due to error