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Chalk is very fine-grained carbonate sediment, comprising skeletal calcitic debris of algae platelets. Porosity in chalk can be high, sometimes as high as 40–50%. Nevertheless, given the very fine-grained nature of the rock, permeabilities are low; 1–7 md is typical of the productive intervals. Factors influencing porosity preservation in chalk are overpressure, early oil migration, burial depth, chalk lithofacies, mud content, and grain size.<ref>Scholle, P. A., 1977, [http://archives.datapages.com/data/bulletns/1977-79/data/pg/0061/0007/0950/0982.htm Chalk diagenesis and its relation to petroleum exploration: Oil from chalks, a modern miracle?]: AAPG Bulletin, v. 61, no. 7, p. 982–1009.</ref><ref>Nygaard, E., K. Lieberkind, and P. Frykman, 1983, Sedimentology and reservoir parameters of the Chalk Group in the Danish central graben: Geologie en Mijnbouw, v. 62, no. 1, p. 177–190.</ref><ref name=DHeur>D'Heur, M., 1986, The Norwegian chalk fields, in A. M. Spencer, ed., Habitat of hydrocarbons on the Norwegian Continental Shelf: London, Graham amp Trotman, p. 77–89.</ref><ref>Brasher, J. E. and K. R. Vagle, 1996, [http://archives.datapages.com/data/bulletns/1994-96/data/pg/0080/0005/0700/0746.htm Influence of lithofacies and diagenesis on Norwegian North Sea chalk reservoirs]: AAPG Bulletin, v. 80, no. 5, p. 746–768.</ref> A correlation is found between the clay content of the chalk and the degradation of reservoir quality; clay hinders early lithification. As a result, clay-rich chalks are less rigid and will tend to undergo more compaction.<ref name=Kennedy>Kennedy, W. J., 1987, Sedimentology of Late Cretaceous–Paleocene Chalk reservoirs, North Sea central graben, in J. Brooks and K. Glennie, eds., Petroleum geology of northwest Europe 1987: London, Graham amp Trotman, p. 469–481.</ref> It is a common pattern in chalk oil fields to find the highest porosity in the crest of the field, decreasing incrementally toward the oil-water contact.<ref name=DHeur /> This character may result from the race for space between oil migration and cementing fluids. The permeability in the water leg can be so poor that chalk fields are unlikely to have significant aquifers.

Chalk is very fine-grained carbonate sediment, comprising skeletal calcitic debris of algae platelets. Porosity in chalk can be high, sometimes as high as 40–50%. Nevertheless, given the very fine-grained nature of the rock, permeabilities are low; 1–7 md is typical of the productive intervals. Factors influencing porosity preservation in chalk are overpressure, early oil migration, burial depth, chalk lithofacies, mud content, and grain size.<ref>Scholle, P. A., 1977, [http://archives.datapages.com/data/bulletns/1977-79/data/pg/0061/0007/0950/0982.htm Chalk diagenesis and its relation to petroleum exploration: Oil from chalks, a modern miracle?]: AAPG Bulletin, v. 61, no. 7, p. 982–1009.</ref><ref>Nygaard, E., K. Lieberkind, and P. Frykman, 1983, Sedimentology and reservoir parameters of the Chalk Group in the Danish central graben: Geologie en Mijnbouw, v. 62, no. 1, p. 177–190.</ref><ref name=DHeur>D'Heur, M., 1986, The Norwegian chalk fields, in A. M. Spencer, ed., Habitat of hydrocarbons on the Norwegian Continental Shelf: London, Graham amp Trotman, p. 77–89.</ref><ref>Brasher, J. E. and K. R. Vagle, 1996, [http://archives.datapages.com/data/bulletns/1994-96/data/pg/0080/0005/0700/0746.htm Influence of lithofacies and diagenesis on Norwegian North Sea chalk reservoirs]: AAPG Bulletin, v. 80, no. 5, p. 746–768.</ref> A correlation is found between the clay content of the chalk and the degradation of reservoir quality; clay hinders early lithification. As a result, clay-rich chalks are less rigid and will tend to undergo more compaction.<ref name=Kennedy>Kennedy, W. J., 1987, Sedimentology of Late Cretaceous–Paleocene Chalk reservoirs, North Sea central graben, in J. Brooks and K. Glennie, eds., Petroleum geology of northwest Europe 1987: London, Graham amp Trotman, p. 469–481.</ref> It is a common pattern in chalk oil fields to find the highest porosity in the crest of the field, decreasing incrementally toward the oil-water contact.<ref name=DHeur /> This character may result from the race for space between oil migration and cementing fluids. The permeability in the water leg can be so poor that chalk fields are unlikely to have significant aquifers.

Chalk reservoirs can show strong permeability layering. Pelagic chalk is usually non-net reservoir although under favorable circumstances it can be productive.<ref>Megson, J., and T. Tygesen, 2005, The North Sea Chalk: An underexplored and underdeveloped play, in A. G. Dore and B. A. Vining, eds., Petroleum geology: Northwest Europe and global perspectives: Proceedings of the 6th Petroleum Geology Conference, Geological Society (London), v. 1, p. 159–168.</ref> Pelagic or autochthonous chalk results from the slow settling of sediment on the sea floor. Pervasive early cementation and extensive bioturbation significantly reduce the porosity and permeability from an early stage.

Chalk reservoirs can show strong permeability layering. Pelagic chalk is usually non-net reservoir although under favorable circumstances it can be productive.<ref>Megson, J., and T. Tygesen, 2005, The North Sea Chalk: An underexplored and underdeveloped play, in A. G. Dore and B. A. Vining, eds., Petroleum geology: Northwest Europe and global perspectives: Proceedings of the 6th Petroleum Geology Conference, Geological Society (London), v. 1, p. 159–168.</ref> Pelagic or autochthonous chalk results from the slow settling of sediment on the sea floor. Pervasive early cementation and extensive [[bioturbation]] significantly reduce the porosity and permeability from an early stage.

Pelagic chalk on the seabed is easily disturbed and remobilized. Clean chalk lacks any significant sediment cohesion as it has no unbalanced interparticle electric charges or platy interlocking grains to hold it together.<ref>Bramwell, N. P., G. Caillet, L. Meciani, N. Judge, M. Green, and P. Adam, 1999, Chalk exploration, the search for a subtle trap, in A. J. Fleet and S. A. R. Boldy, eds., Petroleum geology of northwest Europe: Proceedings of the 5th Conference, Geological Society (London), p. 911–937.</ref> Processes tending to redeposit chalk include debris flows, turbidity currents, slumps, and slides ([[:File:M91FG200.JPG|Figure 6]]).<ref name=Kennedy />

Pelagic chalk on the seabed is easily disturbed and remobilized. Clean chalk lacks any significant sediment cohesion as it has no unbalanced interparticle electric charges or platy interlocking grains to hold it together.<ref>Bramwell, N. P., G. Caillet, L. Meciani, N. Judge, M. Green, and P. Adam, 1999, Chalk exploration, the search for a subtle trap, in A. J. Fleet and S. A. R. Boldy, eds., Petroleum geology of northwest Europe: Proceedings of the 5th Conference, Geological Society (London), p. 911–937.</ref> Processes tending to redeposit chalk include debris flows, turbidity currents, slumps, and slides ([[:File:M91FG200.JPG|Figure 6]]).<ref name=Kennedy />